File:Optical flats in use.jpg

English: Two photos of the use of optical flats to measure the thickness of metal parts within millionths of an inch using interferometry, from a 1921 magazine article. In each photo two optical flats are used, with the top flat resting on a precision gage block on the left and the object to be measured on the right. In the left hand photo a ball bearing is being measured, in the right a cylindrical steel plug gage. If the object to be measured is the same height as the gage block, the upper optical flat will rest flat against the surface of the block and no interference fringes will be seen. However, if the object is a different height, the top flat will rest at a slight angle, and there will be a tiny wedge-shaped gap between the upper flat and the gage block surface. The light waves reflecting from the two surfaces interfere, creating a pattern of bright and dark "interference fringes" (also called Newton's rings). The surface is illuminated by a monochromatic light source with a precisely known wavelength. Each interference fringe represents a difference in height from the previous fringe of one-half the wavelength, so the difference in height can be measured by counting the interference fringes.
In this example, red selenium light with a wavelength of 25 microinches (μin) is used, so each band represents a difference in height of 12.5 μin. The ball bearing in the left photo is in error by 140 μin, while the plug gage is in error by 60 μin. The optical flat itself is flat within 2.5 μin.