RAYLEIGH-SCATTERING LIMITS FOR LOW-LEVEL BIDIRECTIONAL REFLECTANCE DISTRIBUTION FUNCTION MEASUREMENTS

The objective is to estimate the Rayleigh limit in bidirectional refle ctance distribution function (BRDF) measurements caused by air in the laboratory, the wavelength, and the path length of light in the receiv er field of view. Moreover, we intend to show the trend for the reduct ion of this limit by introducing a medium with small refractive index and by using a longer wavelength. Although the BRDF typically describe s the angular distribution of scattered light from surfaces, the expre ssion describing the equivalent BRDF caused by the optical scattering from gas molecules in the optical path is derived through the use of t he Rayleigh scattering theory. The instrumentation is described, and t he experimental results of the equivalent BRDF caused by gas scatterin g from molecules in clear air, nitrogen, and helium gases are reported . These results confirm the trends of the prediction.