Optical loss mechanisms of solar reflectors directly impact the performance of concentrating solar collectors for solar thermal power plants and process heat applications. This thesis presents measurement methods, experimental results and advanced physical models. Results are shown for glass-, polymer- and aluminum-based reflector materials and soiling. In order to characterize the optical losses due to scattering, a camera based instrument and evaluation method is presented. A developed model allows for the prediction of the reflectance for different levels of soiling. Shape and slope of reflectors for parabolic trough and linear Fresnel collectors are assessed. In order to investigate optical losses due to shape deformations, the static fringe reflection technique was developed. It applies deflectometry to characterize local surface slope deviations of concentrator facets. Overall, the findings of this study enhance the understanding of optical loss mechanism of solar reflectors. The presented methods and models contribute to an improved development of concentrating collectors and their components.