The optical transmittance properties, under diffuse light, of semitransparent materials used to fabricate photovoltaic devices have been investigated by using an apparatus provided with two "coupled" integrating spheres, one for producing the incident diffuse light and the other for collecting the transmitted light. A detailed analysis of the measurement conditions has been performed taking into account the perturbing effect each sphere produces on the other, and a final expression for the correct transmittance of the sample is derived. (C) 2002 Published by Elsevier Science B.V.