In the seal business flatness is measured in helium light bands. To measure these small distances we use a mono-chromatic light and a set of optical flats.
A Helium light band is 0.0000116″ or 0,3 microns. Seal faces should be lapped flat to within three helium light bands (a little less than a micron) At five helium light bands you can detect some leakage at the seal faces.
The smallest object that can be seen with the human eye is 40 microns. A human hair has a cross section of about 100 microns.
See face flatness.
See Flatness chart