We present a novel setup to study the scattering properties of mirror coatings. It is designed to measure angle-resolved scattering from coated substrates, with a special emphasis on the study of point scatterers. Near speckle-free imaging is achieved with a rotating diffuser. This setup presents advantages for the identification and analysis of individual defects, which are critical in sensitive optical applications, especially in gravitational-wave detectors.

The properties of optical coatings play an important role in precision experiments. Gravitational-wave detectors not only require the highest quality coatings but also ones withrelatively large diameters. Coatings made with amorphous material can be scaled up easily but for now exhibit relatively high thermal noise characteristics. Crystalline coatings show great promise in terms of their thermal noise properties but cannot easily scale up to the needed sizes. In this paper we explore the possibility of a piecewise coating that includes both amorphous and crystalline material. Specifically, we estimate the scattering losses of such a piecewise coating as a function of the mismatch in the interface between the two coatings. The calculation should be taken as a lower limit to the total losses, as other surface imperfections will play an important role in the final result, but are not considered here. Finally, we present a measurement of the scattering losses from a piecewise coating, to showcase the challenges of realizing such a design.