Infrared Temperature Calibration 101 Using the right tool means better Application Note work and more productivity

Infrared thermometers let you measure a target’s surface temperature from a distance without physically touching it. To point and shoot at a target to measure its temperature may seem a little like magic. This application note demystifies the process of infrared (or “IR”) thermometry and explains why regular calibration is important for maximizing the value of these useful devices.

How infrared hot,” such as the red glow of thermometers the embers of a fire, the yellow measure temperature glow of a candle, and the white glow of an incandescent light Infrared thermometers bulb. The color we perceive is measure the electromagnetic related to the temperature of radiation emitted by an object the heated object. In fact, steel as a result of the object’s workers claim that they can temperature. Until an object accurately judge the tempera- becomes very hot, most of this ture of molten steel to within radiation is in a band of wave- 50 °C based on the color alone. lengths called the infrared Like the eyes of steel work- spectrum. Very hot objects emit ers, infrared thermometers are visible light which is also a form also designed to be sensitive of electromagnetic radiation. around a specific band of While the human eye is very wavelengths. The most sensitive to yellow light with commonly used spectral band wavelengths around 0.555 for general purpose infrared micrometers, it cannot detect thermometers is from 8 µm to light with wavelengths longer 14 µm (8 to 14 micrometers). than 0.700 micrometers (red) Infrared radiation is elec- and shorter than the 0.400 tromagnetic radiation with micrometers (violet). Although wavelengths longer than visible our eyes can’t detect the energy light and shorter than millime- outside of this narrow band of How small is a micrometer? ter wave radiation. Terms like wavelengths, called the visible wavelength and amplitude are A micrometer is very small; in spectrum, we still know it is used to describe infrared and fact, it is one one-millionth of a there, because we can detect it other forms of electromagnetic meter. To put that into perspective, with a radiometer. radiation. For example, wave around 100 micrometers would “Seeing” temperature amplitude describes the inten- equal the thickness of a human We have some experience sity of electromagnetic radiation hair. measuring temperature by and wavelength is used among detecting electromagnetic other things to determine radiation with our eyes. We are whether it is a microwave, visi- familiar with things that “glow ble light, or infrared radiation.

From the Fluke Digital Library @ www.fluke.com/library Calibration is easy with the 4180 Series Precision Infrared Calibrator.

Infrared temperature Converting input mal energy into a resistance calibration temperature to radiance that is measured by a readout The measurement surface of device and used to calculate a Temperature source a calibrator acts as a trans- temperature. It is interesting to In spite of our experiences with ducer by converting thermal note that the sensor is respon- seeing temperature, confidence energy into thermal radiation. sible for most of the error in in IR temperature measurement The intensity of a portion of a temperature measurement, usually requires the use of cali- the infrared radiation emitted which explains the importance brated instruments. Calibration by the measurement surface is of calibrating the temperature can be defined as a set of oper- measured by the infrared ther- sensor. One of these sources ations, performed in accordance mometer to calculate a tempera- of error in the measurement with a definite, documented ture. The measurement surface surface and perhaps the largest procedure, which compares the is analogous to the sensor of in an infrared temperature cali- measurements performed by an RTD which transduces ther- bration is emissivity. an instrument to those made by a more accurate instrument or standard, for the purpose of detecting and reporting, or eliminating by adjustment, Wavelength errors in the instrument tested. An IR temperature calibra- tion starts with a measurement surface that acts as a tempera- ture source, which may be a flat plate or cavity that func- tions as the standard or refer- ence. Calibration geometry, which includes the size of the Amplitude measurement surface and its distance from the thermom- eter being calibrated plays an important role in the measure- ment result. Also critical are the temperature stability and uniformity and the physi- cal properties of the emitting surface such as its emissivity. Temperature uniformity is important for infrared thermometer and thermal imager calibrations.

