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This is a common way of interpreting the results of an interferometer as described the the "fringes" definition above. Counting the fringes from left to right, then top to bottom, the larger number is the "power". Subtract the smaller number from the larger number to get the irregularity. The final number being the size of the aperture, the spec then reads P/I/A. For the example below, the flatness is 8/5/4 (8 fringes power, 5 fringes. As expressed over a defined aperture (ex. 12/12/4 = 12 fringes of power and 12 fringes of irregularity over a four-inch aperture) is measured in newton rings on an interferometer.
Measured by interferometer, a fringe is a distortion in the surface relative to an optical plane (1 fringe = ½ wave)Using an interferometer with a monochromatic light source, the object part is measured against an optical flat. As light reflects in the gap between the object and the optical flat, the light will interfere with itself creating light and dark fringes or bands. As commonly measured on a helium neon laser at a wavelength of 632.8nm, each fringe is equal to .316 microns. These fringes can be counted to express flatness over a given area or evaluated as a contour map and interpreted for shape and flatness