Abstract

In order to predict the performance of electro-optical devices in a winter environment, it will be necessary to understand the effects of falling snow on the propagation of visible and infrared radiation. Several groups of investigators¹,² have measured snow transmittance at visible and infrared wavelengths over the past few years, and have attempted to relate the infrared extinction coefficient β_ir to the visible extinction coefficient β_VIS The usual procedure has been to calculate β from the measured transmittance T by using where R is the optical path length. The result of several such investigations ¹,² has been that β_ir/β_vis ≌ 1.4. This result is somewhat surprising, because most snow crystals are quite large compared to either visible or infrared wavelengths, and hence might be expected to cause approximately the same extinction at either wavelength. However, Eq. (1) is not valid if scattered radiation constitutes an appreciable part of the measured transmitted power. Because all optical instruments have a non-zero field of view, scattered radiation can contribute to received power, causing values of β calculated as above to be systematically low. The error will be wavelength dependent, because the angular pattern of scattered radiation is wavelength dependent.

© 1983 Optical Society of America