Can deflectometry work in presence of parasitic reflections?

Phase Measuring Deflectometry (PMD) is a meanwhile established method to facilitate the measurement of specular freeform surfaces with high dynamical range. However, for transparent objects like progressive eyeglass lenses, this metrology suffers from one severe limitation: the parasitic reflection at the rear side of the specimen renders the conventional phase evaluation impossible. Therefore, such objects currently have to be specifically prepared for deflectometric measurements by roughening or blackening the rear side. In this talk, different approaches to overcome this major drawback are introduced and compared. The performance of these methods is analyzed with respect to the attainable accuracy and measuring speed, underlining aspects of information efficiency. We present first complete measurements of the refractive power of surfaces for progressive lenses whose rear sides have not been blackened or treated in any other special way.