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Emissivity: Understanding thedifference between apparentand actual infrared temperatures

and C. The use clip at the top ophase A indicates 133.4 F, whilethe end o the use, specicallythe metal cap o the top o theuse, appears much cooler witha temperature o 103.6 F andthe use body just below the cap

appears to be 121.9 F.Can this be true? Is the metalcap only 103 F? No. You areseeing an example o the appar-ent temperature and the eecto emissivity. The use end cap isa highly refective metal, in thiscase copper. Notice that the bodyo the use also appears hotterthan the metal cap. The tempera-ture o the cap is actually as hotas the use body its in contactwith.

To explain why the apparent

temperature seen through a ther-mal imager can be signicantlydierent than the actual tempera-ture, lets review our knowledgeo physics.

Thermal radiation andproperties o materials

All objects emit inrared (thermal)radiation. The intensity o theradiation depends on the temper-ature and nature o the materialssurace. At lower temperatures,the majority o this thermal radia-

tion is at longer wavelengths.

As the object becomes hotter,the radiation intensity rapidlyincreases and the peak o theradiation shits towards shorterwavelengths. The relationshipbetween total radiation intensity(all wavelengths) and tempera-

ture is dened by the Stean-Boltzmann law:

Taking inrared temperaturemeasurements is certainly a loteasier than it used to be. Thetricky part is understanding whenan inrared reading is accurateas-is, and when you need toaccount or certain properties o

the materials youre measuring, oror other things like heat transer.The most common use o inra-

red temperature measurementis or the inspection o electricalpower distribution equipment.Lets look at a typical three-phaseused power disconnect (Figure1) and the corresponding inraredimage (Figure IR1) below.

Figure 1 shows a typical3-phase used power discon-nect. The corresponding inraredimage, gure IR1, was taken with

the emissivity setting at 1 on ourthermal imager. The tempera-ture span and color scale or theinrared image is set to 95.5 Freerring to black, with warmertemperatures indicated progres-sively by blue (105 F), green(115 F), red (125 F) and white(133 F and hotter). We alsomeasured the load in phases A,B and C (rom let to right), atapproximately 34 amps each.

A simple analysis o the thermalimage indicates that phase A issignicantly hotter than phases B

Figure 1: Fused power disconnect. Figure IR1: Corresponding inrared image.

By L. Terry Clausing, P.E.ASNT Certifed NDT Level III T/IR,or Fluke Corporation

Q = esT4 where:Q = radiation intensitye = emissivity o material

s = Stean-Boltzmann constantT = absolute temperature

At a given temperature, themaximum radiation is achievedwhen the object has an emis-sivity o 1. This is reerred to asblackbody radiation, becausewith an emissivity o 1, the objectis a perect radiator. However in

our real world, there are no trueblackbodies, that is, no perectradiators. Since real materials areless than perect radiators, therelevant issue is How much lessthan perect are they? Emissiv-ity is dened as the measure ohow much less than perectlya material radiates whencompared to a blackbody.But, emissivity is only one othree actors that cause an objectto be less than a perect radiator.

Application Note

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The thermal natureo materials

Materials (objects in everyday lie,whether they are solids, liquidsor gases) are constantly aectedby their surroundings. Thermally,all objects attempt to exchangeenergy with other objects in theirnatural drive towards thermalequilibrium with their surround-ings. In this search or thermalequilibrium, heat is exchangedbetween objects via three mecha-nisms: conduction, convectionand radiation.

Conduction is dened as heattranser between two solid bodiesthat are in physical contact witheach other. Convection is heattranser, usually between a solidmaterial and a liquid or gas.Conduction and convection aredependent on physical contactbetween materials. Radiationis a process o heat transer,characteristic o all matter (attemperatures above absolutezero). Radiation passes througha vacuum, and can also passthrough gases, liquids andeven solids.When radiative power is

incident on an object, a raction

o the power will be refected (r),another portion will be absorbed(a), and the nal portion will betransmitted through the object.The transmitted raction is t. Allo this is described by the TotalPower Law:r + a + t = 1 where:

r = raction reecteda = raction absorbedt = raction transmitted

The ability o an object to absorbradiation is also related to itsability to emit radiation. This is

dened by Kirchos lawa = e where:

a = absorbance coefciente = emissive coefcient

So in plain English, when thethermal imager observes the ther-mal radiation rom real objects,part o what the thermal imagersees is refected rom the suraceo the object, part is emittedby the object, and part may betransmitted through the object. Inour example o a steel part, the

transmission is zero, but to thedegree that the part is refective,it is less emissive and thereorereal objects will usually appearcooler than they actually are.

