07 Infrared Temperature Measurement Theory

7/31/2019 07 Infrared Temperature Measurement Theory

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Infrared TemperatureMeasurement Theory and Application

Author and Presenter:John Merchant, Sales Manager, Mikron Instrument Company Inc.

ABSTRACTInfrared thermometers for non-contacttemperature measurement are highlydeveloped sensors which havewide-spread application in industrialprocessing and research. This paperdescribes, in non-mathematical terms,the theory upon which the measurementtechnology is based, and how thisis used to deal with the variety ofapplication parameters which confrontthe intending user.

INTRODUCTIONAn infrared thermometer measurestemperature by detecting the infraredenergy emitted by all materials whichare at temperatures above absolute

zero, (0Kelvin). The most basic designconsists of a lens to focus the infrared(IR) energy on to a detector, whichconverts the energy to an electricalsignal that can be displayed in units oftemperature after being compensatedfor ambient temperature variation. Thisconfiguration facilitates temperaturemeasurement from a distance withoutcontact with the object to be measured.As such, the infrared thermometer isuseful for measuring temperature undercircumstances where thermocouplesor other probe type sensors cannot beused or do not produce accurate datafor a variety of reasons. Some typicalcircumstances are where the object tobe measured is moving; where theobject is surrounded by an EM field, asin induction heating; where the objectis contained in a vacuum or othercontrolled atmosphere; or in applicationswhere a fast response is required.

Designs for an infrared thermometer(IRT), have existed since at least thelate nineteenth century, and variousconcepts by Fry were featured byCharles A. Darling (1) in his bookPyrometry, published in 1911.

However it was not until the 1930s thatthe technology was available to turnthese concepts into practical measuringinstruments. Since that time therehas been considerable evolution inthe design and a large amount ofmeasurement and application expertisehas accrued. At the present time, thetechnique is well accepted and is widelyused in industry and in research.

MEASUREMENT PRINCIPLESAs previously stated IR energy is emittedby all materials above 0K. Infraredradiation is part of the ElectromagneticSpectrum and occupies frequenciesbetween visible light and radio waves.The IR part of the spectrum spanswavelengths from 0.7 micrometers to1000 micrometers (microns). Figure 1.Within this wave band, only frequenciesof 0.7 microns to 20 microns are usedfor practical, everyday temperaturemeasurement. This is because the IRdetectors currently available to industryare not sensitive enough to detect thevery small amounts of energy availableat wavelengths beyond 20 microns.

Though IR radiation is not visible to thehuman eye, it is helpful to imagine it asbeing visible when dealing with theprinciples of measurement and whenconsidering applications, because inmany respects it behaves in the sameway as visible light. IR energy travels instraight lines from the source and canbe reflected and absorbed by materialsurfaces in its path. In the case of mostsolid objects which are opaque to thehuman eye, part of the IR energystriking the objects surface will beabsorbed and part will be reflected. Ofthe energy absorbed by the object, aproportion will be re-emitted and part

will be reflected internally. This will alsoapply to materials which are transparento the eye, such as glass, gases andthin, clear plastics, but in addition, someof the IR energy will also pass throughthe object. The foregoing is illustrated inFigure 2. These phenomena collectivelycontribute to what is referred to as theEmissivity of the object or material.

Materials which do not reflect ortransmit any IR energy are know asBlackbodies and are not known to existnaturally. However, for the purpose oftheoretical calculation, a true blackbodyis given a value of 1.0. The closestapproximation to a blackbody emissivityof 1.0, which can be achieved in real lifeis an IR opaque, spherical cavity with asmall tubular entry as shown in Figure 3The inner surface of such a sphere willhave an emissivity of 0.998.

Different kinds of materials and gaseshave different emissivities, and willtherefore emit IR at different intensitiesfor a given temperature. The emissivityof a material or gas is a function ofits molecular structure and surfacecharacteristics. It is not generally afunction of color unless the source of

Wavelength (meters)

Infrared spectrum 0.7 to 1000 micrometers (microns)

ELECTROMAGNETIC SPECTRUM

1012

1010

108

106

gammarays

x-ray

ultraviolet

visible

infrared

radio

104

102

1 102

104

Figure 1

emission absorption reflection transmission

emitted

incident

hot coldreflected

RADIATIVE HEAT EXCHANGE

Figure 2


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the color is a radically different substanceto the main body of material. A practicalexample of this is metallic paints whichincorporate significant amounts ofaluminum. Most paints have the sameemissivity irrespective of color, butaluminum has a very different emissivitywhich will therefore modify the emissivityof metallized paints.

