Informative booklets for non-destructive testing

NDTCompact and understandable

Volume 7

THE X-RAY INSPECTION(RT/RS)

The German Library - CIP-Standard reception

Purschke, MatthiasThe X-Ray Inspection (RT / RS)

Castell Publication Inc., Wuppertal 2004

All rights reserved. No part of this publication shouldbe reproduced, stored in a retrieval system, ortransmitted in any form or by any means, electronic,mechanical, photocopying, recording or translated.

Printed in Germany

Layout, printing and processing:Neusser Werbedruck GmbH, Remscheid

ISBN 3 - 934 255 - 22 - 1

Volume 7

The X-Ray Inspection (RT / RS)by

Matthias Purschke PhDGE Inspection Techologies (GmbH LLC etc.)

X-ray inspection is used:

to discover inconsistencies and defects of all kinds. This applies to both thevolume and the surface of the workpiece through which the radiationpasses. Not only iron and steel can be subjected to an X-ray inspection, butalmost any type of metal and its respective alloys, as well as ceramics,synthetic porcelain, concrete, wood and composite materials (carbon-fiberreinforced and glass-fiber reinforced plastic) can be inspected.

Along with ultrasound and eddy-current methods, X-ray inspection is one ofthe most widely used procedures in Non-Destructive Testing (NDT). It isused to examine welded joints and castings that are used in a diverse arrayof applications such as inspection of vehicle components, pressurevessels, pipelines, chemical systems and many others.

X-ray techniques are divided into 4 types:

Radiography (RT): The X-ray film or digital detector serves as a mediumthat makes the X-ray attenuation profile visible. This technique is commonlyapplied to welded joints, particularly during field examinations with mobilex-ray units.

Radioscopy (RS): An electronic detector is used to make the x-rayattenuation profile visible. This, in turn, enables the inspector to carry outan “on-line inspection” through a video monitor. This technique iscommonly used for light alloy castings and assemblies, as well asstationary inspections (i.e., testing of welded joints during the productionprocess).

Densitometry: This technique is used to measure the thickness of material(i.e., steel plates).

Diffractometry: This technique is used to analyze various materials byexamining the diffraction properties of X-rays.

This book covers radiography and radioscopy exclusively. It is important tonote that the X-ray techniques described are only applicable to inspectionobjects that can be penetrated by X-rays.

The focus of the book on radioscopic applications is not intended as avalue judgement or a bias towards radiography. It is based solely on theauthor’s considerable additional experience with radioscopic applications.

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Contents:

1. Fundamentals of X-ray technology 71.1 Introduction 71.2 Generation of X-ray radiation 81.3 X-ray adjustments 111.4 X-ray attenuation 12

1.4.1 Radiation attenuation through photoabsorption 13

1.4.2 Radiation attenuation through scattering 131.5 Radiation contrast 16

1.5.1 Quantitative observations 171.5.2 Noise 19

2. Industrial X-ray units 222.1 Operating/control module 222.2 High voltage generation 23

2.2.1 AC voltage devices 232.2.2 DC voltage devices 252.2.3 High-stability devices 26

2.3 Tube housing 272.4 Operation of X-ray units 302.5 Verification of device properties 31

2.5.1 Focal spot measurement according toIEC and EN 12543 31

2.5.2 Evaluation of maximum energy accordingto EN 12544 35

3. Detectors 373.1 Radiographic imaging system 373.2 Radioscopic imaging systems 39

3.2.1 X-ray image intensifier video system 413.2.2 X-ray-sensitive line diode arrays 433.2.3 Semiconductor flat panel detectors 45

3.3 Image quality control 503.3.1 Wire penetrameters 503.3.2 Platinum duplex wire penetrameter 53

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4. Radiography - Radioscopy 554.1 Radioscopic optical imaging geometry 56

4.1.1 Ideal imaging geometry 574.1.2 Real imaging geometry 58

4.2 Quantitative description of radioscopy systems 60

5. Radioscopic examination systems 655.1 Requirements 67

5.1.1 Weld examinations 685.1.2 Casting examinations 695.1.3 Recognition of foreign bodies in food 695.1.4 Soft radiation technology 70

5.2 Introduction to digital image processing 715.2.1 Image enhancement 725.2.2 Image evaluation 73

5.3 Computed tomography 78

6. Summary and outlook 86

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1. Fundamentals of X-ray technology

1.1 Introduction

X-ray radiation belongs to the group of electromagnetic radiation. Figure1 shows an overview of the entire spectrum of electromagnetic radiationtypes according to their respective energy. Because of its higher energy incomparison to visible light, the X-ray can also penetrate materials that areopaque to ordinary light. This penetrative ability and the linearpropagation of the X-ray form the basis for all radiographic (withradiographic film) and radioscopic (without film) examination methods.These processes provide useful information by generating descriptiveimages of the thickness and density properties of the workpiece.

Fig. 1: full spectrum of electromagnetic radiation and energyspecifications, as well as fields of application (according to Becker)

In a technical sense, X-rays are generated through the use of X-ray tubes(see Fig. 2). Radioactive isotopes are often used in field radiography.Radiation forms when the atomic nuclei of radioactive substances areconverted. The conversion causes a spontaneous decay; the X-ray is thusgenerated. It is not possible to influence the energy and intensity of theradiation. The type of radioactive substance used is the sole determinant ofthese factors.

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In contrast to the continuous X-ray spectrum (see Fig. 4), radioactiveisotopes have line spectra that vary according to the isotope used. Theisotopes that are most commonly used today are selenium (Se 75), iridium(Ir 192), and cobalt (Co 60). The isotopes are transported in suitablecontainers and used through a mechanical system at the destination. Onone hand, isotopes are easy to handle because no electrical supply isnecessary, but on the other hand, there are substantial legal regulations toensure sufficient protection from radiation that can make use andtransportation difficult. Because it is not possible to adapt the energy of theisotope to the wall thickness of the workpiece, isotopes can only be usedfor limited wall thickness ranges (see EN 1435).

1.2 Generation of X-ray radiation

The principle of X-ray generation is illustrated in Fig.2.

Fig. 2: the principle of X-ray generation

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A thin wire filament (cathode) inside an evacuated glass or metal-ceramictube sends out (emits) electrons. The electrical potential difference (tubevoltage) that exists between the cathode (negative electrical polarity)and the anode (positive electrical polarity) results in a force that causesthese free electrons to accelerate as they move towards the anode. Oncethey strike the anode (target), the electrons decelerate (lose kineticenergy).

The area of the anode that is struck by the electrons emitted by thecathode is the thermal focal spot. Its size depends on how focused theelectron beam is, as well as the arrangement and size of the heating wirefilaments. When the electrons decelerate, only approximately 1% of theenergy generated is X-ray radiation. The remaining 99% of the energygenerated is thermal energy. Consequently, the X-ray tube anode must becooled intensively. There are several requirements that the anode materialand the degree of focus of the electron beam must fulfill.

The anode material must possess a high melting point. In general,tungsten (melting point approx. 3000°C) is used. The mean temperature ofthe focus is about 800°C, however, at its peak, it can be far higher.

There must also be a limited degree of focus of the electron beam. Theintensity requirements determine the degree of focal spot focus. Highintensities require large focal spots.

When a thermal focal spot is projected onto an area that is perpendicular tothe central beam, it is referred to as an optically effective focal spot or anoptical focal spot (see Fig. 3). Particularly in radioscopy, the size of theoptical focal spot is fundamentally important - it affects the quality of theimage. Two factors determine the size of the optical focal spot:

• the size of the thermal focal spot-

• the angle of the anode surface in relation to the central beam, calledthe anode angle β.

This angle is about 20° for usual standard tubes. Anode angles and rayoutlet windows limit the opening angle of the useful bundle of rays to about40°.

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Fig. 3: thermal and optical focal spot

The X-ray that is emitted by the anode during the deceleration processleads to a continuous spectrum, from a process called “Bremstrahlung”(from the German for “braking radiation”). A spectrum is described asradiation intensity I0 over radiation energy E (see Fig. 4). The term“continuous” means that within a certain area, all energies are present -even if they display different intensities. In contrast, gamma rays display aspecific and discrete line spectrum. The highest point of energy is referredto as maximum energy Eg.

In radioscopy, depending on the application, the dimensions of the focalspot vary between 0.3 mm and a maximum of 1.5 mm. In radiography, thedimensions can measure up to several millimeters. To fulfil the highestimage quality requirements, microfocus tubes with focal spot dimensionsthat measure less than 0.1 mm are also used.

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Fig. 4: energy distribution at a specific tube voltage

The maximum energy Eg can only be used to describe radiation in a limitedmanner. This is because it is based on the condition that the kinetic energyof the electrons is fully converted into radiation. This is a purely theoreticaloccurrence. Therefore, we normally use the mean energy or the positionof maximum intensity to describe radiation properties clearly. The meanenergy describes the level of radiation energy at which the shaded areas inFigure 3 below the curve are equal in size. It is not at the point of maximumintensity.

The spectrum's total intensity depends on the tube current I and the tubevoltage U values. For the purposes of radiography and radioscopyapplications, the anode material does not need to be considered further.

1.3 X-ray adjustments

The following discusses what impact X-ray operating desk adjustments canhave on the spectrum:

• Change of tube current

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Changes in current I cause a proportional change of radiation intensity I0 forall energies below the maximum energy. The mean energy, the position ofthe maximum intensity and the maximum energy remain unaffected.

• Change of the acceleration voltage (tube voltage)

Increasing voltage V causes the point of maximum intensity to shift to ahigher level of energy. It also leads to higher maximum and mean energyvalues, as well as a superproportional change of the total intensity.

To affect the radiation quality (penetrative ability and intensity rates),another option is to insert “prefilters” into the path of rays between theradiation source and the workpiece. This is also referred to as hardening ofthe spectrum.

• Hardening through prefiltration

In this process, the total intensity decreases, but the mean energy andmaximum point of intensity shift to a higher energy value. Materialscommonly used for prefiltration purposes are aluminum (Al) with athickness of 1-5 mm and copper (Cu) with a thickness of 0.2 – 2 mmupwards.

1.4 X-ray attenuation

The major characteristic of the X-ray is its ability to penetrate. Some of theradiation passes through a workpiece, but the rest does not. A suitabledetector can record the resulting attenuation and create a radiograph.

In order to understand the interactions that take place when the radiationpasses through the workpiece, it helps to imagine the radiation as“particles” that possess a certain energy. In the case of electromagneticradiation, these particles are called photons or quanta. You can picturethe attenuation as an intermittent interaction with the electrons of the atomor the molecules of the workpiece. For the application energy areascovered in this course (approx. 40-450 KeV), we can describe theattenuation as the sum of two sub-processes: photo absorption andscattering.

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The highly complex secondary processes, which work to undo thetemporary ionization of the atoms, will not be described further at thisinstance. In addition, it must be noted that in reality, the reciprocal actionsbetween X-rays and physical matter are much more complicated. Thesimplified description presented here is, however, fully sufficient for a clearunderstanding.

