8 November 1996 Temperature dependence of the emission factors for beryllium oxide in the upper and lower atmospheric windows
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The standard equation for the radiometric emission factor as monitored by an infrared radiometer includes radiation emitted and reflected by the sample. The derivation of this equation assumes that the temperature variation of the band emittance is neglible which is only valid if the sample is gray within the band of observation. The radiometric emission factor obtained from the standard equation will therefore not agree with the true average emittance if the spectral emittance exhibits spectral structure. Ceramic beryllium oxide has one emittance edge rising with wavelength in the 3 - 5 micrometers atmospheric window and another edge falling with wavelength in the 8 - 13 micrometers window. Broadband radiometer measurements and model calculations are used to compare the temperature variation of the radiometric emission factors with that of the correct band emittance. Comparing the model calculations with experimental heat- camera data confirm the prediction that the band emittance and the radiometric emission factor decrease with temperature in the lower atmospheric window and increase in the upper atmospheric window. An approximation based on linearizing the temperature dependence of the radiometric data to obtain the correct band emittance is applied to these two cases. The agreement with the integrated results of spectral measurements is very satisfactory for the data from the 8 - 13 micrometers window, but less so for the 3 - 5 micrometers range. In the lower atmospheric window the material is partly transmitting and the atmosphere partly absorbing.
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Oerjan Staaf, Oerjan Staaf, Carl-Gustaf Ribbing, Carl-Gustaf Ribbing, Stefan K. Andersson, Stefan K. Andersson, "Temperature dependence of the emission factors for beryllium oxide in the upper and lower atmospheric windows", Proc. SPIE 2815, Optical Radiation Measurements III, (8 November 1996); doi: 10.1117/12.257146; https://doi.org/10.1117/12.257146

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