optical flat, parallel optical flat, plane-parallel plate, optical flat with two parallel faces, plate glass, optical flat gauge, plain glass block, plane-parallel
klaasist plaat mõõtevahendite tööpindade tasapinnalisuse mõõtmiseks, optical-grade piece of glass lapped and polished to be extremely flat on one or both sides, usually within a few millionths of an inch (about 25 nanometres)