1.4.1 Radiation attenuation through photo absorption

Photo absorption is the actual desired and image-creating effect. When theenergy of the X-ray photon hits the test object, it is transferred fully(absorbed). A portion of the energy is emitted again when a photoelectron isreleased, and when X-ray fluorescence radiation is generated (see Fig. 5).

Fig. 5: photo absorption of a photon by an orbital electron

1.4.2 Radiation attenuation through scattering

During the scattering process, the X-ray photon experiences a change ofdirection (scattering) with or without a reduction of energy (Compton orRayleigh scattering). From energy values of 40-60 keV onwards, Comptonscattering dominates significantly in comparison to Rayleigh scattering.Therefore, we will concentrate exclusively on the Compton effect. Here, theintermittent action causes the photon to lose energy while at the same timea Compton electron is released (see Fig. 6). The scattered radiation thusgenerated spreads evenly in all directions and does not generate animage. This can be compared to illuminating an object from all sides; itdoes not produce a silhouette within an optically visible range.

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Fig. 6: scattering of a photon when it comes into contact with an orbitalelectron

The radiation attenuation, which is presented as the sum of both effects,strongly depends on the radiation energy E. In the case of increasingenergy, the attenuation weakens considerably. In addition, the workpieceatomic number Z (see Fig. 7) also determines which effect is dominant. Forlight alloys and plastics, it is the Compton scattering effect.

Fig. 7: dependence on absorption and scattering of energy and atomicnumber

It is not possible to mathematically reconstruct the individual intensityand/or attenuation profile that results from the radiation attenuation oncethe radiation has passed through the workpiece. A quantitative description

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of radiation attenuation can only be made when simplified significantly. Inaddition, two highly theoretical assumptions are made:

• narrow bundling of rays and• monoenergetic radiation (only one line in the spectrum).

However, it can be useful for many applications to use the aboveassumptions to describe the radiation attenuation behavior mathematically(see Fig. 8):

I = I0 ⋅ e - µ ⋅ w

I - intensity behind test objectI0 - non-attenuated radiation intensityµ - attenuation coefficientw - irradiated workpiece wall thickness

The attenuation coefficient µ depends on the:• wall thickness and volume of the workpiece• energy spectrum• material• imaging geometry

Fig. 8: radiation attenuation through absorption and scattering

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As the radiation energy increases, the attenuation coefficient decreases. Inother words, the workpiece becomes increasingly transparent and thusabsorbs less radiation. This means that as the energy increases,multithickness workpiece walls can only be imaged with smaller intensitydifferences (radiation contrast Cr).

1.5 Radiation contrast

The radiation contrast is of special interest for X-ray examinations. This isbecause, in general, there is a significant link between how easy it is todetect material defects and the contrast quality of the X-ray image.

As radiation energy increases, the attenuation coefficient decreases. Thismeans that the workpiece becomes more and more transparent (it absorbsless radiation).

Consequently, as the energy increases, multithickness workpiece walls canonly be imaged with smaller intensity differences ∆I (see Fig. 9).

Fig. 9: dependence of intensity difference on energy

That is to say, as the energy increases, the intensity difference of adetail (a defect) on the radiograph decreases independently of thedetector used. This must be considered for each respectiveexamination. "More is better" is the wrong method when adjustingthe X-ray voltage for detecting defects!

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1.5.1 Quantitative observations

In the following paragraphs, we would like to present a mathematicaldefinition for radiation contrast. It is based on the intensity conditions duringthe X-ray examination. This point of view is intended to explain the majorinfluence factors on radiation contrast for practical work purposes.

In accordance with the physical processes that take place duringattenuation, the total intensity behind the test object always consists ofthe attenuated primary radiation (absorption) and the scattered radiation.

The total intensity Ig behind the test object is the sum of attenuated primaryradiation Ip and scattered radiation Is:

Ig = Ip + Is

Because the scattered radiation spreads evenly in all directions, in caseswhere there is only a small difference w in the thickness of the material, theportion of scattered radiation Is has the same value in all areas behind theworkpiece. The "diffused" character of the scattered radiation appears as abase intensity or background brightness (radioscopy) / base opticaldensity (radiography) in the radiograph.

The intensity difference ∆I, which is important for the ability to detectdefects, and which is brought about through the change in wall thickness wis, in simple terms, the difference between the attenuated primary radiationvalues Ip1 and Ip2.

∆I = Ip1 - Ip2

If we now apply the X-ray attenuation law,

I = I0 • e - µ • w

I - intensity behind the test object,I0 - unattenuated radiation intensity,µ - attenuation coefficient, andw - wall thickness of irradiated specimen,

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then a connection can be made between ∆I and ∆w as well as the energy-dependent X-ray absorption coefficient. The result is:

∆I ≈ Ip • µ • ∆w

We can then calculate the radiation contrast as the relative difference inintensity ∆I to the total intensity Ig.

Cr = ∆I/Ig = µ • ∆w / (1 + Is/Ip)

A more general formulation illustrates the difference between two differentattenuation coefficients (test material (µ) and material defect (µf)) andprovides a definition of radiation contrast as follows:

Radiation contrast: Cr ≈ (µ -µf) • ∆w / (1 + Is/Ip)

This formulation is important for X-ray imaging examinations. It uses thefollowing definition for the scattering ratio k: k = Is / Ip

Three factors affect the radiation contrast directly:• the difference between the attenuation coefficients• the spread of the defect, parallel to the direction of the beam• the scattering ratio

We can thus infer the following statements for practical examinations:

• Air or gas-filled cavities (µf ≈ 0) such as shrinkholes, etc, generate ahigh radiation contrast, which is solely determined by the depth of thedefect ∆w, the workpiece material and the scatter ratio. This alsoapplies to defects with large surfaces and cracks. This is where themain advantage of radioscopy comes into play. In terms of theirorientation, defects can be brought into an optimal position inrelation to the direction of the beam.

• In the case of metallic inclusions, high radiation contrasts can obtained ifthe difference between the attenuation coefficients of the workpiecematerial and the inclusion is large. This is often the case with light alloycastings. As above, an adjustment to the direction of the beam canenable optimal radiation contrast.

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• In general, it is possible to achieve a smaller scatter ratio by adjustingthe tube to fade out the primary beam on the specimen section to beimaged. Such methods should always be used to ensure optimal imagequality or radiation contrast.

1.5.2 Noise

To illustrate the "quantum nature" of radiation, the “particle model” wasused from section 1.4. It illustrated how helpful it is to imagine radiation asa stream of particles. Therefore, a mean value is always obtained, evenwhen we irradiate material with a constant thickness. The intensityfluctuates around this mean value (see Fig. 10). This variation of the meanvalue is called "noise."

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Fig. 10: superposed noise over the mean intensity

The signal-to-noise ratio (SNR) is used to measure the influence of noiseon the actual “useful” signal. Normally, definitions of this term describenoise amplitude in relation to the mean intensity value:

• SNR large → “useful” signal; mean intensity value I0 is large in relationto noise → good image quality

• SNR small → “useful” signal; mean intensity value I0 is small in relationto noise → poor image quality

The SNR depends on intensity. The greater the intensity of the X-ray,the better/larger the SNR.

In contrast to radiography, the signal-to-noise ratio is very important inradioscopy. This is because in radioscopy, the images generated can beviewed “live” through the use of the appropriate detectors. By necessity,radiography uses exposure times that are significantly longer in order to“average” the intensity. This causes a higher SNR and allows the examinerto disregard the impact of noise on image quality. Radioscopy either workswith the corresponding higher intensities or uses digital image processingsystems to “average” the intensity (see Section 3.2).

Intensity

Time

I0

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Conclusions regarding radiographic image quality:

Penetration Contrast Intensity SNR

X-ray voltage/power:

increase more less more more

decrease less more less less

X-ray current:

increase same more more more

decrease same less less less

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2. Industrial X-ray units

Industrial X-ray units typically consist of the following components:

• operating/control module• high voltage generation• high voltage cable• tube housing• cooling system

2.1 Operating/control module

The user can adjust all the necessary parameters through control moduleoperation:

• high voltage (kV - kilovolts - 1000 V)• tube current (mA - milliamperes - 0.001 A)• exposure time (min - minutes)• focal spot (if the tube allows a choice between two focal spots)• operation modes (constant current, constant power, program)• selection of the warm-up program for tube training• focal spot size and focusing (specific to microfocus systems only)

Fig. 11: operating module of a modern X-ray unit

Modern operation and control modules (Fig. 11) are controlled bymicroprocessors. The necessary parameters thus can be configured with

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ease, and the user benefits from the best protection available against faultyoperation. Images of nominal and actual values are displayed on a screensimultaneously. The user can operate the device shown in Fig. 11 via softfunction keys. In addition, the system displays messages alerting the userto all causes of errors that occur through faulty operation or devicemalfunction. For a cost-effective integration of the X-ray unit into amanufacturing process, there is the option to control the entire systemthrough a serial interface. This can also be done without the controlmodule, for example with a PC or an industrial control panel

2.2 High voltage generation

X-ray units can be divided into three groups according to the type of highvoltage generation:

• AC voltage devices• DC voltage devices• high-stability devices

2.2.1 AC voltage devices

AC voltage devices are used for radiographic weld examinations for ship,pipe, and steel construction. They are deployed during field examinationswhere mobility is essential.

To enable mobility, the high voltage source and the X-ray tube are installedin a radiation protection housing called the tube housing assembly.Consequently, high-voltage cables and patch cords are no longer needed.The tube housing assembly is connected to the control unit through afairly long low-voltage cable which allows the configuration of theparameters (voltage, current, exposure time) (see Fig. 12).

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Fig. 12: transportable AC device (built around 1960)

These devices are also called single-tank or half cycle systems. TheX-ray tube is supplied with a high voltage of changing polarity. This meansthat an X-ray can only be generated during every other half cycle (when thehigh voltage adjacent to the anode is positive). The radiation generated ispulsed at the same rate as the supply frequency. This effect does notinterfere with radiography, because the exposure times are generally muchlonger than the net period.

Modern single-tank systems are characterized by the fact that they canproduce a high DC voltage and thus substantially shorten the necessaryexposure times (see Fig. 13).

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Fig. 13: transportable DC voltage device (built in 2003)

2.2.2 DC voltage devices

DC voltage devices are intended for continuous operation and primarilyused for stationary operation. The high voltage is generated in an areathat is separate from the X-ray tube. Such devices have a rectifier circuitwith filter capacitors that is integrated into the process of generating highvoltage. Therefore, the DC voltage can be generated with a low ripple.

The main advantage when using DC voltage X-ray units, especially formobile weld examinations, is that the exposure time is significantly shorterthan for half cycle devices (see Fig. 14). DC voltage devices reduce thenecessary exposure time substantially, particularly when examining largerwall thicknesses.

