HELIUM LIGHT BAND H014
A method of measuring seal face flatness.
One helium light band equals 0.0000116 inches or 0,3 microns.
Seal faces are lapped to less than one helium light band by most seal manufacturers, but after lapping the part relaxes and can go out of flat as much as three helium light bands.
At about five or six light bands, lapped seal faces will begin to show some leakage.