Infrared temperature sensors have been success-fully used for years in process industries forongoing temperature monitoring and control.Although the technology is proven, choosingamong units with different specifications is some-times confusing, leaving the process engineer torely on more traditional temperature measurementmethods (e.g., those involving contact) or on ven-dor recommendations. Recent innovations ininfrared temperature sensor design have provided

process engineers with enhanced functionality, andmore questions about how to integrate and useinfrared temperature sensors in their process.

Infrared technology explainedAn infrared temperature sensor collects radia-

tion from a target in the field of view defined bythe instrument’s optics and location. Theinfrared energy is isolated and measured usingphotosensitive detectors. The detectors convert

By Karen Ackland

Choosing aninfrared temperature sensor can be astraightforwardprocedure.

Selecting the right infrared temperature sensor

June 1998 • InTech 48

TEMPERATURE

What is the temperature range of your process?

What size is the target?

How close to the target can the instrument be installed?

Does the target fill the field of view?

What is the target material?

How fast is the process moving?

Will you measure discrete objects or a continuous process?

What is the ambient temperature?

Are the ambient conditions contaminated (dust, smoke, steam)?

Do you want to connect to existing control equipment?

Do you need to keep records for audits and/or quality programs?

1000¡10¡

250¡35¡

����

�������

��

1

23 4

5 245¡

2890452

29873498

2348792

the infrared energy to an electrical signal, whichis then converted into a temperature value basedon the instrument’s internal algorithms and thetarget’s emissivity (a term referring to the emit-ting qualities of the target’s surface). Infrared ornoncontact temperature sensors are very success-ful in measuring hot, moving, or difficult-to-reach objects, or where contact temperaturesensors would damage the target. A block dia-gram of an infrared temperature sensor is shownin Figure 1.

Understanding the process application helpsdetermine which type of infrared temperaturesensor to use. What is the temperature range ofthe target? How big is the measurement spot?How far away is that spot from the sensor? Theseare the first of several questions to ask to help findthe right temperature sensor for your application.Environmental and operating conditions deter-mine other sensor specifications (e.g., ambienttemperature, display and output, and protectiveaccessories). Finally, ease-of-use, maintenance,and calibration considerations may uncover hid-den costs that will further influence the choice ofan infrared temperature sensor.

Determine temperature rangeInfrared instruments are available for low-

temperature applications (from below freezing) tohigh-temperature applications (over 5,000°F). Ingeneral, the narrower the temperature range, thebetter the resolution of the output signal for mon-itoring and controlling process temperatures.

If monitoring start-up or cool-down tempera-tures is critical, it is necessary to choose a tem-perature sensor with a wider measurement range.

This is critical in heat-treating applications, forexample, where temperature must be held withina specific temperature range for a period of timeto affect a material’s metallurgical properties.

Establish target sizeIn infrared temperature measurement, the area

to be measured (i.e., the target) should fill theinstrument’s field of view. Suppliers of infraredtemperature sensors typically recommend that themeasurement target exceed the field of view by50%. If the target is smaller than the field of view,background objects (e.g., furnace wall) will influ-ence the temperature reading. Conversely, if thetarget is larger than the instrument’s field of view,the instrument will not capture a temperaturevariation outside the measurement area. An illus-tration of field of view is shown in Figure 2.

To collect all the emitted radiation, singlewavelength infrared temperature sensors (i.e.,point sensors) need a clear line of sight betweenthe instrument and the target. Sighting opticsallow the user to visually sight through the instru-ment on the target. Some instruments have abuilt-in laser that pinpoints the target, which isespecially helpful in dark areas. Two-color or ratio

TEMPERATURE

49InTech • June 1998

Object

Infrared temperature sensor

Optics Amplifier

Electronics

Detector

Atmosphere

Figure 1. The infrared temperaturesensor collects energy emitted from theobject based on its optics and location.Detectors measure the energy and con-vert it into an electrical signal.

Sensor

GoodBest Incorrect

Target equalto spot size

Target smallerthan spot size

Target greaterthan spot size

Background

Figure 2. For accurate temperaturemeasurement, the target should belarger than the instrument’s field of view,or spot size. If the spot size is largerthan the target, energy emitted from thebackground or surrounding objects willalso be measured.

instruments, where temperature is determinedfrom the ratio of the radiated energies in two sep-arate wavelength bands, are a good choice whentargets are very small or moving in and out of thefield of view. Energy received from two-colorinstruments may be attenuated up to 95% andstill provide accurate temperature measurement.Two-piece fiber-optic units, where the cable cansnake around the obstructions, may be a goodchoice if a direct line of sight between the instru-ment and the target is otherwise impossible.