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Fig. 14: comparison of exposure times: iridium, standard single-tank andDC voltage device (ERESCO 42 MF)

Depending on the high voltage required, one can distinguish unipolaror bipolar devices. A unipolar system requires one high voltage generator,while a bipolar system requires two. For unipolar operation the maximumhigh voltage is 225 kV, and for bipolar operation it is 450 kV (see Section2.3).

2.2.3 High-stability devices

Until recently, devices with a converted frequency of 500 Hz instead of asupply frequency of 50 Hz were referred to as high-stability DC voltagedevices. They enabled voltage fluctuations at a per mille ratio to thenominal value.

Today, devices that are equipped with the most modern power-electronicsemiconductor components are also considered state-of-the-art for routineradioscopic examinations. The devices work with a switching frequency ofabout 20 kHz and only exhibit high voltage ripples at frequencies of about 40kHz. These ripples amount to fractions of per mille. Therefore, even at theintensity of radiation, only the smallest fluctuations occur.

The application of this technology allows a highly compact and durablepower and control electronics set-up (see Fig .15).

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Fig. 15: high voltage generator cathode with power module (unipolarsystem up to 225 kV)

Connecting the power module directly to the high voltage generatordecreases the required usage space. In the case of bipolar systems, whichconsist of a cathode generator, power module, and anode generator, it ispossible to stack or place the high voltage generators next to each other.

For the usual radioscopy applications, i.e., routine weld and castingexaminations, no additional cooling is needed for the generators and powermodules.

2.3 Tube housing

X-ray tubes differ according to:

• power (maximum 4.5 KW)• nominal voltage (maximum 450 kV)

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• dimension of the focal spot• design (unipolar, bipolar tubes)

Two different types of design are unipolar and bipolar tubes. One majorunipolar tube characteristic is that only the cathode is supplied withhigh voltage. The anode is grounded (0 V), thus only one high voltagecable and one high voltage generator (cathode generator) are necessary. Itis possible to cool the anode of a unipolar tube of up to 225 kV with water(see Fig. 16).

Fig. 16: diagram of a unipolar tube

For isolation reasons, unipolar operation is not useful at voltages greaterthan 225 kV. Therefore, bipolar tubes are used for higher tube voltages.With a -225 kV cathode maximum and a +225 kV anode maximum, thetotal voltage amounts to a maximum of 450 kV (see Fig. 17). Bipolartubes generally use oil for cooling purposes, and need a heatexchanger/cooling unit to re-cool the oil.

Fig. 17: diagram of a bipolar tube

max. -225 kV

oil cooling

max. +225 kV

water cooling

0 Volt

max. -225 kV

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Currently, both tube types are called metal-ceramic tubes. They containceramic parts, which are placed between a metal cover and high voltageparts, providing for electrical isolation. Beryllium is the material used for thetube window. This material has a small inherent filtration rate and only hasa slight effect on the radiation spectrum.

We can further divide the types of tubes into direct and panoramicemitting units. Direct emitting units are generally used for standardapplications. The radiation angle of it normally lies at around 40 degrees. Incontrast, a panoramic unit can emit radiation over 360 degrees. Thesetubes are often used for weld examinations of round welds centrally locatedinside the pipe (the tube is moved into the pipe).

The size of the focal spot, or focus, is an important aspect when selectingthe X-ray tube for a special application. While in radiography, due to specialimage conditions, the focal spot sizes are defined in millimeters. Inradioscopy, focal spot sizes are classified as follows:

standard focal spot: ≥ 0.4 minifocus: 0.1-0.4 microfocus: ≤ 0.1

Geometric magnification of up to 2 and a maximum of 3 can easily beobtained with the standard focal spot 0.4 IEC (0.85 mm EN 12543).Minifocus tubes with a 0.2 focal spot (0.3 mm EN 12543) allow forgeometric magnification of a maximum of 6-8. Microfocus tubes areused for larger geometric magnifications. Depending on the degree offocus, they can enable magnifications of 50 and more. This is also referredto as X-ray microscopy.

In contrast to the tubes described so far, microfocus tubes are opentubes. This means that an additional vacuum pump is necessary toevacuate the tube body. This type of system allows the user to replace thewire filament and anode as needed.

Microfocus tubes have focusing systems that can be used to bundle theelectron beam. In contrast to conventional tubes, microfocus tubes containa focus coil and a special cathode system in order to bundle radiation. Theyalso often contain an XY deflection system in order to be able to positionthe focal spot on the anode. Typical focal spot dimensions lie within a

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range of 10 µm, for an electrical power of up to approximately 40 W.Higher wattages result in larger focal spots.

2.4 Operation of X-ray units

The adjustment of the high voltage depends on the workpiece anddetermines the penetrative ability of the radiation. The adjustment of thetube current determines the intensity of the radiation and must also beadapted to the workpiece.

It is only necessary to pre-select an exposure time in radiography. Theexposure clock is switched off for radioscopic applications.

With double focus tubes, it is possible to switch between a large"radiographic" and a small "radioscopic" focal spot. Often tubes forradioscopy are already equipped with two small focal spots so that if onefocus is damaged, the second focus can be used as a substitute withoutlosing time.

The following modes of operation are available:

Constant current operation: The tube is operated with pre-set operatingvalues for voltage, current and if necessary, exposure time.

Constant power operation: The tube is operated with pre-set voltage andif necessary, exposure times. The tube current is automatically adjusted tothe value that the tube power allows for at the specified high voltage.

Program operation: Operating parameters can be entered into programsto facilitate work on repeated examinations. Entering a program number willlaunch the program. This further reduces the opportunity to enter wrongparameters.

It is necessary to warm-up the tube after each long pause because it is avacuum element. The warm-up speed depends on whether the tube hasbeen out of operation for days, weeks, or months. A microprocessor controlmodule automatically performs a warm-up according to the length of thebreak. Modern X-ray units have a built-in real-time clock. It enables anoptimal adjustment to the new desired operating parameters, while takingthe previous break and operating parameters into account.

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For X-ray units with sealed tubes, it is only necessary to adjust theparameters for current, voltage, and exposure time. For microfocussystems it is also necessary to adjust the focus of the beam. Because theoptimal beam focus changes whenever the high voltage changes, it isnecessary to readjust it for each high voltage value. Since the optimalfocusing current changes whenever the high voltage changes, it must beadjusted anew for each high voltage. Today, CPU-controlled operatorcontrol panels are available to automate this highly complicated procedurealmost entirely.

2.5 Verification of device properties

The properties of an X-ray unit that are important for the user are theimaging properties of the tube, and the relation between the adjusted X-rayvoltage and the actual maximum energy attained. The actual maximumenergy attained is a major factor in determining the necessary exposuretime. Because of this fact, the relation to the adjusted X-ray voltage is anessential factor for quality - especially in radiography. Europeanstandardization authorities took this into account when they drafted EN12544, which is a binding standard on determining this relation.

It is a fact that particularly in radioscopy, the impact of the focal spot on theimage and image quality is of particular importance. We would thereforelike to introduce the method of measurement according to DIN or IEC,which has been in use so far, as well as the new reference method ofmeasurement according to EN 12543. These standards will help the user toassess the various data provided by tube manufacturers in relation to theimage properties.

2.5.1 Focal spot measurement according to IEC and EN 12543

Focal spots are currently measured in accordance with two standards,which are identical in content: DIN 6823 and IEC 336. These standardsspecify the imaging conditions, the structure of the aperture or pinholecamera, and the subsequent evaluation of the measurement results.

In order to measure the size of the focal spot, a lead-encased aperture(pinhole camera) is attached to the front of the tube outlet window. The sizeof the aperture is 30 µm for nominal focal spot values larger than 0.3, and10 µm for minifocus tubes smaller than 0.3. For nominal focal spot values

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larger than 1.2 or 2.5, apertures of 75 µm or 100 µm are permissible. Animage of the focus is projected onto a radiographic film.

For this type of measurement, several problems arise that can considerablyskew the result:

• the aperture produces a blurred focal spot image• fixation of the aperture in the central beam• affect of prefilters on the optical density of the film

The most significant disadvantage of the procedure is that measurementscan only be made in a field of energy that is normally limited to a maximumof 60-70 keV. This is because at higher levels of energy, the radiationpasses through the lead shield of the aperture. Therefore, it is not possibleto measure the focus under nominal conditions.

A magnifying lens is used to evaluate the focus image generated on theradiographic film (see Fig. 18). The result of this measurement is thencompared to the IEC reference table provided to determine the nominalfocus size.

Fig. 18: two X-ray focal spots on a radiographic film (photographed with apinhole camera)

Since it is already possible to weight the measured value towards theanode-cathode-axis with a factor of 0.7 at the focal spot image evaluationstage, substantial discrepancies between the data provided by the tubemanufacturers and the true focal spot dimensions will occur in practice.For example, with a 0.4 focus, the focal spot dimensions that weredetermined at that value may be surprisingly high at values of 0.6 mm or0.85 mm.

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The nominal focal spot values determined are used in radiography tocompute the minimum distances between the focus and the film (accordingto DIN 54111, EN 444 and EN 1435). Naturally, the result of the differencebetween the true focal spot dimensions and the specified nominal valuesindicates that the image quality in radiography is worse than expected atthe specified nominal values. This does not, however, play a crucial rolebecause, thanks to the clearly specified standards, all users will obtain thesame distance ratios and the same image quality.

This situation is not as acceptable in radioscopy. Because it usesgeometric magnification in a very specific manner, a physically meaningfulcharacteristic value that accurately describes the actual focus imageproperties is essential.

In the course of the European standardization activities, this wasimplemented for both the measuring process and the evaluation ofmeasurement results. In the future, manufacturers will base the focal spotdata they provide on the actual focal spot measurements. Because this willforce a declaration of larger focal spot measurements, it will have a lastingimpact on the applicable minimum distances in radiography. It will alsohave a positive effect on image quality.

The European standard EN 12543 concerning X-ray focal spotmeasurement and evaluation also takes into consideration that an accuratespecification of the focal spot size is especially important in radioscopy.That is why part 1 of the draft standard defined a reference measuringprocedure. All procedures that follow must bear comparison to thisdefinition. In addition, this standard defines how to accurately specify thefocal spot size based on the measurement results.

This "scanning procedure" is a radiometric method for measuring focalspots. It scans the focus line by line, computes the size of the focal spot,and then displays it in a three-dimensional image.

The roughly collimated central beam of the X-ray tube is measured with thehelp of a scintillation counter over a double-slit collimator with an effectiveopening of 10 µm x 10 µm. The precollimator, the double-slit collimator, andthe scintillation counter are fitted onto a precision compound slide rest.Stepper motors are used to move them. A PC that is capable of reading outthe measurements records the output signal. At the same time, a line scanis carried out in specified increments. The size of the increment depends

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on the desired resolution. The software facilitates the creation of a highlydescriptive image, and subsequent focal spot processing.

