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Proceedings Paper

Analytical evaluation and correction of port loss error in integrating sphere reflectometers
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Paper Abstract

Incomplete hemispherical irradiation of the sample (or collection of radiation, in the reverse geometry) in an integrating sphere reflectometer is unavoidable due to sample viewing (or illumination) requirements. This means that one measures with nearly hemispherical irradiation and obtains a quantity that is not identical to the hemispherical/directional (or directional/hemispherical) reflectance factor (HDR) of the sample. The assumption that this quantity is identical to the HDR can lead to error in the measurement result, which is not generally corrected for. The error, a.k.a. "port loss error", is minimal for comparative measurements of samples with similar reflective properties, but can easily amount to several percent for a diffuse vs. specular comparison and could be even higher for samples that exhibit strong forward scattering. Few papers in the literature have dealt with this issue, perhaps due to the existence of other larger sources of error. Our approach to this problem has three elements: (1) establish a definition and set of equations that quantify the error due to incomplete irradiation of the sample due to the presence of the viewing port ("port loss" uncertainty); (2) develop algorithms and computer models to predict port loss effects for a specular-diffuse sphere coating and specular-diffuse (or more general bi-directional reflectance distribution function (BRDF)) sample; (3) design a technique and instrumentation to allow routine direct measurements of port loss to correct the error; using reference standards such as an Al mirror and a polytetraflouroethylene (PTFE).

Paper Details

Date Published: 14 November 2003
PDF: 11 pages
Proc. SPIE 5192, Optical Diagnostic Methods for Inorganic Materials III, (14 November 2003); doi: 10.1117/12.508326
Show Author Affiliations
Alexander V. Prokhorov, National Institute of Standards and Technology (United States)
Sergey N. Mekhontsev, National Institute of Standards and Technology (United States)
Leonard M. Hanssen, National Institute of Standards and Technology (United States)

Published in SPIE Proceedings Vol. 5192:
Optical Diagnostic Methods for Inorganic Materials III
Leonard M. Hanssen, Editor(s)

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