filter medium is measured statistically by running a solution with sorted particles and measuring the percentage of pass through to whatever standard is being referenced.
measurements of thickness is done using a Marr indicator that uses the twist of a square shaft about double the thickness of a human hair to amplify the difference in height.
surfaces can be measured quire accurately for straightness using an optical flat, up to 1/10 lambda, which in the middle of the spectrum is less than 1/100 of a micron.
... and there are dozens of other distance measurement techniques. The current quickest way to measure accurately to about 1/5 of a micron cheaply is to use laser inferometry....