In 1991, J. Labed, et al. offered a formula of passive measurement of directional emissivity of object in laboratory. It is based on two preconditions, blackbody background radiation and the temperature of target (Tt) different to background temperature (Tb) and the bigger the differenceΔT(ΔT=Tt−Tb) is, the higher the accuracy of measurement will be. In fact, only approximate black-body exists, asTt≠Tb, it is more difficult to satisfy the precondition of “blackbody background radiation”. Authors of this paper re-deduce this formula under the condition of non-blackbody background and a new concept of effective emissivity. Our results had been proven by Monte Carlo’s simulation and experiments. We re-explain the experimental results offered by J. Labed et al. and whyɛr, 3.7(θ) reduces more rapidly thanɛr, 10.6(θ) andɛr, 12(θ) as θ increases.
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