8 June 2011 Changes in apparent emissivity as a function of viewing geometry
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We reiterate the fact that the apparent emissivity of a solid surface can depend on not only the composition and particle size distribution comprising the surface but also on view-angle relative to the bulk surface geometry. We report on experiments measuring the longwave infrared emissivity of quartz sand samples sorted by particle size and observed at a series of view angles up to 60° away from the normal to the bulk sample surface. We show that particle-size and view-angle effects on the apparent emissivity of our quartz samples can mimic each other. In circumstances where significantly off-normal view-angles are unavoidable and characterizing surface qualities are desired, these two effects could be confused. We discuss the existing explanations for these effects. We argue that the view-angle emissivity dependence is intrinsically a bulk geometric effect, not due to a change in apparent particle-size (via coarse grains obscuring finer ones at larger view-angles). We summarize the predominant qualitative explanations of the particle-size effects and review the literature comparing quantitative models and observations. We argue that the existing quantitative models are inadequate for explaining the observations, pointing to a need for further work in this area.
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Christopher J. Deloye, Christopher J. Deloye, Michael S. West, Michael S. West, John M. Grossmann, John M. Grossmann, } "Changes in apparent emissivity as a function of viewing geometry", Proc. SPIE 8040, Active and Passive Signatures II, 80400J (8 June 2011); doi: 10.1117/12.887737; https://doi.org/10.1117/12.887737

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