Optical Flats and Optical Parallelsr Optical Flat Open or Close Ø 45 mm for single side use only for wringing test performed on gauge blocks KOBA- and DAkkS calibration certificate available in Optical Parallels Open or Close Ø 30 mm single, size 12 mm set with 4 pieces for checking parallelism of measuring faces on micrometer calipers KOBA- and DAkkS calibration certificate available