The purpose of this article is to study the accuracy of a number of simple approximate equations for the spherical albedo r (or the hemispherical reflectance under diffuse illumination conditions) of semi-infinite light scattering absorbing media, using numerical solutions of the nonlinear integral equation for the reflection function of a semi-infinite turbid medium, as formulated by V. A. Ambartsumian. We find that the van de Hulst approximation provides the most accurate approximation for the diffuse reflectance r under diffuse illumination conditions. The value of r depends almost exclusively on the value of the similarity Parameter s = root/(1 - omega(o)) (1 - g omega(o))(-1), where wo is the single scattering albedo and g is the asymmetry parameter. A simple approach to derive the normalized absorption spectra of particulate matter from reflectance measurements is proposed. (C) 2006 Wiley Periodicals, Inc.