Just as is the case with visible light,the more highly polished some surfacesare, the more IR energy the surface willreflect. The surface characteristics of amaterial will therefore also influence itsemissivity. In temperature measurementthis is most significant in the case ofinfrared opaque materials which havean inherently low emissivity. Thus ahighly polished piece of stainless steelwill have a much lower emissivity than

the same piece with a rough, machinedsurface. This is because the groovescreated by the machining prevent asmuch of the IR energy from beingreflected. In addition to molecularstructure and surface condition, a thirdfactor affecting the apparent emissivityof a material or gas is the wavelengthsensitivity of the sensor, known as thesensors spectral response. As statedearlier, only IR wavelengths between0.7 microns and 20 microns are usedfor practical temperature measurement.Within this overall band, individualsensors may operate in only a narrowpart of the band, such as 0.78 to 1.06,or 4.8 to 5.2 microns, for reasons whichwill be explained later.

THEORETICAL BASIS FOR IRTEMPERATURE MEASUREMENTThe formulas upon which infraredtemperature measurement is based areold, established and well proven. It isunlikely that most IRT users will needto make use of the formulas, but aknowledge of them will provide anappreciation of the interdependencyof certain variables, and serve to clarifythe foregoing text. The importantformulas are as follows:

1. Kirchoffs LawWhen an object is at thermalequilibrium, the amount of absorptionwill equal the amount of emission.

a = e

2. Stephan Boltzmann LawThe hotter an object becomes themore infrared energy it emits.

W = T4

3. Wiens Displacement LawThe wavelength at which themaximum amount of energy isemitted becomes shorter as thetemperature increases.

max = 2.89 x 103mK/T

4. Plancks EquationDescribes the relationship betweenspectral emissivity, temperature andradiant energy.

C2W = C1[

5(T-1)]-1

INFRARED THERMOMETERDESIGN AND CONSTRUCTIONA basic infrared thermometer (IRT)design, comprises a lens to collect theenergy emitted by the target; a detectorto convert the energy to an electricalsignal; an emissivity adjustment tomatch the IRT calibration to the emittingcharacteristics of the object beingmeasured; and an ambient temperaturecompensation circuit to ensure thattemperature variations within the IRT,due to ambient changes, are nottransferred to the final output. For manyyears, the majority of commerciallyavailable IRTs followed this concept.They were extremely limited inapplication, and in retrospect didnot measure satisfactorily in most

circumstances, though they were verydurable and were adequate for thestandards of the time. Such a conceptis illustrated in Figure 4.

The modern IRT is founded on thisconcept, but is more technologicallysophisticated to widen the scope of itsapplication. The major differences arefound in the use of a greater variety ofdetectors; selective filtering of the IRsignal; linearization and amplificationof the detector output; and provisionof standard, final outputs such as4-20mA, 0-10Vdc, etc. Figure 5 showsa schematic representation of a typicalcontemporary IRT.

Probably the most important advancein infrared thermometry has been theintroduction of selective filtering of theincoming IR signal, which has beenmade possible by the availability ofmore sensitive detectors and morestable signal amplifiers. Whereas theearly IRTs required a broad spectralband of IR to obtain a workable detectoroutput, modern IRTs routinely havespectral responses of only 1 micron.The need to have selected and narrowspectral responses arises because it isoften necessary to either see throughsome form of atmospheric or otherinterference in the sight path, or in factto obtain a measurement of a gas or

other substance which is transparent toa broad band of IR energy.

Some common examples of selectivespectral responses are 8-14 microns,which avoids interference fromatmospheric moisture over long pathmeasurements; 7.9 microns which isused for the measurement of some thinfilm plastics; and 3.86 microns whichavoids interference from CO2 and H2O

theoretical blackbody practical blackbody

EMISSIVITY

0

1

1

Figure 3

INFRARED TEMPERATURE MEASUREMENT

A.T.C.E

DET

Figure 4


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vapor in flames and combustion gases.The choice between a shorter, or longerwavelength spectral response is alsodictated by the temperature rangebecause, as Plancks Equation shows,

the peak energy shifts towards shorterwavelengths as the temperatureincreases. The graph in Figure 6illustrates this phenomenon. Applicationswhich do not demand selective filtering

for the above stated reasons may oftenbenefit from a narrow spectral responseas close to 0.7 microns as possible.This is because the effective emissivity

of a material is highest at shorterwavelengths and the accuracy ofsensors with narrow spectral responsesis less affected by changes in targetsurface emissivity.

It will be apparent from the foregoinginformation that emissivity is a veryimportant factor in infrared temperaturemeasurement. Unless the emissivity ofthe material being measured is known,and incorporated into the measurementit is unlikely that accurate data will beobtained. There are two methods forobtaining the emissivity of a material:a) by referring to published tables andb) by comparing the IRT measurement

with a simultaneous measurementobtained by a thermocouple or resistancthermometer and adjusting the emissivitysetting until the IRT reads the same.Fortunately, the published data availablefrom the IRT manufacturers and someresearch organizations is extensive,so it is seldom necessary to experimentAs a rule of thumb, most opaque,non-metallic materials have a highand stable emissivity in the 0.85 to 9.0range; and most un-oxidized, metallicmaterials have a low to mediumemissivity from 0.2 to 0.5, with theexception of gold, silver and aluminumwhich have emissivities in the order of0.02 to 0.04 and are, as a result, very

difficult to measure with an IRT.

