Optical Parallels Series 157 FEATURES Designed to inspect parallelism and flatness of measuring faces of micromters. Each set consists of 4 thicknesses. Supplied in fitted wooden case. Technical Data Flatness: 0.1µm/.000004”
Parallelism: 0.2µm/.000008”
Diameter: 30mm/1.18”
Parallelism Check between Measuring Faces by Means of Interference Fringe Produced by an Optical Parallel
The parallelism between the measuring faces can be determined as follows; place the optical parallel to the anvil and observe the number of interference fringes produced on the spindle side under the measuring force of the micrometers.
The parallelism is about 1ìm (0.32ìm x 3 = 0.96ìm).
Fringe on the anvil side must not be more than one.

Mitutoyo 157-903 Optical Parallel, 0-25mm Range, 0.1micrometer Flatness, 0.2micrometer Parallelism, 30mm Diameter