2 Fluke Corporation Infrared Temperature Calibration 101 Emissivity same temperature. Emissivity is Emissivity, The radiant energy coming from greatly affected by the type of an opaque target is a combina- material and surface finish of blackbodies tion of emitted radiance caused an object. Metals with smooth and graybodies by the target’s temperature and surface finishes tend to have reflected radiance coming from low emissivity and high reflec- Emissivity can be any value elsewhere in the environment. tivity while long narrow holes between and including zero Transmission through the have relatively high emissivity and one. Zero emissivity means object is another source of radi- and very little reflectivity. The that no matter what an object’s ant energy that must be consid- sum of emissivity, reflectiv- temperature, no light will be ered when objects are not ity, and transmission is always radiated. An emissivity of one equal to one. means that the surface will Infrared radiate perfectly at all wave- temperature cali- lengths. Scientists give these reflectivity + emissivity + transmissivity = 1 brators must be special perfectly radiating designed to have objects the name “blackbody.” a known emis- Objects with emissivity very The relationship between emissivity, reflectivity and transmission. sivity, which must close to one are also usually remain constant called blackbodies. A flat plate opaque. The amount of light over the full operating tempera- calibrator with an emissivity emitted at a particular tempera- ture. Unfortunately, emissivity around 0.95 is referred to as a ture is determined by the emis- is neglected in the calibration graybody if the emissivity is sivity of the surface. Emissivity of most IR calibrators. These uniform across all wavelengths. is the ratio of the radiant energy calibrators are themselves cali- emitted by a surface to that brated by inserting a contact emitted by a blackbody at the thermometer such as a platinum

Effect of a 1 % increase of emissivity on apparent temp TBG=23 °C, ε=0.95, λ=8 μm to 14 μm

) 4.000 C ° ( 3.500 e g

n 3.000 a h C

2.500 e r

u 2.000 t a r

e 1.500 p

m 1.000 e T

t 0.500 n e r

a 0.000 p -50 0 50 100 150 200 250 300 350 400 450 500 p -0.500 A

-1.000

-1.500 Target Temperature (°C)

Temperature errors equivalent to a one percent error in emissivity. A 1 % error in emissivity could lead to a 3 °C error in temperature at 450 °C.

3 Fluke Corporation Infrared Temperature Calibration 101 resistance thermometer (PRT) These corrections are based unless the emissivity setting of physically into the target. This on the difference between the the thermometer does not match technique also neglects temper- actual measured emissivity of the calibrator. Look for a calibra- ature losses at the surface of the the target and the tor that can compensate for the infrared target. With this type of emissivity setting of the ther- emissivity settings of infrared calibration, the user may not be mometer. A one percent error in thermometers. aware that complicated emis- the emissivity at 500 °C would As shown in the Spectral sivity-related corrections are result in a 3.5 °C error in the Radiance and Temperature required at each temperature in calibration. On the other hand, graph below, when temperature order to achieve the accuracy emissivity values offered by causes an object to emit light, claimed by the manufacturer. manufacturers are usually the light comes out in many typical values only and are not different wavelengths. This is actually verified by calibration. called spectral radiance Tech tip: This can lead to a loss of (see graph below). If you could traceability and inconsistent line up all the waves from If the target is at a tempera- results over time and incon- shortest to longest, you would ture below the dew point, sistent results among different see that the brightest waves ice may form in crystal calibrators. are somewhere in the middle. patterns that will cause the To correct for temperature If you then increased the emissivity of the surface to errors in the measurement temperature of the object change, introducing errors surface, an infrared tempera- emitting the light, you would into the calibration. Purging ture calibrator should be cali- notice that the shorter waves with dry gas is one method brated with a radiometer, which were getting brighter than of preventing the growth of measures the amount of radiant the longer waves. When the crystals which eventually may energy coming from the calibra- object becomes hot enough, form a sheet of ice that actu- tion target at each temperature. even the very short waves from ally masks the temperature A temperature display that has 0.400 µm to 0.700 µm start to of the target, creating even had a radiometric calibra- be bright enough that the radia- larger errors. tion does not need additional tion emitted by the object is emissivity-related temperature, visible to the human eye and

Spectral Radiance and Temperature Spectral Radiance (W m-2 μm-1 sr-1)

Wavelength (μm)

The peak on the graph corresponds to the brightest wavelengths. The peak moves to the left and the whole curve moves up as the temperature increases.

