LIGHTING CONSIDERATIONS IN MACHINE VISION

Light is Vision

Ian AldertonAlrad Instruments Ltd

www.alrad.co.uk

Light is Vision

Agenda

• A brief introduction to Alrad Instruments Ltd

• What is Light?

• The Techniques- darkfield, brightfield, backlight..

• Cold Light – Fibre optics a brief description

• Application Examples

• pictures of products installation

Company Profile

Alrad are a Distribution company for Imaging, Electro Optic, Analytical components and instruments including:

CCD and CMOS camerasFrame Grabbers and Digital Acquisition cardsMachine Vision Lighting SolutionsImaging SoftwareNational Instruments Compact Vision System

Trading since 1970, 40 years in business.Two divisions – Imaging and Electronics

Company Profile

Based in Newbury, BerkshireMember of the UK Industrial Vision Association [UKIVA] special interest group of PPMA

ISO 9000 Approved

Light Characteristics

X-RAYS ULTRAVIOLET VISIBLE NEAR-INFRARED MID-INFRARED FAR-INFRARED

450-700 nm

Light is EnergyTravels in Wavelengths (Measured in Nanometers)

Angstrom = 1/100,000,000 CentimeterNanometer = 10 Angstroms = 0.000001 mm

Lighting Technique

Backlight (Transmitted) - Opposite of CameraIncident - Same Side as the Camera

Bright-Field - High-Angle IlluminationDark-Field - Low-Angle IlluminationAxial - Approaches From the Camera AxisStructured - Geometry is ControlledContinuous Diffused - Complete, uniform envelope of lightMulti-axis

Light Characteristics

Diffused - Illumination from multiple directionsDirected - Illumination from single direction

Diffused Light Directed Light

Surface Characteristics

Diffuse Surfaces reflect Light at all Angles.

Specular Surfaces reflect Light at the Angle of Incidence.

Absorptive Surfaces do not reflect Light

Lighting Technique

Field of View

Dark-Field Dark-Field

Bright-Field

Bright-Field

Lighting Technique

Measuring the Light Environment

Point Souce<1% CoverageHighly AngularHighly Asymmetrical

Ring Light–<15% Coverage–Highly Angular–Radially Symmetrical

Point Source<1% CoverageHighly AngularHighly Asymmetrical

Dark-Field

Lighting Technique

Bright-Dark-Field

Lighting Technique

Dome Illumination

Lighting Technique

Measuring the Light Environment

Ringlight with Diffuser15-30% CoverageAngularRadially Symmetrical

Continuous Diffuse Illumination–90-100% Coverage–Non-Angular–Radially Symmetrical

Coaxial Illumination System

Lightsource

Object

Diffuser

BeamSplitter

Diffuse LightILP ACIS

Lighting Technique

Reflekto r Reflekto r

Ball Application

Advanced Coaxial IlluminationRinglight

Back Light

Lighting Technique

BackLight

Edge and ContourPrinting on Clear Containers

Location of Filament in LampLocation of Object for Robot Guidance

Translucency of Materials

Backlight Applications

Line Light

Lighting Technique

Zeilenförmiger Querschnitt

Runder Querschnitt

Optical Fibers -Randomization Techniques

VOLPI provides 2 different basic randomization techniques to compensate inhomogeneities of the light source

automated randomizationhomogeneous randomized multifurcated fiber bundlessuperior uniform distribution

manual randomizationvarious qualities of manually randomized fiber bundles available according to re-quirementscustomized combinations of active area geometry (cross-section convertion)

Optimized Lighting for Machine Vision

• Maximum Contrast for Features of Interest• Minimum Contrast for Features to be Ignored• Maximum Light Energy• Increases Depth of Field• Increases Signal to Noise Ratio• Minimizes Ambient Lighting Effects

key point - machine vision is analyzing reflected light

Components for Machine Vision

backlights – application

high contrast with opaque objects, especially flat onesgives the silhouette / contour of the object translucent objects become transparent (upper / lower surfaces cannot be separated)

Lead frame silhouette glass rod seems to only diffuse light allowsshine by itself inspection inside the light bulb

Diffuse backlight for lamp inspection and adjustment in a round table machine.

Components for Machine Vision

incident lighting – application

Diffuse incident lightfor homogenous (poor contrast) lighting with weakly structured surfacesto suppress surface structure (e.g. machining marks)suppression of rounded edges

Directed incident lighthomogenous illumination of diffusely reflecting surfacesconscious utilization of reflections to emphasize structural imperfections (e.g. scratches, edges)

Shadow free lighting to find cracks in safety relevant parts (ball joint).

Left: Identification of em-bossed numbers on a machined surface.

Right: motor bearing plate with directed light at a shallow angle.

Components for Machine Vision

darkfield lighting – application

field of view is not directly illuminated by the lighting (stays dark)

homogeneous objects appear dark, defects appear bright

emphasizes edges

Engraved shiny metal surface / printing on integrated circuit.

Reading text and codes rivet identification bottle inspectionon CD-ROMs

Components for Machine Vision

telecentric lighting – application

functions only in combination with a telecentric lensfor transparent to opaque 3D objectsvery good extraneous light suppressionbest suited to measurement applications

Precision measurement of problematic materials: metal and glass rods are imaged with a high contrast due to the telecentric lighting. Telecentric online

measurement of shiny precision parts.

Components for Machine Vision

One object – different lighting techniques

What do you see? What do you want to see? What does the camera see?Above all a question of lighting:

Dark field incident light Diffuse backlight Telecentric backlight

Directed incident bright Diffuse incident bright Dark field backlightfield light field light

Test object:

transparent glass plate with chamfer

Machine Vision Applications