11 - Reflectance spectroscopy

Summary

Introduction

One of the objectives of studying a planet by reflectance is to infer certain properties of the surface by inverting the remote measurement. In the laboratory, the objective of a reflectance measurement is usually to determine the spectral absorption coefficient of the material or, at least, some quantity proportional to it, by inversion of the reflectance.

There are at least three reasons why reflectance spectroscopy is a powerful technique for measuring the characteristic absorption spectrum of a particulate material. First, the dynamic range of the measurement is extremely large. Multiple scattering amplifies the contrast within very weak absorption bands in the light transmitted through the particles, while very strong bands can be detected by anomalous dispersion in radiation reflected from the particle surfaces. Hence, the measurement of a single spectrum can give information on the spectral absorption coefficient over a range of several orders of magnitude in α. Second, sample preparation is convenient and simply requires grinding the material to the desired degree of fineness and sieving it to constrain the particle size. Third, reflectance techniques are effective in the range k ∼10⁻³–10⁻¹, where both transmission- and specular-reflection techniques are very difficult. By contrast, if α(λ) is measured by transmission, the sample must be sliced into a thin section that must then be polished on both sides; also, the range by which α(λ) can vary is limited to about one order of magnitude.