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Introduction
The easiest and best way to test the flatness of a flat lapped or polished surface
is with an optical flat. Such surfaces are found on micrometers, measuring machines,
gage blocks, snap gages, ring seals, valve seats and precision flat lapped parts.
The surface must be sufficiently finished to reflect light; ordinary ground surfaces
are too irregular to show light wave interference bands. However, unless the material is extremely soft or porous, a few rubs on a flat lap will smooth off the top of
the grinding ridges and enable bands to be seen in monochromatic light.
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