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.