Except when there is somethinghotter in the vicinity; since withopaque materials, the lower theemissivity, the higher the refec-tivity. The result in this case ismaterials appear to be hotter thanthey actually are! Lets examinesome real objects to illustratethese eects.

Applying emissivity toreal objects

In the gure IR1 example, notonly is the use end cap tempera-

ture actually much hotter than the103.6 F that it appears, the hotspot above it is most assuredlyhotter than the 133.4 F that itappears.

So, how much hotter might itbe? This used power disconnectis electrically energized, so letsconduct a simple experiment witha metal part that is not electri-cally energized. Note: While thisexperiment may not be shock-ing, it can still burn you.

Picture a round stainless steel

block sitting at ambient tempera-ture. Observed with our thermalimager (with emissivity set to1), the metal appears to vary intemperature rom about 74 F to87 F. This seems to make sense,since the block could have pickedup a little heat rom our handsduring handling.

Actually, the metal block isvery uniorm in temperature. Theapparent hot spot is a refec-tion o my ace on the surace o

the metal. Can you see my eyeglasses in the image? (Figure IR2)

Now lets take this block andplace it in a warm oven and bakeit or three hours. We remove theblock rom the oven and inspectit with the thermal imager [seeFigure IR2a]. The block appearsto vary in temperature rom about92 F to 110 Fand you can seethe image o my ace in the warmmetal surace even more clearlythan beore. Using a thermo-couple, we measure the surace

temperature and nd that itsactually 169 F (see Figure 2a).

How can the thermal imagersreadings appear reasonablewhen the metal part is at roomtemperature and be so wrong(still producing a mirror image omy ace on the hot surace) whenthe part is 169 F?

2 Fluke Education Partnership Program Inrared cameras and emissivity

Figure 2: Stainless steel block.

Figure IR2: Thermal image o stainless steel block.

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At room temperature, the blockappears to be room temperature

because the block is primarilyrefecting the thermal radiationrom everything around it. Sincethe ambient temperature in theroom is in the 70s, the refec-tion rom the surace o the blockappears also to be similar. Whenthe same part is heated in theoven, the part becomes muchhotter than the surroundings, sothe thermal imager is able to seean increase in radiant energy,albeit much lower in apparenttemperature because o the low

emissivity value o the surace.Lets modiy our experiment tobetter demonstrate what thethermal imager sees.

We take another stainless steelblock and paint hal o it with a

fat black paint (fat black painthas an emissivity o 1) and bakeit (in a slightly warmer oven)another three hours (Figures 3and IR3). When we remove theblock rom the oven this time, theunpainted side appears to be92 F but the thermal imagernow indicates the painted sideto be 198 F. We can make avery good estimation o theactual emissivity o this mate-rial by observing the unpaintedsurace with our IR camera and

adjusting the emissivity valueon the thermal imager until thereading matches the temperatureobserved on the painted side. Inthis case, the emissivity is oundto be approximately 0.12.


Figure IR2a: Thermal image o steel block ater heating. Figure 2a: DMM with thermocouple, measuring surace temperature othe steel block.

Emissivity is acantankerous variable

As weve seen, emissivity variesby surace condition, but alsoby viewing angle, and even bytemperature and by spectralwavelength. A table o commonemissivity values is published inthe operating manual or yourthermal imager. The table shouldbe considered only a roughguide in estimating an emissivityvalue to use with any particularmaterial. I actual temperaturevalues are required, it is best toperorm experiments as describedhere, to properly characterize theemissivity or the material andits application. The two most

Figure 3: Steel block, let side painted black. Figure IR3: Corresponding thermal image o steel block.

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common techniques or provid-ing a higher emissivity reerencesurace are the application o afat black high emissivity paint tothe surace (as discussed in the

previous section), or applicationo common black electrical tape tothe materials surace. Both blackelectrical tape and fat black tapehave an emissivity o approxi-mately 0.96. Another option isto use an inrared thermometerwith adjustable emissivity, anda contact probe, adjusting theemissivity until the contact probeand inrared temperature displaysequilibrate.

