Model 198A
Mixed Beam Light Chopper
The model 198A is a dual frequency optical chopper, using a thin rotating metal blade with an inner set of 11 apertures and an outer set of 18 apertures to simultaneously chop two light beams. The blade is driven by a precision DC motor whose speed is controlled via a phase locked loop that is referenced to either an internal quartz crystal oscillator or an external reference signal. The chopper has three reference frequency outputs, one at each of the frequencies generated by the inner and outer apertures, and one at a frequency equal to their sum.

It is ideal for use in measurements where two modulated beams give rise to a third optical signal at a frequency which is the sum of the two chopped frequencies.

Models 651-1, 652-1 & 652-2
Precision Light Choppers
The 650 series of light choppers features a control box with separate chopping heads for remote operation. There are three different models within the range, the 651-1 using a standard dual port chopping head and 652-1 and 652-2 employing a single port micro head for use where space is a prime consideration.

All the models in the range offer precision frequency control via digital push-buttons or by the application of an AC signal to the sync input on the control unit. One special feature of the 650 series choppers is the ability to add extension leads to increase the remote distance between the chopping head and the control unit.