If you work in the sealing industry, then you're probably accustomed to using optical flats to measure seal face flatness, as that is the only accurate way of going about it. Unfortunately, optical flats are restricted to measurements based on monochromatic light. More specifically, an optical flat allows you to determine the number of helium light bands that encompass a seal face's flatness. The problem which this is that helium light bands themselves are useless measurements -- or at least they're useless until you convert them to inches or microinches.
Set up an equation relating helium light bands (HLB) to inches (IN). One helium light band equals 0.0000116 inches, or HLB * .0000116 = IN.
Plug the number of helium light bands into the equation from step 1 to solve for inches. Given 3 helium light bands, for instance, the equation would read 3 * .0000116 = IN, or .0000348.
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Convert the value obtained in step 2 to inches by multiplying it by a million, because one microinch is merely one millionth of an inch. Given .0000348 inches, for instance, the final value in microinches would be 34.8.