Figure 19 shows the result for a typical focal spot with a clearly pronouncedsaddle structure. According to DIN specifications, it is a focal spot with anominal value of 1.5. The distribution of intensity displayed in the threedimensional image has maximum values that correspond to the highestintensities, and also to the largest optical density that can be measuredwhen using the pinhole camera method.

Fig. 19: result of the scanning procedure

Figure 20 displays the focal spot's distribution of intensity in the form of“contours.” This isodose image shows the isodoses between 10% and 90%of the maximum intensity.

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Fig. 20: result of the scanning procedure in the form of an isodose image

In the new standard, the focal spot size is defined as the maximumexpansion in length and width that is evident between the 10% isodoses inthe isodose image. In radiography, the larger of these twomeasurements is defined as the nominal focal spot. In the near future, twofocal spot sizes will be declared in the specification provided bymanufacturers: the size according to the new EN, and another according tothe old IEC/DIN.

2.5.2 Evaluation of maximum energy according to EN 12544

The relationship between the specified X-ray voltage and the actualradiation maximum energy attained (the time required to attain themaximum energy) is predictable when considering exposure times inradiography. The user should be able to assume that the maximum energyspecified by the manufacturer of the X-ray unit is an accurate measure ofthe required exposure time. This, however, is not guaranteed.

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Figure 21 outlines comparative data for five different transportable X-rayunits. There are remarkable differences in the necessary exposure timesspecified for a steel wall thickness of 65 mm. Four of the units providemaximum energy specifications of 300 keV. However, it is evident from thetable that the necessary exposure times specified vary considerably.Interestingly enough, one of the units specifies 275 keV as the maximumenergy value. The same unit (ER 60 MF) also specifies the second shortestexposure time, and not, as one would expect, the longest.

Fig. 21: comparison of required exposure times for various transportable X-ray units for 65 mm

EN 12544 will ensure that the data provided by manufacturers will becomemore transparent and easier to compare. It is an important step forward inensuring that industrial X-ray unit properties are standardized.

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3. Detectors

It is possible to use radiation examinations to detect differences in densityor thickness in a test specimen. The distribution of intensity behind the testspecimen must be converted by suitable detectors into a visible image forfurther evaluation.

It is for this reason that the detector of the X-ray system is of particularimportance. It is also referred to as the actual imaging system because itconverts an invisible radiation image into a visible X-ray image forevaluation. Different imaging systems are available for various testingpurposes. The most important are:

Radioscopy:• luminescent screen (also with video camera)• X-ray image intensifier video systems• X-ray-sensitive linear diode arrays• semiconductor flat panel detectors

Radiography:• X-ray film

The purpose of the examination determines which specially adaptedimaging system will be used. The most important parameters of thedifferent detectors are:

• spatial resolution or inherent detector unsharpness - this factordetermines how easy it is to detect small details with a certaingeometric magnification.

• contrast - difference in intensity when imaging shows slight differencesin wall thickness

• dynamic range - maximum wall thickness difference that can still beshown

• examination speed• cost

3.1 Radiographic imaging system

In radiography, the intensity ratios are recorded on an X-ray film.

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After the exposure process, the intensity distribution exists as a latentimage. The film is then processed into a document for further evaluation.The relation between the amount of radiation absorbed by the film (dose)and the resulting optical density produces the characteristic curve. Theoptical density difference ∆S resulting from the intensity difference ∆Idepends on the slope (gradient G) of the characteristic curve and is aproperty of the film system used (see Fig. 22). Prolonging the time ofexposure leads to a higher contrast because, according to J∼tB, theintensity difference ∆I is smoothed in an area in which the characteristiccurve displays a higher gradient.

Radiographs are evaluated through illumination generated by a filmilluminator according EN 25580. This leads to a negative image.Commonly used film illuminators allow an evaluation of radiographs up to afilm density of S ≈ 4, while some more powerful devices allow an evaluationof up to S = 5. The irradiation standard (EN 444) demands a minimumdensity of S = 2.0 for testing class A and S = 2.3 for testing class B.

Fig. 22: characteristic curve

The human eye can still safely perceive density differences of ∆Smin ≈ 1%provided that these changes in absorption occur abruptly. Every imagingsystem has a limited spatial resolution or a certain inherent unsharpness ui.

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This value is energy-dependent; with a voltage of U = 200 kV, the valuereads ui ≈ 0.1 mm.

Another property of the film system (EN 584) that determines image qualityis the graininess of the processed X-ray film. A film exposed to a certainoptical density that is viewed locally will display density variations around amean value that is analogous to the noise of the X-ray image (see Section1.5.2). High image graininess hinders the detection of the defect, especiallywhen displaying small defects optically (very small ∆S). Changing the filmtype to one with lower graininess, however, means that the exposure timemust be prolonged.

Radiography is especially advantageous when mobile operation is aconcern. When using a suitable radiation source, the procedure offers highcontrast sensitivity and sharpness in the optical display of defects.However, radiography is a static examination method, and the detection ofdefects is easily complicated by an unfavorable alignment of the defect inrelation to the direction of the beam.

Film, of course, is seldom worth considering as a detector duringproduction processes because of the required exposure and developmenttime. In such cases, radioscopic X-ray systems that allow online viewing ofthe X-ray image must be used.

3.2 Radioscopic imaging systems

When using radioscopy, an image of the intensity distribution is produced inits simplest form on a screen. The ability of a certain material to convert X-rays or gamma rays into visible light is called X-ray fluorescence.

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Fig. 23: radioscopy with luminescent screen

Analogously to the characteristic curve, there is a connection between theradiation intensity and the brightness (luminescence). The intensitydifferences behind the test object are shown as brightness differences onthe luminescent screen. In contrast to radiography, a positive image isproduced. The relation between radiation intensity and brightness isapproximately linear within a very large range. Compared to thecharacteristic curve, the small slope of the transfer function must beconsidered, and accordingly, the contrast sensitivity of the luminescentscreen is lower. The internal unsharpness is also higher than that of a filmsystem. Advantages are:

• dynamic examination and optimization of beam directionregarding defect alignment

• serial testing of parts• immediate ACCEPT / REJECT decision• low investment and follow-up costs

The X-ray image intensifying screen with a video camera currently has thelargest practical importance as a detector system. Drastic improvements incontrast and spatial resolution have made the image quality of this systemcomparable to the image quality generated in radiography.

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3.2.1 X-ray image intensifier video system

Inside the vacuum tube of the X-ray image intensifier (see Fig. 24), an X-ray sensitive, luminescent coating made of CsI (cesium-iodide) is locatedon the front side. In contrast to conventional luminescent screens, thiscoating is very thin and composed of specially aligned CsI crystals. Thisluminescent coating converts the X-ray image into a visible image. Theresulting light causes electrons to emerge from the photocathode. They areaccelerated through an electric field that exists between the cathode andthe electrodes of electron optics located on the output screen. The outputscreen converts the electrons into a visible image.

Fig. 24: design of the X-ray image intensifier

The light intensity is amplified by about 10,000. Despite being an “online”image, the image contrast that can be achieved is comparable to that of anX-ray film. This, however, causes a relatively high noise content in theimage. Nevertheless, in radioscopy it is possible to apply appropriate digitalimage processing methods to reduce noise considerably, which is similar toprolonging exposure time in radiography.

With its inherent unsharpness rate of approx. 0.3 mm, the imageintensifier delivers a distinctly lower spatial resolution compared to the X-

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ray film. Geometric magnification (see also section 4) can compensate forthis deficiency. Increased geometric magnification only leads to animprovement in detail recognition if the focal spot size is sufficiently small.Another way to improve detail recognition is the “zooming” of the imageintensifier. This means that the input field of the image intensifier isdecreased, yielding an enlargement effect in the output window.Consequently, the area of the specimen to be tested at an angle is smaller,and longer examination times are required. This is especially true in thecase of weld examinations, where a high spatial resolution is required todetect defects. “Zooming” helps improve defect recognition in this situation.

The output image of the X-ray image intensifier is scanned through a videocamera and transmitted to a monitor or a digital image processing system.The transmission is done according to the CCIR standard (or NTSC inNorth America)

The light intensity on the output screen of the image intensifier is scannedline by line and converted into an electrical voltage. In the video signal (seeFig. 25), a high light intensity (white) corresponds to a high voltage level.The individual video lines are displayed on the monitor in the line feedprocess. The whole image comprises 625 (640) lines. The full imagefrequency is 25 (30) Hz and the half image frequency is 50 (60) Hz. Theattainable resolution is 5 MHz-bandwidth horizontally, and approximately400 (480) pixels vertically.

Fig. 25: CCIR video signal

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Depending on the application, different camera types are used. Twodifferent types of cameras are available for a large range of applications:

CCD Cameras and the Conventional Tube Camera.

Tube cameras are used only – if at all – for test positions that require a highspatial resolution, i.e., resolution of small details. Tube camera propertiesgenerally depend on the camera tube used. However, certain properties,such as inertia, are the same across all tube types. This means that whenexamining specimen that are in motion (i.e., casting examinations),dragging effects or blurs appear, which can considerably reduce therecognition of small details. In the past, this has increasingly lead to theprimary usage of CCD Cameras, which, along with other positiveproperties, have very low inertia.

The CCD camera has a fundamentally different design compared to that ofa tube camera. It is composed of discrete light sensitive diode elements.The typical number of pixels (picture elements) is 756 x 581. Compared tothe tube camera, it has a somewhat lower threshold of resolution, but aconsiderably better contrast transmission in the middle spatial frequencyrange (see section 4). It is characterized by a very low inertia and a lowtendency towards glare. It is presently used as the standard camera forweld and casting examinations.

The properties and main configuration of the video monitor used are ofparticular importance for the ability to detect defects in X-ray images.Interestingly enough, they are not taken into consideration in mostdiscussions of radioscopic image quality. This is probably because monitorproperties are highly intangible and an examination of these propertieswould occur under considerable subjective influences. It should be noted,however, that no changes should be made to a configuration afteroptimizing the monitor adjustments (i.e., with a test image). However, thisdoes not correspond to common practice. Usually, the testing personneladjust the monitor according to their own subjective criteria.

3.2.2 X-ray-sensitive line diode arrays

X-ray sensitive line diode arrays are composed of coated photodiodes thatconvert the radiation into the electrical signal. The photodiodes arearranged along a line so that, in contrast to the X-ray image-intensifying

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screen, only one-dimensional information about the test object is obtained.This means that the test object must be guided past the line diode array ata constant speed. This leads to the test set-up shown in Figure 26.

Fig. 26: schematic examination set-up with linear diode array

An image processor processes the electrical signal and enables its displayon the video monitor. The read out speed of the line diode array is variable,which means that the signal-to-noise ratio can be improved. However, theread out speed must be synchronized with the movement speed of the testspecimen, otherwise the image will show geometric distortions. This meansthat a fast-moving object requires a higher read out speed, and only a slightimprovement in the signal-to-noise ratio is possible.

