Fringe reflection technique for heliostats: A comparison between two phase retrieval methods

In order to characterize the optical quality of the heliostat facets, a fringe reflection technique is proposed.
The fringe reflection technique consist in patterns projected at night on a Lambertian target. The image of the
pattern reflected by the heliostat is recorded with a camera. Distortions in the fringes, due to mirror stress and
canting, allows the characterization of the surface error of the facets.
In order to obtain the surface slope deviation from the facets, two phase retrieval methods have been
proposed in this article. A phase shifting technique widely used in 3D surface reconstruction and a novel
Direct Phase Detection technique. In addition, two phase unwrapping techniques have been tested.
Our aim in this research is to highlight some of the key factors involved in the phase retrieval process and
identify vulnerabilities present in each method.