Difference deflectometry with coupling mirrors for flatness measurements of rough or scattering surfaces

For flatness measurements of specular optical surfaces, the Extended Shear Angle Difference technique (ESAD) has been established [1], which measures the difference of surface slopes at two laterally separated positions. The surfaces slopes are determined by an autocollimator and a pentaprism. From the slope differences measured along a scanning direction, the surface topography is reconstructed. Since ESAD uses autocollimators and thus requires specular surfaces, this technique cannot be applied directly to scattering (technical) surfaces such as granite. This can be overcome by using mirrors coupled mechanically to the surface under test. We have set up a demonstrator using two mirrors with a fixed distance to realize the shear, and a common autocollimator measures the slope difference. We expect to achieve uncertainties lower than those attained by commercial inclination sensors which reach sensitivities down to 1 µm/m. The principle and setup of the shearing unit will be described and exemplary measurements will be presented. [1] I. Weingärtner, M. Schulz, C. Elster, ”Novel scanning technique for ultra-precise measurement of topography,” Proc. SPIE 3782 (1999), 306–317