The optical behaviour of photovoltaic (PV) modules from different fabrication technologies was investigated under diffuse light by a novel characterization method. The optical apparatus, hemispherical/hemispherical reflectometer, allows both reflectance and transmittance measurements under an incident diffuse light which simulates the distributed outdoor irradiation from the sky or from the albedo. This paper presents only the configuration for reflectance measurements. The apparatus is provided with a single integrating sphere of 40 cm diameter, which acts as a lambertian source of diffuse light and spontaneously collects the diffuse light reflected by the sample in the front hemisphere. The hemispherical/hemispherical reflectance, R-hh, expresses the optical loss of the PV device under diffuse light, and is obtained by measurements of light irradiance inside the sphere in correspondence with the sample and with a selected number of standards of diffuse reflectance. The best optical performances, in terms of optical loss, were achieved by modules realized with blue mono-Si cells and having textured front surfaces in the cover sheets. (C) 2002 Elsevier Science B.V. All rights reserved.