The TREND Networks C995-OH Optical Chopper is a microprocessor-based control system that utilizes direct-digital-synthesis to deliver precise optical chopping rates from 4 Hz to 5000 Hz. Equipped with a large five-digit LED readout, the C995-OH controller enables digital entry of the desired chopping rates from the front panel. Additionally, the C995-OH is equipped with a bidirectional RS-232 port that permits the user to set the desired chopping rate to a resolution of .001 Hz and to read the status of the instrument. The C995-OH, designed with a phase-locked-loop control system, allows the chopping rate to also be synchronized to a user-supplied external clock ranging from 4 Hz to 5 KHz. The controller can then be used to measure and display the frequency of the external clock. The C995-OH chopping head is attached to the controller by means of a 10 foot coiled cord. The precision etched blade is fully enclosed for protection from inadvertent damage. There are two apertures and two sections on the blade (30 slots and 3 slots) for the high and low frequency ranges, respectively. The aperture diameters are 15mm with a slot width of 4.5mm (30 slot section) or 30mm (3 slot section). The small 4.75 inch square outline and two inch maximum depth permits easy integration into compact optical setups. Dual #8-32 mounting holes permit the apertures to be placed at a height as low as 0.75 inches above an optical bench or with the 1/2 inch rod and stand, as high as 13 inches above the mounting surface.