Determine optical resolutionOptical resolution is specified by the D:S

ratio, which is determined by comparing the dis-tance from the object to the sensor (D) with thesize (i.e., diameter) of the spot being measured(S). For example, a 1-inch spot on a target beingmeasured at a distance of 10 inches has a D:Sratio of 10:1. Infrared sensors on the markettoday have D:S ratios ranging from 2:1 (lowoptical resolution) to more than 300:1 (highoptical resolution). The higher the optical reso-lution, the more expensive the instrument opticstend to be. The choice of D:S ratio reallydepends on the size of the object to be measuredand the distance the sensor is from the target.For example, high resolution is needed for high-temperature applications (e.g., heat treating)where the sensor must be mounted far awayfrom the target but must still measure a smallspot. Optical charts help determine the targetspot size at a specific distance for fixed-focus

instruments. An optical chart for one sensor isshown in Figure 3.

Infrared temperature sensors are availablewith both fixed- and variable-focus lenses. Theinstrument’s focal point is the smallest spot it canmeasure. On a fixed-focus instrument, there is asingle focal point at a set distance. While it ispossible to accurately measure temperature at adistance closer to or farther from the focal point,the spot size will be larger than at the focal point.Variable-focus instruments have a minimumfocal point that can be adjusted to correspond tothe distance from the target.

Target material impacts measurementThe target material’s emissivity and surface

characteristics determine the spectral response orwavelength needed in a sensor. Highly reflectivemetals with different alloy compositions tend tohave low or changing emissivities. Thus, theoptimum wavelength for measuring high-temperature metal is the near infrared, around0.8 to 1 micron. Because some materials aretransparent at certain wavelengths, choose awavelength at which the material is opaque. Forexample, 5 microns is a good choice for surfacemeasurement of glass. Plastic films have trans-mission coefficients that vary according to thewavelength and thickness of the materials.Choosing 3.43 microns for polyethylene or poly-propylene or 7.9 for polyester allows measure-ment of thin films (less than 10 mils). The typicalspectral response for low-temperature applicationsis 8 to 14 microns. If there is any doubt, the man-ufacturer can test a sample of the material todetermine the optimum spectral band to use.

If processes are run with different target mate-rials, select an instrument with adjustable emis-sivity. Fixed-emissivity instruments are sufficientfor some materials, especially in low-temperatureapplications.

Fast response timeInfrared temperature sensors reach 95% of the

final temperature reading—a common definitionof response time—much faster than contact tem-perature sensors (e.g., thermocouples). This isparticularly important when measuring movingor quickly heated objects. New infrared sensorson the market have response times selectabledown to 1 millisecond. However, a fast responsetime is not desirable for all applications, especial-ly in those where a fast sensor may exceed thecapability of existing control instruments. Inaddition, when there is significant thermal lag inheating a process, speed in the instrument may beunimportant.

June 1998 • InTech 50

TEMPERATURE

750600450300150

6 12 18 24 30

6.4 mm @ 200 mm

0.25 in @ 8 in

0.9

0

0

Distance: sensor to object (in)

Distance: sensor to object (mm)

0.3 0.61.3

2.02.6

23 8 1532

5065

IR sensor

Target spot sizeat focal point

Diameter of target spot size

Distance from sensor to object

Sp

ot

dia

mete

r (i

n)

Sp

ot

dia

mete

r (m

m)

D:S = Distance to spot

Spot diameter

Figure 3. The smallest spot this instru-ment can measure is 0.25 inch at a dis-tance of 8 inches. It would still be possi-ble to accurately measure from a dis-tance of 24 inches, but the minimumspot size would increase to 2.0 inches.

Signal-processing needs varyDiscrete processes (e.g., parts manufacturing),

as opposed to continuous processing, requireinstruments with signal processing (e.g., peak orvalley hold and averaging). Peak hold may beused, for example, to measure the temperature ofglass bottles on a conveyor belt with temperatureoutput fed into a controller. Without peak hold,the temperature sensor would read the lowertemperature between the bottles and respond byincreasing the process temperature. With peakhold, the instrument response time is set slightlylonger than the time interval between bottles sothere will always be at least one bottle represent-ed in the temperature measurement. A sensitivecontrol system can be fine-tuned by averaging thetemperature output.

Ease of use is importantInfrared temperature systems should be easy

and intuitive for plant operators to use. Today,user interfaces may be located directly on the sen-sor, on a remote monitor panel, or through a soft-ware program. Sensors with a built-in display anduser interface are easy to install and set up. A sep-arate, more accessible monitor is appropriate forongoing temperature monitoring when sensorsare installed in hard-to-reach locations. A typicalinstrument with display is shown in Figure 4.

The simplest monitors provide a remote dis-play of the current temperature. Additional fea-tures include adjustable set points that generatean alarm or process correction. Digital displays,which are replacing traditional analog displays,provide averaging and trend plotting and helpminimize operator error. LED displays are easierto read in low light, but may be difficult to see inbright light. Graphical displays that plot temper-ature data over time are also available.

Infrared smart sensors house microproces-sors and support bidirectional, serial communi-cations between a sensor on the plant floor anda PC. Software available with smart temperaturesensors, often running on the familiar Windowsplatform, makes it easy to remotely monitortemperature data and modify sensor parametersfrom the safety of the control room, as shown inFigure 5.