In this experiment we see thatthe dierence between the appar-

ent temperature on the unpaintedside and actual temperature isan error o 106 F. I we wereto conduct a similar experimentwith a high temperature inraredsensor, and examine steel at2000 F, the error between theactual and apparent temperaturescould be more than 400 F. Ocourse, neither black paint or tapecould survive 2000 F.

Its oten useul to use a narrowspectral band similar to thewavelength o the objects radiant

energy. Wiens DisplacementLaw helps us determine the peakwavelength o the objects peakradiant energy or an object at acertain temperature.lmax = b / T where: lmax = peak wavelength

o radiant energyb = 2897 micron / KT = temperature (Kelvin)

When you are working withhigh temperature materials, youcan greatly reduce the errors dueto uncertainty in emissivity byselecting inrared detectors that

operate at narrow wavelengthbands at shorter wavelengths.

The math and physics necessaryto prove this is beyond the scopeo this application note. However,

calculations demonstrate thatby choosing an inrared sensorwith a wavelength band closeto one micron (rather than the8-14 micron spectral band usedby most thermal imagers), themaximum dierence betweenthe 2000 F actual and apparenttemperatures would be closerto 50 F, (without knowing theprecise emissivity o the materialwith better certainty).

To summarize: Temperaturemeasurement without knowl-

edge in this case would resultin an error o more than 400 F.Making the same measurementwith knowledge would reducethe error to 50 F, with no betterdetermination o the materialsemissivity.

Emissivity, the variablevariable!

Back to our steel block example,lets discuss another very signi-cant phenomena. We will takeour unpainted metal block and

drill three holes part way into thebody. All three holes are 1/8 inchdiameter. The rst is 1/8 inchdeep, the second is 1/4 inch deep,and the third is 3/8 inch deep.Bake the block or another threehours, then remove the block andobserve it again with the camera.[See Figure IR4.]

Interestingly, the hot blocksurace appears to be about 84 F,and now appears to have three

hot spots. The 1/8 inch deep holeappears to be 106 F. The1/4 inch deep hole appears tobe 112 F; and the 3/8 inch deephole appears to be 125 F.We know that the metal block is

actually about 175 F (measuredby a thermocouple) and thesurace nish is uniorm and hasan emissivity o approximately0.12. The reason the temperatureappears to be higher in the holesis that a hole in a body enhancesthe emissivity. The greater the

depth/diameter ratio o thehole, the greater the emissivityenhancement. By adjusting theemissivity on the thermal imagerto match the actual temperatureat each hole, we nd that theemissivity appears to be 0.25or the 1/8 inch deep hole. Theemissivity o the 1/4 inch deephole appears to be 0.35 and the3/8 inch deep hole appears tohave an emissivity o 0.45.

This is an extremely importanteect. Lets look at another piece

o electrical equipment to see why.


Figure IR4: Thermal image o steel block with three holes.

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Emissivity and electricalequipment

In Figures 5 and IR5, you seeanother power disconnect withthe conductors bolted in placeusing Allen head bolts. The corre-sponding inrared image showsa hot connection on the middlephase. Notice the apparent hotspot in the hot Allen socket head.The well o the bolt head appearshotter primarily because the wellillustrates the blackbody eect oa hole.

In manuacturing processes,steel or aluminum rolls are oten

used to heat or cool a materialsuch as in paper or plastic lmprocessing. These rolls are usuallypolished metal suraces, and itsimportant to understand the ther-mal prole since the manuactur-ing process depends on thermaluniormity across the rolls. Thetemperature o these rolls can bedicult to measure with a ther-mal imager because they have

very low emissivities. However,there are oten points where thematerial passes between two

rolls. The tangent point betweentwo rolls also tends to simulatethe blackbody eect, allowing oreective temperature measure-ment in an otherwise dicultsituation.

This eect is illustrated incommon electrical equipmentas well. Look at Figure 6. In thiscase, we have another powerdisconnect with knie bladeswitches. This type o switchutilizes shiny metal blades, andthe proximity o the blades with

narrow gaps simulates the black-body eect or greatly improvedeective emissivity.

The important message here isto develop your understanding oapparent and actual temperaturemeasurement. Actual temperaturemeasurement requires an intimateunderstanding o physics, heattranser and characteristics omaterials.

