How Infrared Detector ResponseCan Wreak Havoc onEmissivity/Reflectivity/Transmissivity
By: Joe DeMonte ASNT/PdM TIR LevelIII
AsThermographers,wehavealwaysstrivedtoproducethemostaccurateresultsusingourthermalimagers and our spot radiometers. Those of us that have had formal training consider manydifferentissues when performing qualitative (temperature measurement) or quantitative (thermalpatterns)inspections:
1.Emittance of the surface
2.Reflection source and intensity
3.Focus of the imager
4.Minimum focus distance
5.Total field ofview
6.Spot size ratio and the distance to the target, considering the targetsize
7.Transmissivity of a protective lens or infraredwindow
8.Load on the equipment
9.Atmospheric makeup
10.Solar loading
11.Forced convective cooling/heating
12.Etc…
18-1031-0028 Rev.A
Peoplehavetriedtocreatecomputerprogramstohelpuswithsomeoftheabovechaos,buttherehavebeen difficulties that kept the results from being “accurate” due to some of the effects listed above.Forinstance,itiswidelyacceptedthatattemptingaccuratetemperaturemeasurementsonobjectswithatotal emittance less than 50% is not possible in most cases. Correcting for the wind is nearimpossible.There are rules of thumb for load correction and some great software programs, but it isn’t anabsolute.Recently,IfoundaprogramthatcouldhelpLWIR(LongWaveIR)ThermographerscorrectforthetransmissioninstabilitythatCaF2(CalciumFluoride)materialscause,duetothatmaterialsspectraltransmission curves and the reality of surface temperature versus wavelength (as simply put byPlanck’sBlackbodyCurves).Therearemanygreatarticleswrittenonthissubject,sowewon’trecreatethathere.I,however,wouldliketoexpoundupononefactthatwillmakeaprogramlikethisanother“approximation” that should be used with greatCAUTION!
NOTallLWIRdetectorsareequal:theycanhavedifferentspectral responsecurves.
1.Actual Response Curve from aManufacturer
2.Several Generic ResponseCurves
3.Typical Transmission Curve for 2mm ThickCaF2
If one LWIR camera responds best closer to 9 microns and that camera is used to measurethetransmissivity of a CaF2 window, that result CANNOT be used in a LWIR camera that responds best at10microns. It is more complex than it seems at firstglance.
Thesimpleansweris…allinfraredwindoworopticalmaterialsneedtobetestedusingthesensorsoropticalimagerinquestion.Sensorresponsecurvesdifferfromonetype/manufacturerdetectortoanother.Allmaterialsexhibitsomesortof“selectiveradiance”,meaningthatthetransmittancefromonewavelengthtothenextcananddoeschange.WhencomparingtwodevicesthatarebothLWIR(8-12um) infrared detectors that have differing response curves, we can almost guarantee that theywillhave two results using any infrared window. This is why reputable infrared window manufacturerswillnot give out an “exact” transmission number, and a simple computer program may not be the answer.
Inourownexperiences,wehaveseenashighas12%variationsintransmissivityusingtwoLWIRcamerasfrom the same manufacturer. Why? From one year to the next, this particular manufacturerchangeddetectors.
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