The read out speed also depends on the structural design of the lineardiode array. Linear diode arrays with few photodiodes allow for higher readout rates and object speeds than those with more photodiodes.

The usual width of a photodiode is around 0.45 mm. For specialexamination tasks that require a higher spatial resolution, line diode arrayswith a photodiode width of 0.225 mm are also available. The total width of

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an X-ray-sensitive line diode array depends on the number of photodiodesbuilt into the sensor (i.e., 1024 photodiodes x 0.45 mm yields a sensorwidth of 460 mm).

In practice, X-ray-sensitive line diode arrays exhibit several advantages.The tube-sided collimation of the ray cone on a fan beam that illuminatesthe linear diode array slightly, exposes only a narrow area of the testspecimen to radiation. Consequently, the resulting scattered radiation in thetest object is minimized, and the radiation contrast of the image isincreased considerably.

Moreover, because the individual layers of the test object are only exposedto radiation for the duration of one image line recording, the exposure of thetest object to radiation is low compared to other imaging systems. For thisreason, such detectors are installed wherever it is necessary that theexposure to radiation be minimal, such as baggage checks in airports (nofilm damage allowed in the test) and inspection for foreign bodies in food.

3.2.3 Semiconductor flat panel detectors

The desire to combine the advantages of an X-ray image intensifier (highluminance on output, good spatial resolution) with the advantages of afluorescent coating coupled directly to an image recorder led to thedevelopment of flat panel detectors or FPD’s (see Fig. 27).

Fig. 27: flat panel detector

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In flat panel detectors, a fluorescent screen is applied directly to a largesurface made of light-sensitive and radiation-resistant semiconductormaterial. Usually, the size of the input screens is within the same range asthe size of the X-ray intensifier input screens. However, the distortionstypical of X-ray intensifiers do not appear.

The working principle of flat panel detectors is based on the conversion ofincident X-ray radiation into an electrical charge, which can then be readout electronically. Amorphous silicon is used as an electrical semi-conductive material for this process.

The two principles of conversion are:• scintillator method (indirect conversion)• photoconductor method (direct conversion)

Each method has special advantages and disadvantages, as well as limitsfor practical use in imaging systems. For all methods of conversion, chargecarriers are integrated over a certain period before they are read outelectronically. The measurement data is then recorded using appropriatemethod, and displayed as an image.

With the scintillator method, a fluorescent screen is applied to theamorphous silicon to convert the X-ray radiation into visible light. Variousmaterials are available for this method. The basic design of a flat paneldetector with a conversion layer of phosphor compounds (Gadolinium Oxy-Sulfide (Lanex) or similar) is shown in Figure 28.

The incident X-ray radiation produces flashes of light in the phosphorcompound that are transformed into an electrical charge by a photodiode.Here again, the charge carriers are read out with the help of thin filmtransistors, which are approached line-by-line or column-by-column.

By using various chemical compounds and particle sizes in the scintillatormaterial, different properties concerning spatial resolution, light output ornoise can be enhanced or optimized. A drawback of all phosphorcompounds is the effect of light scattering in the scintillator. A compromisebetween image sharpness and signal-to-noise ratio must be madedepending on the material used. In spite of this, it is currently the mostwidely used technology for flat panel detectors.

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Varian comp.

Fig. 28: scintillator method (phosphor)

A flat panel detector consists of millions of light-sensitive pixels that arearranged in a grid on a rectangular surface. (see Fig. 29).

Varian comp.

Fig. 29: electrical design

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It is possible to also view this structure on the flat panel detector through amicroscope. (see Fig. 30).

Varian comp.

Fig. 30: actual design

Flat panel detectors with varying pixel sizes and dimensions are available(see Tab. 3.1). A flat panel detector weighs about 8 - 10 kg. Therefore, it iswell below the weight of an X-ray image intensifier.

FhG-IZFP Detector 1 Detector 2

Pixel width 0.4 mm 0.127 mm

Spatial resolution 1.2 LP/mm 4 LP/mm

Contrast resolution @200kV < 1% 3 %

Dynamic range 14,000 : 1 1,000 :1

SNR * 2,500 : 1 1,100 : 1

* calculated for: 200 kV, 1mm Cu pre-filter, 1 mA, 1 sec. integration time, 1 mm² detectorsurface

Tab. 3.1: overview

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There are distinct differences in spatial resolution between the two flatpanel detectors, depending on the width of the pixels. With its distinctly lowspatial resolution, detector 1 has considerable advantages in dynamicrange and signal-to-noise ratio. Accordingly, a compromise must always bemade between spatial resolution and dynamic range, depending on theusage, and the appropriate detector must be implemented.

One major difference between a flat panel detector and an X-ray imageintensifier video system is that some flat panel detectors do not have areal-time function. In addition, there can be problems when it comes todisplaying grey values on a normal VGA monitor, because many detectorshave a very high dynamic range of 14-16 bits (Tab. 3.1, Detector 1). Thishas considerable consequences for visual radioscopic examinations. Forexample, the test object can only be moved in a stop-go mode;consequently, the object examination requires a “manual drive” through theindividual grey value ranges that correspond to the various wallthicknesses.

A flat panel detector (detector 2) is shown in Figure 31.

Fig. 31: flat panel detector

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The entire system consists of the actual X-ray-sensitive detector, aconnection cable and an electronic read out system with data acquisition todisplay the X-ray image on a monitor.

When flat panel detectors are manufactured, the individual pixels exhibitvarying sensitivities. Furthermore, temperature changes influence thebehavior of the detectors. For this reason, the detector must be calibratedperiodically (gain and offset calibration).

Different manufacturer-specific data acquisition solutions are available forthe flat panel detector read out and image display on a monitor. The data isimmediately processed digitally, and the image is transmitted to a computermonitor directly (see Fig. 27).

Flat panel detectors are higher in price than X-ray image intensifiers. Thefact that prices are acceptable despite the expensive mechanics andmanufacturing process is largely because electronics and a-Si structuresare mass produced. The manufacturing process is also the basis fornotebook screens (albeit with a reversed working principle), which helpscontrol pricing in the market.

3.3 Image quality control

The Image Quality Indicator (IQI) generally specifies how good or poorthe image quality is. It is determined using an Image Quality test piece.

Important Note: Image Quality Indicators serve only to determinea metric for the image quality or the unsharpnessof the system.

BUT:

There is not a perfect correlation between thedetectability of Image Quality Indicators and thedetectability of natural defects!

3.3.1 Wire penetrameters

The question of how good or poor an X-ray image is can be answered withthe specification of an Image Quality Indicator (IQI). To determine the

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quality index, for instance, wire penetrameters according to EN 462-1(see Fig. 32) are laid out on the workpiece and imaged. The diameter of thewires decreases according to a geometrical progression from 3.5 mm (wireW1) to 0.05 mm (wire W19). The indicator of the thinnest detectable wirepenetrometer yields the Image Quality Indicator. For example, if wire W11is still visible, the image has an Image Quality Indicator of IQI 11. The wireshould be of the same material as the irradiated workpiece.

Frequently, the quality of an X-ray image is also specified as a relativewire detectability, or IQI sensitivity. This is especially true in radioscopy.This means that a ratio is composed of a wire diameter and a irradiatedwall thickness and then specified as a percentage. For example, a wirediameter of 1 mm on a 100 mm wall thickness yields a wire detectability of1%.

Fig. 32: wire penetrameter

Figure 33 shows the wire detectability of an X-ray image intensifier videosystem (in percent) taken from the irradiated wall thickness for aluminum(low percent values mean high wire detectability).

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Fig. 33: wire detectability with aluminium

Figure 34 shows the wire detectability of an X-ray image intensifier videosystem (in percent) taken from the irradiated wall thickness for iron (lowpercent values mean high wire detectability).

Fig. 34: wire detectability with iron

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Fig. 35: comparison of an image intensifier and flat panel detectors(aluminum)

A comparison of the wire detectability (aluminum) between an imageintensifier and flat panel detectors (see Fig. 35) clearly shows that both flatpanel detectors 1 and 2 deliver a better wire detectability than an imageintensifier system without digital image processing. Except for detector 2 inreal-time mode, wire detectabilities are attained that are nearly equal inquality to the image intensifier with digital image processing.

3.3.2 Platinum duplex wire penetrameter

The double wire penetrameter (acc. to EN 462-5) (see Fig. 36) iscomprised of a row of 13 elements. Each element is comprised of a pair ofwires with a circular cross-section. The distance between the individualwire penetrometers in a pair corresponds to the wire diameter.

The largest pair of wires whose separate wire images become blurred intoa single shape is considered the threshold of differentiability. The totalunsharpness is then obtained by using the size of the double wirediameter. The duplex wire IQI serves only to determine unsharpnessand should always be used in conjunction with wire penetrameters.

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Fig. 36: platinum double-wire penetrometer

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4. Radiography - Radioscopy

The essential differences between radiography and radioscopy lie in thedifferent detector properties and the resulting inspection conditions (seeTab.4.1).

While noise is of little importance in radiography, it plays a considerablerole in X-ray image intensifier video systems. This is because in radioscopyit is possible to carry out X-ray image examinations online. Because inradiography exposure times sometimes reach several minutes, thequantum noise of x-ray radiation is reduced. In contrast, when usingimage intensifiers the exposure time is limited to the 40 ms (0.04 seconds)specified in the video standard. This leads to a distinct “graininess” in thelive image. Using digital image processing, it is possible to considerablyreduce the noise in a static image by summing up a number of videoimages, similar to prolonging the exposure time in radiography.

Furthermore, the spatial resolution of common detectors in radioscopy isdistinctly lower than the spatial resolution of film (film approx. 0.1 mm,image intensifier camera approx. 0.3 mm). This means that inradioscopy an optical imaging geometry is required to permit geometricmagnification. Common factors of magnification are around approx. 1.5 -2.5 in casting examinations. The geometric magnification compensates forthe low spatial resolution or higher inherent unsharpness in comparison toX-ray film.

There are also considerable differences in the flexibility of examinationperspectives. Because of the arrangement of the film, only a fixed, rigidexamination perspective directly behind the test subject is possible inradiography. However, very few restrictions exist in radioscopy, dependingon the handling system and the geometry of the casting.

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Radiography Radioscopy• Film detector: X-ray image intensifier detector:

high spatial resolution medium spatial resolutionhigh contrast resolution high contrast resolutionlow noise (with reduction strong noisethrough long exposure time) digital image processing: integration

• Imaging geometry 1 : 1 Imaging geometry 1 : 1.5 and higher:geometric magnification approx. 1 geometric magnification larger than 1influence of optical focal spot: influence of optical focal spot:low if minimum distance is kept large with strong magnification

Test perspective: fixed Test perspective: flexible

Tab. 4.1: comparison of radiography and radioscopy

Despite the lower spatial resolution of radioscopic detectors compared toX-ray film, it is possible to attain an imaging quality at least equivalent tothat of film. The prerequisite for this is the optimization of the radiationimage using the flexible options that radioscopy systems offer, especiallygeometric magnification and noise reduction through the use of digitalimage processing methods. This is of special importance for the field ofradioscopic weld examinations, because it has been clearly definedthrough standards.

