A procedure based on diffuse reflectance spectroscopy was developed to determine the ultraviolet screening effect of a benzotriazole light stabilizer in wool fabric. Expressions that enable the protective effect of the stabilizer to be calculated directly from reflectance data were derived. Two reflectometers with quite different optical geometries were used to measure the reflectance spectra, and the merits of the two systems are discussed. A theoretical expression for the optical screening phenomenon in highly scattering materials was evaluated for substrates of different reflectivity and the general implications of this relationship are considered. -- AATA