TECHSPEC® Single Surface 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. TECHSPEC Single Surface Optical Flats ship in a durable storage case for permanent protection. λ/20 flats that are 76.2mm or larger also include a certificate of calibration.