Conclusion: Assuming that sufficient geometric magnification is used andthe focal spot is correspondingly small, it is possible to achieve an imagingquality and detail detectability equivalent to that of film.

4.1 Radioscopic optical imaging geometry

The schematic design of a radioscopy system is shown in Figure 37. Thedevelopment of the radiation image can be divided in two components: thegeneration of a visible X-ray image and the subsequent image transfer. Allimaging systems used in radioscopy produce a positive image of theattenuation profile (low attenuation equals high intensity in radiation image).

The X-ray source produces the radiation image of the test object throughcentral projection. An X-ray image intensifier converts the radiation imageinto a visible X-ray image. The transfer system that follows consists of a

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lens system, camera, and monitor. Its only task is to make the output imageof the X-ray image intensifier visible to the operator on a monitor. Throughdigitization it is possible to send the image to a digital image processingsystem. Digital image processing is used both to enhance the imagequality, and to obtain an automatic image evaluation.

Fig. 37: radioscopic system set-up and imaging system

A consequence of the geometric magnification of test objects is that theproperties of the X-ray focal spot have a decisive influence on the imagequality. To better understand the influence of the focal spot, the idealimaging geometry is discussed here, even though it is irrelevant in practice.

4.1.1 Ideal imaging geometry

Figure 38 shows the ideal imaging geometry of a point-shaped X-ray focalspot. The optical display of the test object is dependent only on its positionbetween X-ray focus and detector. Thus theoretically, any small objectdetail could be displayed or visualized as a radiation image through theappropriate increase in geometric or direct magnification. Themagnification factor M is consequently calculated as the ratio of thefocus-detector distance (FDD) to the focus-object distance (FOD):

geometric or direct magnification factor M = FDD / FOD

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• Therefore, the magnification factor M tends towards 1 when the object ispositioned directly on the detector (corresponds to the radiographicimaging situation)

• The closer the object moves to the X-ray source, the greater themagnification factor M

• The magnification factor is exactly 2 when the object is located in themiddle between focus and detector

Fig. 38: ideal imaging geometry

4.1.2 Real imaging geometry

The real imaging situation however, is somewhat different because of theX-ray focus size (see Fig. 39). The focal spot now causes a geometricunsharpness ug, which is dependent on focal point size d and on theselected geometric magnification M. The geometric unsharpness iscalculated as:

geometric unsharpness ug = d ⋅ (FDD - FOD) / FOD or

ug = d ⋅ (FDD/FOD - 1)

Employing geometric magnification yields: ug= d ⋅ ( M - 1)

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Fig. 39: real imaging geometry

Geometric unsharpness has such a pronounced impact on the intensityprofile that an edge or sharp structure will have a blurred appearance in animage, and will not appear as a jump in intensity. For a small object detail,this means that the loss of contrast due to blurring can lead to the detailbeing drowned out by noise. It is then no longer detectable on the image.The geometric unsharpness caused by the focal spot leads to a limitedresolution of the whole radioscopy system, regardless of the detectorused. The larger the focal point, and the larger the geometricalmagnification M, the more distinct this effect becomes.

Since a geometric magnification must generally be used in radioscopy tocompensate for the low detector resolution compared to film, there arehigher requirements as to the size of the focal spot. Focal spots inradioscopy are approximately 0.4 to 1.5 mm maximum, while inradiography focal spots are up to several millimeters in size. Thus inradiography, a minimum distance between focal point and film is specifiedin order to achieve a certain optical imaging quality (EN 1435 / EN 444).

Increasing the distance between tube and detector is not sensible inradioscopy for several reasons. First of all, a magnification of the FDDwould lead to a decrease in geometric magnification. A further significantargument against increasing the distance lies in the observation of theintensity conditions.

The inverse square law states that doubling the FDD means only aquarter of the surface element intensity will reach the detector. This loss ofintensity would have to be compensated for through a longer integration

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time with digital image processing in order to achieve a reasonable signal-to-noise ratio in the radiation image.

Conclusion: Since, due to the thermal conditions of the focal spot, acompromise must always be found between required intensity and focalspot size, it is always necessary to set up a usage-specific system that hasa fixed minimum detectability value.

4.2 Quantitative description of radioscopy systems

For several years, scientists have been working on procedures that willenable the quantitative capture and assessment of radioscopy systemproperties. The definition of the modulation transfer function has proven tobe especially suitable for this technology. With the modulation transferfunction (MTF), it is possible to establish a relationship betweenunsharpness and the threshold resolution of a system, as well as describethe transfer of different image details.

MTF describes the relationship between contrast and spatial frequency.The spatial frequency is defined as the number of line pairs per millimeter:

1 mm 1 mm 1 Lp/mm 4 Lp/mm

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How is MTF generated (see Fig.40)?

Fig. 40: generation of the modulation transfer function

In diagram a), a sinusoidal intensity distribution of increasing spatialfrequency is displayed as it would appear on the input screen of an X-rayimage intensifier.

Diagram b) displays the optical image of the intensity distribution fromdiagram a), just as it would appear in the video signal. As expected, themore the spatial frequency increases, the more the contrast decreases untilthe threshold resolution is reached.

In diagram c), the contrast found in diagram b) for each spatial frequency isdisplayed. The diagram is applied to 1 or 100%. This diagram represents

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the modulation transfer function. The threshold resolution is, by definition,reached when the contrast has faded to 10%. The MTF describes both thethreshold resolution of the whole system or the individual components, andthe contrast transmission of small details of the whole system or of theindividual components.

All components of the imaging system are described with an MTF.However, for the practical user, only the MTF of the entire system isimportant in the end. Not only is the threshold resolution important in orderto describe the radioscopy system (spatial frequency, with the MTF fadedout to 10%), but also the whole process - even when spatial frequencies liebelow the threshold resolution. X-ray systems with identical thresholdfrequencies may exhibit differing MTF courses, and thereby, differconsiderably in their transmission properties.

Fig. 41: schematic diagram of the optical imaging process

Even if this description of a radioscopy system seems abstract, the MTF isthe quantitative description of what the user may observe in the daily use ofthe radioscopy system. To illustrate, the imaging procedure for an “ideal”detail (i.e., a pore) is schematically displayed over the whole image chainup to the monitor in Figure 41. Through every component of the system,the first ideal orpin-shaped detail, which may quantitatively be describedwith the appropriate MTF of the respective components, is blurred.

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The diagram shows how, due to its specific transfer properties, each newcomponent widens the detail that was originally ideal with a simultaneousreduction of contrast. The result of the diagram, i.e., the MTF of the wholesystem, may be described as the product of the MTF’s of the individualcomponents. Figure 42 demonstrates this through an example with twoMTF’s.

Fig. 42: total MTF as product of individual MTF’s

The curves a) and b) in Figure 42 represent the MTF’s of the imageintensifier and the camera. The resulting total MTF curve c) is generatedfrom the product of curve a) and curve b) and is clearly below curve b).Curve d) is the result of what happens when the output signal of thecamera is reproduced by the monitor.

The diagram makes clear that the total MTF is always worse than theworst MTF of an individual component of the image chain. This meansthat for an optimized system, the best transfer properties possiblemust be used for EACH COMPONENT.

To get the complete picture, an approach should be introduced which givesan extensive description of imaging systems.

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Image quality is typically thought of in terms of resolution, signal contrastand noise. There is a contribution to these effects that are determined bythe X-ray spectrum, the part to be inspected and the inspection time,whichputs a limit on the best possible image quality to be expected. In addition,there is a loss of image quality during the detection process. The ability ofthe detector to accurately measure the information present in the X-raysduring the imaging process is the Detective Quantum Efficiency (DQE).

DQE can best be thought of as the square of the signal-to-noise ratio of thex-rays incident on the detector, divided by the square of the signal-to-noisemeasured by the detector for a given frequency:

2

2

)()(

)(In

Out

SNRSNRfDQE =

DQE is the ability of the detector to accurately represent all of theinformation present in the X-ray field as a function of frequency. A perfectdetector will give a DQE of one over all frequencies, while a poor detectorwill have a DQE that approaches zero. The frequency dependence ensuresthat spatial resolution and signal-to-noise are contained in DQE leading toimage quality and probability of detection (POD) estimates.

With DQE, the final image quality can be estimated through this formula:

InOut SNRfDQESNR ⋅= )( .

For example, assume there are two detectors with different DQE. For thesame X-ray exposure conditions, the detector with the higher DQE will givethe larger SNROut and better image quality. The same image quality can beachieved in both detectors, but the detector with the higher DQE wouldrequire fewer X-rays, which translates to shortened exposure times and/orlower tube current.

In general, DQE consolidates the detector performance covering manyindividual parameters (resolution, efficiency, noise, etc.) into a singleparameter. Therefore, specifying the DQE for a detector will also helpdetermine the final image quality and inspection times required for a givenapplication.

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5. Radioscopic examination systems

Apart from the technical properties of imaging systems, economicproperties, such as cost, reliability of examination, capacity for automation,and the speed of inspection, are also important when it comes to practicaluse. This realization led to the development of single-function customizedsystems in the start of the 1980’s.

The radioscopic workstation DP 351 for the automatic examination of lightalloy vehicle components in the production line (see Fig. 43) is used as anexample to illustrate the design and operation mode of an examinationsystem of this kind.

Fig. 43: radioscopic examination workstation for automatic inspection ofcastings

The manipulator principle enables radiation without shadow effects for allnecessary angles of inspection. Additional features (type recognition, sort

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function, etc.) facilitate integration into the production process, up to thecomplete automation of the Feed, Examine, and Sort operations.

In order to achieve a maximal output of parts, high-performanceexamination systems of this kind are equipped with a twin manipulator. Thismeans that two part manipulators are installed. While one loadedmanipulator is inside the radiation protection cabinet and the examination isin process, the loading, unloading, and marking of the examined parts cantake place simultaneously outside of the cabinet with the secondmanipulator. The time required to change parts and the idle time (where noexamination takes place), are solely determined by the rotation of the twinmanipulators. This takes only a few seconds.

Apart from such single-function systems optimized for maximum output,standard or universal systems have been widespread for years. Flexibleconcepts, easy handling and an advantageous price-performance ratiohave led these systems to use in the most diverse areas. Universalsystems are operated either manually or using programmed sequences.

Fig. 44: the universal radioscopic inspection system X-cube compact

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Internal illumination, an “Emergency stop” combination, and a safety switchwith isolated contacts are all part of the electrical equipment of an X-raycabinet. The protection cabinet, which is made of layers of steel, lead andanother layer of steel, has the dimensions and equipment of a fullprotection apparatus based on the current version of official X-rayregulations (RöV, in Germany).