4 Fluke Corporation Infrared Temperature Calibration 101 Manufacturer recommended large target

IR thermometer spot size

199.3

Smaller target

= IR thermometer field of view

Peripheral vision in infrared thermometers makes large targets necessary for many calibrations. the object appears to glow. sometimes called scatter, can warm objects in the environ- The emissivity at each wave- account for between 1 % and ment, such as windows, walls, length, called spectral emissiv- 35 % of the total measured and people. For an opaque ity, determines how bright each energy, depending on instru- object, emissivity plus reflectiv- wave will be. The thermometer ment quality. While the spot ity is equal to one. uses the total brightness of a size may be adequate for practi- Consequently, if the emis- selected band of these waves to cal field measurements, it is not sivity of an opaque target is determine the temperature. always adequate for the labora- zero, then none of the radiated Detecting radiance with tory accuracy required during energy coming from that target a thermometer a calibration. That is why cali- is caused by its own thermo- The radiated light energy from brating most IR thermometers dynamic temperature. All of the calibrator’s surface is trans- requires a target the light detected by the ther- mitted through the air until it that is significantly larger than mometer is radiation reflected is detected by a thermometer. the size suggested by the from the target that originated Dust, smoke and glass surfaces distance-to-spot ratio of the elsewhere, striking it from can impede the light and affect thermometer. elsewhere in the room. If the the results of the measurement. The infrared energy collected target’s emissivity is 0.95, then The detector has to collect the by the thermometer will be its reflectivity is 0.05. In other light from the precise area of filtered by its optical system, words, the target absorbs 95 % the intended target to avoid which is only sensitive to a of the energy in the room and measuring unintended objects particular spectral band such reflects the other 5 %. It also in the background. The as 8 µm to 14 µm. Included in means that the energy emitted distance-to-spot ratio (D:S) the collected energy is reflected by the target caused by temper- assists the user in finding the radiation caused by the ambi- ature is 95 % of what would be correct position of the ent temperature in the room. expected from a perfect black- thermometer. Reflection is related to emis- body. IR thermometers attempt However, spot size is only a sivity in an interesting way. to compensate for the energy percentage of the thermometer’s The detectable infrared radia- reflected by a target; however, entire field of view. Like tion coming from the calibra- when a target becomes very the human eye, an infrared tor is usually a combination of cold compared to the environ- thermometer has some emitted thermal radiation from ment, or if the emissivity is very peripheral vision. Peripheral the measurement surface and low, then the reflected energy vision in IR thermometers, reflected energy from other may make it difficult to read

