An experimental procedure based on the use of a Lambertian diffuser is described, which allows the characterization of Fresnel lenses in the field or the laboratory, whatever their characteristics. Lambertian reflectors have already been successfully used in combination with CCD cameras for the optical characterization of solar concentrators (mainly reflective mirrors). The procedure used in this work for characterizing Fresnel lenses is based on the use of a CCD camera taking pictures of the light intensity profile through an opal diffuser which operates like a Lambertian source placed at the focus of the optical concentrator; it also requires power and light spectrum measurements. Key optical characteristics such as optical efficiency, lens transmittance spectrum and concentration ratio profile in the focal area have been obtained for a linear and a point-focus Fresnel lens. (C) 2010 Elsevier Ltd. All rights reserved.