Figure 45 shows an example of an ergonomic operating desk. The displaylevel contains a monitor with a live image, as well as a monitor fordisplaying digitally processed images.

Fig. 45: operating console of an X-cube compact

The control level contains sets of buttons for choosing operation modes,additional functions, parameter specifications, manual control, buttons forthe examination decision as well as the necessary signal lamps.

5.1 Requirements

To fulfill specific inspection needs, it is always necessary to optimize theradioscopy system for a particular application. The individual components

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need to be selected so that an optimal image quality can be attained as aprerequisite for a reliable examination result. The requirements for differentinspection tasks, such as weld and casting examinations etc., will bediscussed further on.

5.1.1 Weld examinations

The radioscopy standard EN 13068 (Radioscopic Testing) was published inthe year 2001. In recent years, radioscopic weld examinations haveconsiderably increased in importance. Consequently, radioscopicexaminations have become indispensable in the serial production oflongitudinally or helically welded pipes. Various supervisory institutions(TÜV in Germany etc.) have released this examination technique for anumber of users as well.

In weld examination it is often important to be able to discover thesmallest defects (i.e., pores), as well as cracks. Except for the weldelevation, these components are even. For radioscopic systems, theseconstraints mean that high demands are placed on the spatial resolution.Generally, geometric magnifications of 2.5 or more are used depending onthe wall thickness to be penetrated. This requires a suitable X-ray tube,often with focus dimensions of 0.3 mm. The imaging system must be ableto transmit even weak contrasts. This requires a high contrast sensitivity,which is normally tested using image quality indicators (IQI). In addition tothe good contrast sensitivity, the signal-to-noise ratio must be high in orderto be able to assess even the smallest of details. Therefore, theexamination of an item in motion is generally not possible because thesignal-to-noise ratio often must be improved using digital image processingto integrate the stationary image.

In economic terms, radioscopic examinations generally perform very wellcompared to radiography, because there are no film, film processing, ordevelopment costs. However, there is a possibility that it will become arequirement to fully document all examination results, which could lead toproblems. Normal documentation methods such as video recorders or hardcopies generally have an insufficient image quality. Storing the resultswithout loss of quality can only be performed digitally. This, however,requires considerable volumes of storage media, such as magnetic-opticalstorage drives, CD’s or the like.

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5.1.2 Casting examinations

Castings generally have distinctively large differences in wall thickness.Moreover, the size of permitted defects is relatively high in comparison toweld examinations.. Defects may however, be relatively “flat,” in regards towall thickness. In other words, the depth range may be relatively low. As aresult, the radioscopy system is required to have a large dynamic rangeand high contrast sensitivity. The spatial resolution requirements are moremoderate. In any case, the contrast transmission of the system at mid-range spatial frequencies, (for mid-range defect sizes) should be excellent.Over a sufficiently large area the spatial resolution should only decreaseslightly with relation to the modulation transmission function. It shouldnot decrease to the threshold resolution until higher spatial frequencieshave been reached. The signal-to-noise ratio requirements are also high, inaddition to the high contrast resolution.

Examination speed is of particular importance for the serial inspection ofcastings. For this reason, examinations are generally performed on movingcastings. The examination speed, and thus the casting output, is furtherincreased by the use of specially designed single-purpose systems whichhave special casting manipulators, casting feeds and outlets.

An alternative to a programmed examination procedure is a fully-automatic examination, which involves an image evaluation for castingassessments. This “unmanned” fully-automatic examination is now theindustrial standard for various castings made of light alloys (wheels,suspension components, etc.). This objective examination guarantees highand constant examination reliability, since it lacks the possibility of normalhuman error. When considering the possible cost increases resulting fromthe delivery of defective workpieces, computer-controlled inspection hasgreat potential for lowering costs by eliminating this problem.

5.1.3 Recognition of foreign bodies in food

Due to the product liability law enforced in the European internal market,new questions have risen relating to quality guarantees for food production.Alongside the criteria of taste, appearance and adherence to the legalrequirements for provisions, packaged foods must also be kept free offoreign bodies. Radioscopic examinations offer a solution for both metallicand non-metallic foreign bodies.

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The largest technical challenges for this type of examination system are thevast quantities in which foodstuffs are produced, as well as the highproduction rate at which they are produced and packaged. In order toobtain a 100% examination in a production line, the inspection must be inmotion at a very high speed. A line diode array is usually used instead ofan X-ray image intensifier. Apart from the speeds at which the test itemstravel, the need for a low radiation exposure of the test items also plays asignificant role. One of the most important requirements for a radioscopysystem is a good contrast resolution, since foodstuffs containpredominantly non-metallic foreign bodies, which are only visible via weakcontrasts. Due to the factors mentioned above, visual evaluation by aninspector is generally impossible. Production line rates also placeconsiderable demands on the computer-based fully automatic evaluation,since this also has to be performed on a test piece in motion.

5.1.4 Soft radiation technology

The term “soft radiation technology” indicates that, for such applications,low accelerating voltages are used (starting from 20kV). These techniquesare generally used in the examination of plastics or electronic components.An area of application which has gained importance significantly in therecent years is compound materials (e.g., carbon-fiber, glass-fiberreinforced plastics), as well as “honeycomb structures” (used for theconstruction of airplanes). The latter are made of plastic and are built tocarry honeycomb-like structures of aluminum. A typical example ofapplication is the testing of bonds in critical areas.

The major demand on the radioscopy system is the requirement to produceimages with high contrast and low noise, even at low tube voltages.

Conditions may be entirely different for applications in electronics. Theinspection of integrated circuits (ICs) under certain circumstances makesthe use of microfocus tubes necessary. The reason being, to inspect theconnection wires, which have a diameter of just 2 µm, extrememagnifications of 50 - 80 are required. This is often termed “X-raymicroscopy”. The tube voltages required for this may also be considerablygreater than for “normal” soft radiation applications.

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5.2 Introduction to digital image processing

Digitization provides the opportunity to make the output image of an X-ray-sensitive detector accessible to an image processing system. Digitalimage processing is used to enhance image quality or evaluate imagesautomatically.

The first step involves converting or transforming the image obtained fromradioscopy to make it computer-compatible, so that it can be “understood”by a computer. This conversion involves the digitization and quantizationof the image.

Digitization refers to the analysis or scanning of the image into individualpoints called Pixels (Picture Elements). The image is divided into mosaic-like individual values. Common formats are 512 x 512, 640 x 512, and1024 x 1024 pixels.

Moreover, each pixel has to be assigned a value, called a grey value,according to the intensity of the spot in question. This process is known asquantization. X-ray image intensifier video systems divide the intensityscale from black to white into approximately 256 grey values. The greyvalue “zero” corresponds to black in general, and the grey value 255 towhite.

The image that has been digitized and quantified in this way is also calledthe image matrix or digitized image. This process is carried out on thevideo signal through the use of an analogue-digital converter. The actualimage processing is, put simply, performed between these two convertersin the image processing system. In image data processing, it is possibleto distinguish between two areas of the application:

• image enhancement• image evaluation or image analysis

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5.2.1 Image enhancement

Various image enhancement techniques have become standard inradioscopic inspection:

• arithmetical averaging or integration• contrast stretching• filtering

The output images from radioscopy systems have a high proportion ofnoise. This is mainly due to the quantum nature of radiation. Theintegration or arithmetical averaging of several individual video imagesduring recording leads to a considerable reduction in noise, and thus to animprovement in the signal-to-noise ratio in the image. This improvement ishighly desirable since it allows details to be recognized more easily.However, this operation is only possible on a frozen image, because anymovement of the test piece would lead to long streaking and blurs in theimage.

Contrast stretching allows certain grey value areas in the image to beemphasized and thereby brings low-contrast details of the image into theforeground. However, in radioscopic weld examinations, this application isoften only useful after a shading correction.

In digital image processing, filtering has a particular importance. Itspossible applications range from noise reduction to the smoothing of smallimage details, as well as the emphasis of structures and small details.Another area of application with this filter operation is the detection ofsimple structures, such as edges, borders of objects, corners and points.Furthermore, it is possible to change the image into a vivid, three-dimensional graph. Attempts to achieve automatic defect detection forwelded joints and castings are also based on special filter structures.

The main problem with filtering image data relates to the deliberatemanipulation of the image content. Just as it is possible to make materialdefects more visible in the radioscopic image, using a different filter canproduce the opposite effect. This means that users of the digital filters musthave a fundamental understanding of the relations and effects of thevarious filters. At the very least, however, the use of filters should bepracticed thoroughly on model parts before actual examination begins.

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5.2.2 Image evaluation

Image evaluation or image analysis is understood both as the measuring ofstructures or intensities in the image, and as automatic defect recognition inweld and casting examinations. The current performance capacity of suchsystems will be illustrated here by taking a closer look at, for example, thefast automatic image evaluation system (SABA) for the fully automaticinspection of castings.

Castings which are used in the automotive industry such as wheels, gearboxes, and suspension components are often important for security andhave to satisfy specific quality regulations. In the correspondingspecifications, details are given on critical values concerning the type, sizeand number of defects. Generally a 100% X-ray examination is required toguarantee the functionality of the part.

The number of parts to be checked of the same type is very high, becauseof their mass character. Visual examinations by qualified operators, whichare still common today, are a tiring and monotonous activity that can behindered by various distracting subjective influences. For this reason, therehas been a desire for years to automate the X-ray examination of light alloycastings. These castings are important for security, and therefore makethe examination more objective. The fully automatic serial examination oflight alloy castings has now become the “state-of-the-art” process forindustrial practice.

The set-up of the fully automatic inspection system only differs from atraditional radioscopy system in that it has an image evaluation computer.The design of the examination machine is largely determined by the kind ofcastings to be examined. However, the component’s X-ray source, imagingsystem and manipulator for the automatic handling of the workpiece arealways part of the examination system. The image evaluation computerdecides, without any human participation, whether or not the workpiecesatisfies the quality regulations.

In practice, a very diverse set of high demands are made on a fully-automatic examination system for defect detection in radioscopy images:

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• A fully-automatic system must guarantee that all defects whichwould lead to exclusion according to the quality regulations arerecognized and classified as rejects by the system.

• The false reject rate, or the number of parts which the examinationsystem mistakenly classifies as reject parts, must be negligible, ifonly from an economic point of view. This also includes therequirement for higher system reliability.

• The unhindered functionality of a fully-automatic examinationsystem must be periodically ascertained through an independentlyperformed test of the system.

• The fully-automatic examination should not last longer than avisual inspection of the part. On the contrary, it should be fasterthan the visual inspection, if possible, to gain further economicadvantage.

