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.