An optical accessory for absolute reflection and transmission measurements in the wavelength region from 0.24μm to 25μm

Abstract

Five methods for the measurement of absolute reflectance are described and compared. Four of the methods, the V-W, V-N, integrating sphere and goniometer methods, have been in use for a number of years. The fifth is a new STAR GEM method. The acronym of STAR GEM is from the capital letters of Scatter, Transmission, and Absolute Reflection measurements using a Geminated Ellipsoid Mirror. Only the goniometer and STAR GEM methods can be used to measure reflectance and transmittance at almost any angle of incidence. The STAR GEM is used in conjunction with an FTIR (Fourier-Transform Infrared) spectrophotometer and also with a grating spectrophotometer to make reflectance and transmittance measurements in the wavelength region from 0.24mm to 25mm. Ordinate errors of the FTIR spectrophotometer are estimated from measurements in the overlapping wavelength region made by both spectrophotometers. A reflectance measurement of a non-plane surface, such as a surface of micro-ball lenses, can also be made using the STAR GEM.