While it is almost always possible toestablish the emissivity of the basicmaterial being measured, a complicatioarises in the case of materials which haveemissivities that change with temperaturesuch as most metals, and other materialssuch as silicon and high purity, singlecrystal ceramics. Some applicationswhich exhibit this phenomena can besolved using the two color, ratio method

TWO COLOR-RATIOTHERMOMETRYGiven that emissivity plays such a vital

role in obtaining accurate temperaturedata from infrared thermometers,it is not surprising that attempts havebeen made to design sensors whichwould measure independently of thisvariable. The best known and mostcommonly applied of these designsis the Two Color-Ratio Thermometer.This technique is not dissimilar to theinfrared thermometers described so far,but measures the ratio of infraredenergy emitted from the material at twowavelengths, rather than the absoluteenergy at one wavelength or waveband. The use of the word color in this

lenschopper

led opticalinterrupter

filter

thermistora.t.c. amp

signal ampprw. amp prw. amp

signal ground

power supply12 vdc

15 vdcphase control

d.c. motor

MODERN INFRARED THERMOMETER

motorcontrol

Figure 5

Infrared Temperature Contd

2 4 6 8 10 12 14 16 18Wavelength (microns)

Blackbody Spectral Distribution Curves

900K

800K

700K

600K

500K

Spectralradiantemittance(W

cm2

1)

1

0.75

0.50

0.25

0

Figure 6


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context is somewhat outdated, butnevertheless has not been superseded.It originates in the old practice ofrelating visible color to temperature,hence color temperature.

The basis for the effectiveness of two-color thermometry is that any changesin either the emitting property of thematerial surface being measured, or inthe sight path between the sensor andthe material, will be seen identically bythe two detectors, and thus the ratio andtherefore the sensor output will notchange as a result. Figure 7 shows aschematic representation of a simplifiedtwo-color thermometer.

Because the ratio method will, underprescribed circumstances, avoidinaccuracies resulting from changing orunknown emissivity, obscuration in the

sight path and the measurement ofobjects which do not fill the field of view,it is very useful for solving some difficultapplication problems. Among these arethe rapid induction heating of metals,cement kiln burning zone temperatureand measurements through windowswhich become progressively obscured,such as vacuum melting of metals. Itshould be noted however, that thesedynamic changes must be seenidentically by the sensor at the twowavelengths used for the ratio, and thisis not always the case. The emissivity

of all materials does not change equallyat two different wavelengths. Thosematerials that do are called Greybodies.The ones that do not are calledNon-Greybodies.

Not all forms of sight path obscurationattenuate the ratio wavelengths equallyeither. The predominance of particulatesin the sight path which are the samemicron size as one of the wavelengthsbeing used will obviously unbalance theratio. Phenomena which are non-dynamicin nature, such as the non-greybodynessof a material, can be dealt with bybiassing the ratio, an adjustmentreferred to as Slope. However, theappropriate slope setting must generallybe arrived at experimentally. Despitethese limitations, the ratio method workswell in a number of well establishedapplications, and in others is the best,

if not the most preferred solution.

SUMMARYInfrared thermometry is a mature butdynamic technology that has gainedthe respect of many industries andinstitutions. It is an indispensabletechnique for many temperaturemeasurement applications, and thepreferred method for some others.When the technology is adequatelyunderstood by the user, and all therelevant application parameters areproperly considered, a successful

application will usually result, providingthe equipment is carefully installed.Careful installation means ensuringthat the sensor is operated within itsspecified environmental limits, and

that adequate measures are taken tokeep the optics clean and free fromobstructions. A factor in the selectionprocess, when choosing a manufacturer,should be the availability of protectiveand installation accessories, and alsothe extent to which these accessoriesallow rapid removal and replacementof the sensor for maintenance. If theseguidelines are followed, the moderninfrared thermometer will operatemore reliably than thermocouples orresistance thermometers in many cases.

REFERENCES1. Darling, Charles R.; Pyrometry. APractical Treatise on the Measurementof High Temperatures.Published byE.&F.N. Spon Ltd. London. 1911.

Reproduced with permission ofthe author, John Merchant,Sales Manager, MIKRONINSTRUMENT COMPANY, INC.

ratio

D1

D2

O/P

beamsplitter

colimator

target

TWO COLOR THERMOMETRY(ratio thermometry)

Figure 7

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07 Infrared Temperature Measurement Theory