Theory and measurement of bidirectional reflectance for signature analysis

This paper presents a description of a fully automated, computer-controlled hemispherical directional reflectometer (HDR). It fills the need in many fields of research and development for a device with HDR measurement capabilities which is state-of-the-art in wavelength coverage to 25.0 micrometers and higher, angular polarization resolved coverage 20 to 80 degree(s), partition of reflected radiation into specular and scattered components, and scattered transmittance. This performance is made possible using an 18′ major axis electroformed gold-plated specular hemiellipsoid with a 1.8′ foci separation. The radiance throughput to the FTIR of this design exceeds by a factor of more than 200 that of the usual diffuse gold integrating spheres. Derived data, based on reflectance and using provided software, includes the IR component of solar absorptance, the index of refraction n and k for dielectrics and conductors for Fresnel materials, and both directional and hemispherical emittance.