In 1963, Melamed proposed a model that expressed reflectance of a powder described as a population of spherical particles of unique diameter as a function of size, shape and optical characteristics of the powder. This paper shows how, assuming particles to be ellipsoids of revolution, one dimension can be added. We show that the mean value of the shape coefficient of the model tends to converge if many particles are accounted for. (c) 2005 Elsevier B.V. All rights reserved.