A method is proposed allowing the physical interpretation of the sun image generated by reflecting systems, e.g., heliostats, on a target plane. The instantaneous flux distributions reflected from the target are measured by a distant video-type camera system in the visible spectrum and analyzed in terms of their constituents (sunshape, astigmatic aberration, mirror glass waviness, canting, and curvature errors). The single contributions to the total image are parmetrized by appropriate physically-based model functions and composed to a theoretical image to be compared with the measured image. Within a fit procedure, the parameter set is determined for every experiment allowing to check the consistency and uniqueness of the approach. A series of measurements on GAST heliostats performed between 1984 and 1987 was analyzed. They demonstrate the good optical performance of these systems, thus justifying their use in large solar thermal central receiver tower plants.

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