The C-995 Phase-Locked Loop Optical Chopper provides crystal controlled chopping rates of 4 Hz to 5 KHz using only one blade. The use of direct digital synthesis permits the rate to be set to a precision of 0.001 Hz and provides quartz crystal stability. Frequency settings may be made via front panel controls and its bi-directional RS-232 port. Additionally, it may be phase-locked to a user supplied external clock. Both enclosed and open style chopper heads are available.