Qualitative vs. quantitativeinrared thermography

Emissivity diculties are nota barrier to eectively usinginrared thermography orpredictive maintenance (PdM).ASTM standards exist to guidethermographic PdM inspections.These standards describe the useo thermal imagers or qualitativeand quantitative inrared inspec-tions.

Quantitative inrared inspectionsrequire determining the emissiv-ity o each component, to makeaccurate temperature measure-

ments possible. This practicemay not always be necessary orroutine inspections, unless theexact temperature value is neededor long term tracing. Qualitativemethods, in contrast, allow youto leave the emissivity at 1.0 andevaluate the equipment on a rela-tive basis: Has it changed, or is itdierent? The basis or qualitativeevaluation is comparing similarequipment under similar loads.


Figure 5: Three-phase power disconnect. Figure IR5: Corresponding thermal image.

Figure 6: Power disconnect with knie blade connectors. Figure IR6: Corresponding thermal image.

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Looking back at Figures 1 andIR1, you can see that there is littlevalue to be gained in spendingtime estimating or debating theemissivity o the various parts in

the power disconnect. The valueis in understanding that phase Ais hotter than phases B and C. Inaddition to realizing that a phaseis hotter, it is essential to measurethe load o the three phases.Greater electrical load inherentlymeans more heat is present:

P = I2

R where:P = power in watts (heat)I = current in ampsR = resistance in ohms

First rule o inrared

thermography:Comparable equipmentunder comparable loads

The rst rule o thermographyin predictive maintenance isto compare comparable equip-ment under comparable loads.In electrical power distribution,comparable equipment is usuallythe easy part since each electricalphase is usually similar in materi-als to the phase next to it. Load isa very dierent matter. Figure 7illustrates an electrician measur-

ing the electrical load.So, just observing that there

is a hot spot does not indicate aproblem. Electrical componentscan be appropriately hot or theelectrical load and conditions.I you measure the loads, youcan determine i the presenceo a thermal anomaly indicatesa problem. Thermal imagers do


not identiy thermal problemstrained, knowledgeable, qualiedpeople make educated assess-ments o equipment. This leads toreal value in preventive mainte-

nance and reduced requency oequipment breakdowns.

Summary

Predictive maintenance with athermal imager can be eectivelyperormed by utilizing qualitativeanalysis o equipment. Qualitativetechniques allow the emissivitysetting on the thermal imagerto be kept at 1.0 and apparenttemperatures used or compari-sons between similar equip-ment under similar load. With

basic training, most technicianscan reliably perorm qualitativeanalysis.

Quantitative inrared analysisrequires a deeper understandingo thermal theory and applica-tion to be truly eective. It reersto the attempt to measure actualtemperatures o materials usinginrared thermography. Actualtemperature measurementinvolves more than simply adjust-ing or emissivity. Total incidentradiance requires dealing with

the eect o refection and trans-mission in addition to emissivity.

Todays thermal imagers arebecoming increasingly aord-able and easy to use. But whatdoes easy mean? The practiceo inrared thermography looksstraight orward and simple; but ithas its tricks. It is much like mostendeavors in lie: The more youlearn, the more you discover thatthere is more to learn.

Fluke CorporationPO Box 9090, Everett, WA USA 98206

2005, 2007 Fluke Corporation. All rights reserved.Printed in U.S.A. 7/2007 2563251 A-EN-N Rev B

Web access: http://www.fuke.com

Figure 7: Measuring the loads on a power disconnect.

Reerences

The American Society or Nondestruc-tive Testing publishes the NondestructiveTesting Handbook, 3rd Edition, Volume 3,Inrared and Thermal Testing. This workis reerenced as the general source or theequations and technical data or the contento this paper.

Qualitative and quantitative inrared ther-

mography are reerenced to ASTM E1934Standard Guide to Inspecting Electricaland Mechanical Equipment Using InraredThermography. This industry consensusdocument describes the recommendedprocedures or conducting inrared inspec-tions or predictive maintenance.

Saety note: Inrared thermographyis oten used to inspect electrical powerdistribution equipment. This paper discussesthe technical aspects o perorming inraredanalysis, especially as it relates to electri-cal equipment predictive maintenance. Allpersons working on or around energized

electrical equipment should consult NFPA70E or OSHA saety requirements.

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