15 March 1996 Broadband emission factors: temperature variation for nongray samples
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Abstract
The general problem of extracting the correct emission factor from broadband radiometric measurements on non-gray samples is treated with emphasis on polycrystalline beryllium oxide and BeO with a coating of silicon. These samples exhibit a strong spectral variation in their emittance functions where the Planck function has large weight. Under these circumstances the band-averaged emission factor will be temperature dependent, even if the spectral emittance is temperature independent. The consequences of this for the conventional expression which includes a correction for radiance from the surroundings reflected by the sample are investigated. It is concluded that the observation of a temperature variation in this emission factor not only violates an assumption of the derivation, it is also a criterion indicating that the numerical value is incorrect Two algorithms, based on linearization and iteration of the temperature variation are introduced and applied to an emittance step model and the experimental radiometer values for the reststrahlen band materials. It is found that the emission factors obtained after this correction procedure are in significantly better agreement with values obtained from weighted integration of spectral emittance over the spectral window of the radiometer. The room-temperature value of the upper TIR emission factor is 0.40 and 0.22 for BeO and the Si-BeO double layer respectively. A sand-blasted aluminum sample had almost perfectly gray emittance and the emission factor is 0.39 and temperature independent.
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Oerjan Staaf, Oerjan Staaf, Carl-Gustaf Ribbing, Carl-Gustaf Ribbing, Stefan K. Andersson, Stefan K. Andersson, } "Broadband emission factors: temperature variation for nongray samples", Proc. SPIE 2766, Thermosense XVIII: An International Conference on Thermal Sensing and Imaging Diagnostic Applications, (15 March 1996); doi: 10.1117/12.235389; https://doi.org/10.1117/12.235389
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