A high-speed optical chopper based on 102-mm-diameter disks and manufactured by Scitec Instruments Ltd. is available from Boston Electronics Corp. The Model 310CD produces a chopping frequency of 120 kHz for a 445-slot disk, via the 50-W electrically commutated motor and drive unit. The system is designed to minimize loss to wind resistance and to overcome heat and vibration effects. The chopping disk is enclosed in a blade protector, and blanking plates cover unused apertures. Plates to reduce the size of the apertures in use also are provided.