Optical Flats are used as a reference (test plate) against which the flatness of an unknown surface may be compared. Each piece is precision polished by our master opticians and tested on a Zygo interferometer to insure the surface flatness. The secondary surfaces of our single surface flats are pitch polished to window quality for viewing the interference pattern. Each flat ships in a durable storage case for permanent protection. λ/20 flats that are 75mm or larger also include a certificate of calibration.