On Absorbance Measurements in Spatially Inhomogeneous Fields

Abstract

The measured absorbance of a spatially inhomogeneous sample may be strongly affected by the relative spatial distribution of the absorbing medium and the optical beam. The measured absorbance is, in general, not the average of local absorbances across the beam, except when the maximum range of local absorbance values across the entire field is small (<0.1 AU). The measured transmittance of an optically inhomogeneous field is equal to the average transmittance only if incident intensity is everywhere uniform. The general problem is outlined and is illustrated by examples of transient spectroelectrochemical experiments monitoring the time course of reactant consumption or product generation as they traverse a thin optical beam parallel to the electrode.