Introduction

Abstract

The term absorption spectroscopy is used to denote the qualitative or quantitative measurement of the absorbance of a material as a function of the wavelenght or wavenumber. With quantitative measurements using a parallel beam of light the so-called ``transmittance'' of a plane-parallel layer, $$ T\left( \lambda \right) \equiv \frac{I} {{{I_0}}} $$is measured, where I and I0 denote the radiation flux after and before the transmission of the radiation through the absorbing layer1. Even a measurement of this order of simplicity, however, involves certain complications. If a continuous beam of light enters into a homogeneous medium bounded by plane-parallel windows, it is partially reflected at each phase boundary, while within the medium it is partially absorbed and partially scattered.