5 Fluke Corporation Infrared Temperature Calibration 101 the correct temperature. This is accommodate the thermometer’s Distance to spot ratio: because the reflected energy peripheral vision. You probably The distance-to-spot ratio (D:S) is a relatively large proportion need the same size target the is the ratio of the distance to of the signal received by the manufacturer uses for its own the object and the diameter of thermometer when the target calibrations; thermometer manu- the area containing a specified temperature is below ambi- facturers also recommend using percentage of the total energy ent temperature. This situation the same calibration distance picked up by the infrared ther- is often referred to as a low that they do to achieve labora- mometer. The D:S ratio is used signal-to-noise ratio. tory levels of accuracy in a cali- as a guide to determine the IR thermometers measure bration. appropriate distance for making a group of wavelengths called If your calibrator does not practical infrared temperature a spectral band. IR ther- have a radiometric calibration, measurements. mometers are spectral band you need to know the emissivity thermometers because they of its target so you can calcu- Electromagnetic radiation: measure spectral radiance, late the appropriate corrections. Energy emitted by a surface that the collective radiance of all These calculations are difficult, travels through space as a wave of the wavelengths inside a so getting the right calibra- with electric and magnetic particular spectral band, such as tion to begin with makes a big components. Examples include: 8 µm to 14 µm. difference. Similar calculations radio waves, microwaves, milli- meter wave radiation, infrared Calculating temperature must be made if the emissiv- radiation, visible light, ultraviolet from radiance ity settings on the thermometer radiation, X-rays and gamma Finally, an accurate temperature do not match those of the cali- rays. calculation requires the emissiv- brator, and that is why a well ity setting of the thermometer, designed calibrator that can make those calculations for you Emissivity: to match the real emissivity of The emissivity of a surface indi- the objects being measured. saves worry and time that you can spend being productive. cates how efficiently it emits Alternatively, in the case of radiation compared to a black- a target with a radiometric body at the same temperature. calibration, the thermometer’s It is measured by the ratio of emissivity should match the Glossary energy radiated by the mate- effective calibrated emissivity Absorption: rial to the energy radiated by a of the target, so that a direct The process of converting blackbody at the same tempera- comparison can be made radiation energy that falls on ture. between the infrared tempera- a surface into internal thermal ture of the calibrator target energy. Field of view: and the infrared thermometer The field of view (FOV) is the under test. Blackbody: region in space containing a Conclusion A blackbody is an ideal surface specified amount of the radiant Infrared thermometers are used that emits and absorbs elec- energy collected by the optical in a variety of situations where tromagnetic radiation with the system of an infrared thermom- contact measurements are maximum amount of power eter. FOV is usually expressed impractical. Applications involv- possible at a given tempera- in angular degrees (e.g., 1 °). ing these useful devices are ture. Such a surface does not The measurement surface must often misunderstood, leading to allow radiation to reflect or pass completely fill the thermometer’s a reduction in the usefulness of through it. In the laboratory a field of view to ensure accurate the resulting measurements. But blackbody is approximated by a temperature measurements. confidence in these measure- large cavity with a small open- ments increases with calibra- ing. Reflection is prevented Graybody: tion. When the stakes are high, because any light entering the A surface that emits radiation or the application is important, hole would have to reflect off with constant emissivity over all this additional confidence is the walls of the cavity multiple wavelengths and temperatures well worth the investment in times, causing it to be absorbed is called a graybody. Although regular testing or calibration of before it could escape. graybodies do not exist in prac- the IR thermometer. tice, they are a good approxima- All calibrations are not Dew point: tion for many real surfaces. The dew point is the tempera- created equal, and the right Infrared thermometer: equipment choice can make all ture to which air must be cooled, at a given barometric pressure, A device that calculates the the difference. When choos- temperature of an object by ing a calibrator, look for one for water vapor to condense into water. measuring the infrared radia- with a large enough target to tion emitted by the object. It is 6 Fluke Corporation Infrared Temperature Calibration 101 sometimes called a non-contact Reflectivity: Transmission: thermometer because of its abil- The fraction of incident radia- The process by which some ity to measure the temperature tion reflected by a surface. fraction of electromagnetic of a surface without making energy reaching a surface is contact with that surface. Scatter: allowed to pass through the An effect caused by non-unifor- material belonging to that Infrared radiation: mities in a medium that forces surface. Transmission assumed Infrared (IR) radiation is elec- light to travel in a direction that to be zero for surfaces that are tromagnetic radiation with deviates from a straight path. opaque at the measured wavelengths longer than visible spectral band. light and shorter than millimeter Spectral band: wave radiation. All surfaces with A well-defined, continuous Transmissivity: temperatures above absolute wavelength range electromag- A measure of how much of an zero (-273.15 °C) emit infrared netic energy. The range from electromagnetic wave passes radiation. 8 to 14 microns used by many through a surface. It is calcu- infrared thermometers is an lated by taking the ratio of the Opaque: example of a spectral band. value that passes through the An opaque object does not allow surface to the value that reaches electromagnetic radiation to Spectral radiance: the surface. pass through it. A surface may A measure of the intensity of be opaque for some wave- electromagnetic radiation at a Uniformity: lengths and transparent for specific wavelength emitted by A uniformity specification is an others. For example, glass is or passing through a particular expression of the maximum opaque to infrared radiation area in a specified direction. It allowable difference between with wavelengths longer than is defined as radiance per unit two measurement results ~3 microns but is transparent to wavelength. Spectral radiance within the region contained visible light. has SI units W·sr--1·m-3 when by a measurement medium or measured per unit wavelength. surface. Radiance: A measure of the intensity of Spot size: Wavelength: electromagnetic radiation emit- The diameter of the measure- The distance in the direction ted by or passing through a ment spot of a radiometer. It is of a moving wave that it takes particular area in a specified usually defined as the diameter to complete a single cycle. In a direction. It is defined as power that collects a defined percent- wave, a property such as elec- per unit area per unit solid age of the total power collected tric potential varies cyclically angle. The SI unit of radiance is by the radiometer, e.g., 95 %. with position. In a moving wave watts per steradian per square This spot size will vary with the position also changes with meter (W·sr--1·m-2). distance from the radiometer. time. Radiometric calibration: Thermodynamic temperature: A calibration that compares A temperature value based on a the measurements made by an scale that is consistent with all instrument to the measurements the known laws of thermody- made by a reference radiometer, namics. for the purpose of detecting and reporting, or eliminating by adjustment, errors in the instru- Fluke Calibration. Precision, performance, confidence.™ ment tested. Radiometer: A device used to measure the power, also called radiant flux, Fluke Calibration Fluke Europe B.V. in electromagnetic radiation. PO Box 9090, PO Box 1186, 5602 BD See also infrared thermometer. Everett, WA 98206 U.S.A. Eindhoven, The Netherlands For more information call: Reflection: In the U.S.A. (877) 355-3225 or Fax (425) 446-5116 Electromagnetic radiation is In Europe/M-East/Africa +31 (0) 40 2675 200 or Fax +31 (0) 40 2675 222 In Canada (800)-36-FLUKE or Fax (905) 890-6866 reflected when it changes direc- From other countries +1 (425) 446-5500 or Fax +1 (425) 446-5116 tions at a surface while remain- Web access: http://www.fluke.com ing in the first medium without ©2007-2011 Fluke Corporation. Specifications subject to change without notice. passing into the second. Printed in U.S.A. 1/2011 3187781C A-EN-N Pub_ID: 11329-eng, rev 03 Modification of this document is not permitted without written permission from Fluke Corporation. 7 Fluke Corporation Infrared Temperature Calibration 101