Environmental considerationsSensors are specified for performance within

certain ambient temperature ranges. Dust, gases,or vapor can cause inaccuracies in measurementand/or damage sensor lenses. Noise, electromag-netic fields, or vibration are other conditions thatshould also be considered before installationbegins. A protective housing, air purging, and/or

water cooling can protect the sensor and ensureaccurate measurements. These accessories are avail-able from most manufacturers. In choosing acces-sories, consider the cost of bringing services (e.g.,power, air, and water) to the unit. When possible,choose accessories that require standard services tominimize installation costs. The manufacturer willspecify cable lengths, and all cables must be ratedfor the required ambient environment.

Two-color instruments are a good choicewhen smoke, dust, or other particulates degradethe measurement signal. Fiber-optic sensors,where the optical head is separated from the sen-sor electronics with a fiber-optic cable, provide asolution around electromagnetic fields or otherharsh environments.

In applications involving hazardous materials(e.g., vacuum chambers), the sensor is mountedto look through a window into the enclosure.Window materials must be able to transmit thewavelengths used by the sensor. When specifyingwindow materials, it is important to determine ifthe operator also needs to be able to see throughthe window. For example, in low-temperatureapplications, the window may make the targetinvisible to the eye, since it is often made of anopaque material such as germanium or amor-phous material transmitting infrared radiation.If the operator needs to see through the window,zinc selenide or barium fluoride windows arerecommended.

TEMPERATURE

51InTech • June 1998

Figure 4. Typical instrument withdisplay.

Maintainability importantThe cost of an infrared sensor is usually

minor compared to the risk of process down-time. A sensor is a long-term investment that,for the most part, can be expected to providereliable use for 10 years or more. With thisexpectation, product reliability and vendorresponsiveness become important evaluationcriteria. If a unit needs repair, what kind ofturnaround can you expect from the vendor?What is the average cost of repair compared tothe cost of a new unit? Are spare or loaner unitsavailable? Does the vendor provide on-site oper-ator training? While these issues are harder toquantify, they potentially represent expensesbeyond the cost of the unit.

The new smart sensors offer functionality thatextends the life of the sensor. Because smart sen-sors contain processing capabilities at the sensingnode, if something goes wrong (e.g., a high ambi-ent condition or failed component in the sensor),fail-safe conditions are automatically used to pro-tect the sensor. Updates to the sensor firmwarecan be downloaded from a PC without removingthe sensor or returning it to the factory for anupgrade.

Vendors recommend that infrared tempera-ture sensors be recalibrated to a known tempera-ture source at least once a year. This is required inISO 9001 plants where infrared temperature sen-

sors are part of the control instrumentation. Mostmanufacturers offer calibration services for theircustomers. Smart temperature sensors can be cal-ibrated on-site using calibration software and ablackbody calibration source.

New uses for temperature dataInfrared temperature sensors are available with

voltage, current, and digital outputs. In manycases, the choice of output will depend on theexisting control equipment. The most commonoutput is the 4-20 mA current loop, which caneasily be integrated into the control environment.When an infrared temperature sensor is replacinga thermocouple, choose an instrument with theappropriate thermocouple output. Many smartinfrared systems support simultaneous analogand digital output using RS-232 or RS-485 seri-al communications. The analog output is usuallyintegrated into an existing control environment,while the digital output is used for ongoinganalysis and quality control.

Digital output supports sophisticated analy-sis where temperature can be combined withother process variables. For example, on a paperline a process engineer may be concerned withthe temperature, moisture, and weight of theproduct. In this case, the temperature data isused as a variable in a model to optimizeprocess efficiency. Digital output capabilities ofsmart infrared units have recently caught theattention of quality managers who can nowcapture temperature data for each product orrun. This data can be archived, graphed, orprinted to document that the job was per-formed according to specifications. If docu-mentation of process temperatures is requiredfor IS0 9001 or other quality programs, digitaloutput should be considered.

Choosing an instrument is straightforwardInfrared temperature measurement is based

on field-proven technology. With a basic under-standing of infrared theory and the main selec-tion criteria, choosing an infrared instrument isa straightforward procedure. In addition to thekey specifications, ease-of-use, installation, andmaintenance requirements help determine if theinstrument is a good match for the process. Inaddition, new process requirements (e.g., ISO9001 documentation or statistical process con-trol) can benefit from the latest smart sensors. Awide variety of infrared temperature sensors onthe market today provide ongoing, accurate tem-perature measurement, regardless of whether theinstrument is selected to replace a thermocoupleor to integrate into a multivariable process. IT

Behind the byline

Karen Ackland is marketingmanager for Raytek Corporationin Santa Cruz, Calif. She has anM.B.A. from UCLA, and hasmore than 15 years of experiencemarketing technology products,including instrumentation, dis-tributed systems, and robotics.

June 1998 • InTech 52

TEMPERATURE

Figure 5. PC software provides remote tem-perature monitoring, sensor configuration,and data analysis for smart infrared sensors.