Abstract

A nonperfect surface is here defined as a surface whose reflectance varies with the angle of incidence of a unidirectional beam of light. This directional reflectance usually differs in magnitude from the diffuse reflectance obtained for perfectly diffuse incident light. The relations between directional and diffuse reflectance are derived. These are then used in the investigation of the case of a specular, nonperfect sample at a port in an integrating sphere. Expressions are derived for L1s/L1st, the measured ratio of the luminous emittances of the sphere wall, with nonperfect sample illuminated L1s, and with perfect standard illuminated L1st. This ratio is used in previously derived expressions for the reflectance of a perfect sample to compute a reflectance ρ for the nonperfect case. By means of examples, the reflectance ρ computed in the comparison method is shown to be the same as the directional reflectance ρD(5) for the light coming in through the entrance port. In the substitution method the difference between ρ and ρD(5) may be significant, depending on the diffuse reflectance and the size of the sample. These conclusions are believed to be valid also for nonperfect samples for which the reflection is predominately diffuse.

© 1965 Optical Society of America

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