Inverse illumination method for characterization of CPC concentrators

The optical characterization of a CPC concentrator is typically performed by using a solar simulator producing a collimated light beam impinging on the input aperture and characterized by a solar divergence (± 0.27°). The optical efficiency is evaluated by measuring the flux collected at the exit aperture of the concentrator, as function of incidence angle of the beam with respect to the optical axis, from which the acceptance angle can be derived. In this paper we present an alternative approach, based on the inverse illumination of the concentrator. In accordance with this method, a Lambertian light source replaces the receiver at the exit aperture, and the light emerging backwards at the input aperture is analyzed in terms of radiant intensity as function of the angular orientation. The method has been applied by using a laser to illuminate a Lambertian diffuser and a CCD to record the irradiance map produced on a screen moved in front of the CPC. Optical simulations show that, when the entire surface of the diffuser is illuminated, the "inverse" method allows to derive, from a single irradiance map, the angle resolved efficiency curve, and the corresponding acceptance angle, at any azimuthal angle. Experimental characterizations performed on CPC-like concentrators confirm these results. It is also shown how the "inverse" method becomes a powerful tool of investigation of the optical properties of the concentrator, when the Lambertian source is spatially modulated inside the exit aperture area.