The Dark areas are as result of normal use. Note the wear areas are on both sides of the plate. This is good and indicates that the Inspector is using different areas to prevent major wear in one area. These wear areas are only a few tenth's, ( .0001's ), however if all of this wear was to occur in one place, it would be thousandths, ( .001's) of an inch.
After lapping, all of the plates are inspected using a repeat-o-meter. This simulates an indicator on a height stand but uses a .000010 resolution indicator to determine the ability to duplicate a reading anywhere on this surface. This is only one half of the calibration procedure due to the fact that a repeat-o-meter can not determine flatness, only repeatability.
An Autocollimator is used to measure the flatness of the surface plate. This instrument uses collimated light that is bounced off of a mirror mounted on a sled selected for its pad spacing. Sleds work like sine bars and convert
arc. seconds into millionths of an inch, rise or fall when moved in increments equal to the pad spacing. This data is assembled into an actual contour plot of the surface plate and calculates the overall flatness.