A flat glass, also known as plan test glass or plane-parallel test glass, is used for testing small flatness deviations.
It is a manual test device with lapped or polished surfaces and a test method based on the interference of light.
A flat glass is a round, high-precision round glass which is fitted with a frame made of plastic or metal.
This is done to protect the glass from fingerprints.
During the test, the glass is slightly pressed onto the test specimen on one side,
whereby a wedge-shaped slit is formed in which the light forms interferences through reflections.
Visible straight stripes appear on flat surfaces, while concave and convex deviations form ring-shaped stripes,
which, however, stand in opposition to each other.
Between the interference fringes, the measuring surface to the glass pane is about 0.27 µm away in white light.
It is suggested to use monochromatic light for better results.
The prerequisite for the use of flat glass is that the surface to be measured reflects sufficient light.
There are flat and spherically curved test tubes which are specified in DIN 58161 Part 1 and Part 2 respectively.
For example, the measuring surfaces of micrometers can be checked for flatness and parallelism with plane glasses.