  Concentrated solar power is a promising technique enabling renewable energy
production with large scale solar power plants in the near future. Estimating
quantitatively the reflectivity of a solar concentrator is a major issue, since
it has a significant impact on the flux distribution formed on the solar
receiver. Moreover, it is desirable that the mirrors can be measured during
operation in order to evaluate environmental factors such as day night thermal
cycles or soiling and ageing effects at the reflective surfaces. For that
purpose, we used a backward gazing method that was originally developed to
measure mirror shape and misalignment errors. The method operates in quasi
real-time without disturbing the heat production process. It was successfully
tested at a solar tower power plant in France. Its basic principle consists in
acquiring four simultaneous images of a Sun-tracking heliostat, captured from
different observation points located near the thermal receiver. The images are
then processed with a minimization algorithm allowing the determination of
mirror slopes errors. In this communication, it is shown that the algorithm
also allows one to get quantitative reflectivity maps at the surface of the
heliostat. The measurement is fully remote and is used to evaluate surface
reflectivity that depends on optical coatings quality and soiling. Preliminary
results obtained with a Themis heliostat are presented. They show that
reflectivity measurements can be carried out within repeatability about 10
percent Peak-to-Valley (PTV) and 1 percent RMS. Ways to improving these numbers
are discussed in the paper