For practical use, these requirements led to a solution that satisfied theeconomic and reliability requirements. While considering the prior-knownstructures and shapes of a casting, an optimum defect detection filter(see Fig. 46) can be used for each area of the workpiece. For a practicalinspection, it is necessary to create a type-specific set of parameterscontaining information on the geometry of the workpiece, the inspectionpositions and the limits permissible according to the quality specification.

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Fig. 46: automatic image evaluation approach

In principle, this approach offers several opportunities to develop fastexamination programs. During the practical implementation, attention waspaid to optimized software structures and a cycle-displaced evaluation wascreated. This means that a large portion of the operations were necessaryfor the image evaluation take place at the same time as the positioning ofthe workpiece. The results concluded that, for Accept parts only, the totalexamination time was determined by the time needed to actually capturethe image, as well as the time needed to position the workpiece.

To make certain that the unit is in working order, the system periodicallychecks the most important components of the unit. These include theimage evaluation computer, the imaging system (X-ray tube, imageintensifier) and the manipulator. Clearly, these system solutions requirecooperation of the image evaluation and the control of the inspectionsystem.

Since many of the inspection machines used today were created with therequirements of visual inspection in mind, there are additional requirementsfor the examination machines themselves. The differences betweenvisual and fully automatic inspection must be considered here.

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If the examination procedure and the inspection requirements of the visualand fully-automatic examination are compared, clear differences emerge:

• Unlike a visual examination, in a fully-automatic system the workpiece isnot examined while in motion, but rather in pre-specified examinationpositions. It must be possible to enter these positions and reproducethem with sufficient accuracy.

• Both computers and humans rely on high image quality for reliabledefect detection. However, the respective requirements on image qualitydiffer considerably. Strong contrasts are required for automaticexaminations. This makes a spectral adaptation of the X-ray radiationnecessary through the use of various prefilters to harden the radiation.Therefore, swivel mechanisms should be included on the examinationmachines. The same basic need of strong contrasts for better defectrecognition exists for visual examinations. However, the operator prefersa lower-contrast image to avoid their eyes tiring too quickly. Anotherdifference is in the tolerance of noise in the radioscopic image. A humancompensates for and accepts considerably higher levels of noise if thisallows them to gain additional information for the inspection. However,for the computer a low-noise image is a prerequisite for reliable defectdetection. Therefore, for fully-automatic examinations, noise reductionmust be provided through digital pre-processing of the image.

• The examination specifications and quality standards currently in usewere formulated with regard to visual examinations. They are of limiteduse for fully-automatic examinations, since occasionally, algorithmicconversion of the rejection criteria required is not possible.

• Currently, casting designers, when designing a part, do not often takeinto consideration the ease with which that particular part can beautomatically examined. This sometimes causes problems for humaninspectors as well. In an automatic examination system, this can beproblematic if labels or stamps are added to areas that require rigorousexamination.

• Examining various types of castings (for instance, cast wheels ofvarious styles, etc.) poses a few problems for human inspectors. A fully-automatic examination must first be run to recognize which of thevarious types of examinations is necessary before the actualradioscopic examination is performed. This can be affected, for

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instance, by using a casting code that is removed again in the finalprocessing of the casting.

• The image processing computer controls almost all functions of theexamination system via a digital interface. The harmonious coordinationof the examination system and the computer must be guaranteed for themost diverse examination units (e.g., for wheels, gear boxes, engineblocks etc.). This places high demands on the examination and controlsoftware of the fully-automatic examination system.

The first fully-automatic examination systems were tested in practice invarious installations and for several years. Since then, they have becomethe industrial standard in the serial examination of a diverse range ofcastings. In the test phases, interesting results were also obtainedcompared to the reliability of visual examinations.

During a comparative practical test, three operators performed a spotcheck on 100 wheels. This spot check contained 18 parts that wereobviously defective and had been certified unanimously as suchbeforehand by several operators.

The inspection occurred under similar conditions to those in practice, overa period of two and a half hours. The result was impressive proof of theweaknesses of human operators compared to a fully-automatic system(see Tab. 5.1).

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Comparative practical test100 wheels with 18 obvious rejects

Operator 1 Operator 2 Operator 3 SABA

Overall acceptance 96 94 94 100

Rejects missed 4 6 6 0

Agreement on rejection 77.8% 66.7% 66.7% 100%

Incorrect defect detection 0 0 0 0

Time taken 2.4 h 2.3 h 2.5 h 2.4 h

Tab. 5.1: comparative practical test (1989)

No operator was able to find all of the reject parts in the spot check. On thecontrary, all three inspectors let 4-6 of the 18 parts pass. It becameapparent that towards the end of the examination, the number of partsmissed rose strongly as a result of increasing exhaustion. The inspectiontimes of the fully automatic system were of the same order as those of theinspectors.

It is also worth noting that, in all of these tests, the operators knew thenature and the goal of the examinations. We may therefore conclude,without a doubt, that the operators were highly motivated and concentratedduring the tests.

Thanks to an optimal coordination of the examination system and high-performance image processing, the systems used in practice today forfully-automatic examinations allow for a considerably higher output of partsthan visual examinations, with significantly higher examination reliability.

5.3 Computed tomography

High demands on the quality of materials and components make itnecessary to apply non-destructive inspection mechanisms to optimize theproduction process. The need to determine the existence of defects inmaterials and components, and to measure and characterize them requires

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examination procedures with a high spatial resolution. Computedtomography is particularly suitable for this, and has become increasinglysignificant due to the development of computers in the last decade.

Computer tomography (CT) enables a reconstruction of object profiles fromX-ray inspections. This involves irradiating the object from at least 180°, butnormally 360°, and capturing the individual attenuation profiles (theprojections from various angles). A corresponding object profile can thenbe reconstructed via mathematical reconstruction procedures. Theprocedure most often used for this is a filtered back projection. Themathematical basis for this was described in 1917 by Radon. Figure 47shows the principle of data capture with a CT with parallel ray geometry.

Fig. 47: measuring principle of CT and the use of parallel ray geometry

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The measuring set-up of CT is displayed in Figure 48. The object to beexamined is rotated step-wise in a fan-shaped X-ray beam and theindividual projections are recorded. By using the fan-shaped beam, theobject has an enlarged image on the detector, which improves the spatialresolution.

Fig. 48: working principle of the 2D CT using fan beam geometry and directmagnification.

The complete examination of a component with two-dimensional computedtomography (2D CT) takes a great deal of time, since many sections of theobject have to be captured. Because the object has to be turned through360o for each section, several hours or days may be required to measureits entire volume.

Unlike two-dimensional CT, three-dimensional computed tomography onlyrequires one 360o rotation of the object. An array detector is used insteadof a detector line to take the radioscopic images (projections). This resultsin short inspection times in comparison to 2D CT. The three-dimensionaldistribution of the X-ray attenuation coefficients in the object volume iscalculated from the projections using a mathematical algorithm.

The 3D-CT requires a high level of calculation. To achieve short calculationtimes, despite the efficient reconstruction algorithms used, multi-processorsystems are necessary in order to calculate the volume while measuring.The algorithms were implemented on dual Pentium PCs in such a manner

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that adding further PCs could easily raise the calculation efficiency. ThePCs are connected to each other over a fast network (see Fig. 49).

Fig. 49: schematic structure of an industrial 3D CT system

Various 3D CT systems were developed with this concept. Depending on theresolution and X-ray absorption required, 450 kV tubes with a focal spot of 1.5mm, minifocus tubes with a 0.3 mm focal spot, and microfocus tubes are used.Voxel sizes vary according to the source from (0.35 mm)3 to (0.01 mm)3.

Reconstructionvolume[voxel]

Projec-tion

number

Measure-ment data

[Mbyte]

Volumedata

[Mbyte]

Typicalmeasuringtime [min]

Recon-struction

time2 [min]

255x255x255 200 100 16 2.5 2.5

511x511x511 400 200 128 5 8

1023x1023x511 800 800 512 201 601 With extension of measuring area, using a detector array of 512 x 512 Pixels2 Using 4 dual Pentium processors with 500 MHz

Tab. 5.2: measuring and calculation time according to reconstructionvolume

Table 5.2 provides data volumes and calculation times according to thesize of the reconstructed volume. The calculation times refer to the use of a450 kV or a 225 kV minifocus tube and a 16-bit detector array with (512)²detector elements. Shorter calculation times can be achieved by using

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several computers for the reconstruction. More precisely, the calculationtime is inversely proportional to the number of reconstruction computers.

The volumes are evaluated on a separate visualization station and placedin an archive.

Figure 50 shows the volume visualization of an aluminum casting, whichoriginates from a series of experiments for optimizing the manufacturingprocess.

Fig. 50: cast aluminum part, 3D visualization

The diameter of this component is about 15 cm. At a (0.3 mm)³ voxel sizeand 5 minutes measuring time, pores measuring 0.4 mm are detected in

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this object. The visual examination of the volume indicated the presence ofseveral pore groups, which is attributable to the disadvantageous way inwhich the casting was carried out. Based on the distribution of the pores,experts can draw conclusions about the casting process and optimize it.

Figures 51 and 52 provide us with two xy-planes calculated from thereconstructed volume through points 1 and 2. Figures 53 and 54 displaytwo xz-sections through points 3 and 4.

In figure 51, single pores and chains of pores can be seen arranged aroundthe central borehole. The size of the pores varies between 0.5 mm andapproximately 3 mm. Figure 52 shows a second xy-plane at a distance of10 cm from the first level through point 2. This image displays a pore group,as well as a single pore (a). The diameter of the single pore is around0.5 mm.

Fig. 51: section through the castingxy-plane through point 1

Fig. 52: section through the castingxy-plane through point 2

Figures 53 and 54 show the depth range of the pore groups. The poresextend up to 1.5 cm in the z-direction in Figure 53 and 1 cm in Figure 54.

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Fig. 53: section through the castingxz-level through point 3

Fig. 54: section through the castingxz- plane through point 4

A 17 cm-long area of an aluminum cylinder head is volumetricallyexamined through the use of 3D CT. Because of the high level of X-rayabsorption, the examination is done using 450 kV, with the voxel sizeequalling (0.35 mm³). Figure 55 shows a cross-section of the cylinder head,and figure 56 shows a 3D display of a 2 cm high area.

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Fig. 55: CT-section of cylinder head Fig. 56: 3D display

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6. Summary and outlook

For decades, X-ray examination has been one of the most importantprocedures in non-destructive testing technology. The major advantages ofthis procedure are the direct creation of an image of the test object and, inthe case of radiography, the ability to store the results of the examinationas an image on an exposed X-ray film.

Radioscopy cannot and does not intend to replace classic radiography inthe future. Because of high-performance system technology and thetechnology of new X-ray sensitive detector systems, entirely newapplication areas will emerge for X-ray examination in the future. Three-dimensional computed tomography, for instance, will be used increasinglyfor product optimization, reverse engineering, and to measure structureswithin the test object.

Even with a history already spanning 100 years, X-ray examination willcontinue to play an important part in NDT and other fields in the future.