VISproLF: Self-calibrating instrument for measuring 3D shape of linear Fresnel facets

Concentrating solar power plants use mirrors to redirect solar radiation toward receivers. Mirrors (facets) are suitably shaped to optimize the radiation capture ratio. Because deviations from ideal shape/slope degrade optical efficiency, the availability of reliable instruments for 3D shape measurement is essential. This paper deals with an innovative instrument, named VISproLF, specific for the case of facets for the primary reflector of linear Fresnel collectors. The instrument is simply composed of a LCD screen and a digital camera; to avoid artifacts, the camera must be equipped with a high quality lens and its characteristic matrix and distortion coefficients have to be accurately evaluated. The arrangement of the LCD screen and camera around the facet must follow the simple rule to view from the camera the facet surface entirely covered by the image of the screen. Then, the position and attitude of the camera (first) and LCD screen (second) are accurately evaluated by a simple procedure based on the pinhole camera model, which makes the instrument self-calibrating. The shape and slope of the facet surfaces are obtained by processing five binary images by means of a completely digital method of which a detailed description is given. In the experimental arrangement used to prepare the present paper, the slope accuracy is about 0.2 mrad. With respect to the most common instruments based on the Fringe reflection technology, VISproLF offers two great advantages: full-digital image processing and a self-calibration feature.