An optical flat is a flat surface which is precisely polished.It can be used as a reference,against which the flatness of unknown surface may be compared.
Measurement of the surface flatness of polished surfaces can be determined visually by comparing the variations between a work surface and the surface of an optical flat.
These optical flats can be used in many applications,such as,inspecting of gauge blocks for wear and accuracy,as well as testing various components,including windows,prisms,filters,mirrors,etc.
They can also be used as an extremely standard flat in interferometers.