Flat glass
The plane glass (or plan sample glass or plane-parallel test tube ) is a manual test equipment for testing low flatness tolerances lapped or polished surfaces, which is based on the interference of light.
The plane glass essentially consists of a round, highly precisely cut glass, which in some versions includes a frame made of plastic or metal to protect it from fingerprints.
The test method is based on the superposition ( interference ) of light waves . One side of the glass is pressed lightly onto the test specimen, creating a wedge-shaped gap in which the light forms interferences due to its reflection. Flat surfaces result in visible straight stripes, while concave and convex deviations form annular stripes, which are, however, opposite to one another. From one interference fringe to the next, the measuring surface moves away from the glass pane by about 0.27 µm (with white light). Monochromatic light works best . The prerequisite for the use of a flat glass is that the surface to be measured reflects sufficient light.
Flat and spherically curved test glasses are specified in DIN 58161 Part 1 and Part 2.
For example, the measuring surfaces of micrometers can be checked for flatness and parallelism with plane-parallel test glasses .
Individual evidence
- ↑ DIN 863-1: 1999-04 .