Coating and method for testing plano and spherical wavefront producing optical surfaces and systems having a broad range of reflectivities

Claim 1

In a test system comprising a Fizeau spherical or plano wavefront interferometer, said interferometer comprising a source of light for providing a wavefront, a plano or spherical test surface, an optical element located in said wavefront and having a reference surface normal to said wavefront, said reference surface comprising a last surface with respect to the direction of said wavefront, said reference surface producing a reflected reference wavefront and a transmitted test wavefront which is directed at a common center of curvature of said reference surface and said plano or spherical test surface, said test wavefront interacting with said test surface and being returned from said test surface by reflection back through said reference surface, and means for interfering said reflected wavefront and reflected test wavefront for producing two beam interference fringes; the improvement comprising a partially reflective, partially absorbtive, partially transmissive beam splitting coating on said reference surface, said coating having a fixed reflectivity and transmittance such that there will be only two beam interference and the contrast of said two beam interference fringes is substantially equalized with respect to said test surface reflectivity.