There is a synergy among design, production, and measurement. You cannot produce optics any better than you can test it. The test methods in the workshop environment have to be rugged and should meet the tolerances specified in the design. This chapter introduces the basic principles of optical testing of surfaces.
Many optical components have at least one flat surface; several of them have many. These components are part of an optical system or an instrument and thus directly influence its performance. It is therefore necessary to measure them; that is, how flat are they? For this purpose, they are to be compared with a reference surface, whose surface flatness is known.