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. The three frequencies f1, f2 and f1 + f2 are chosen to be relative primes which significantly reduces mutual harmonic interference. 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. The model 198A acts as a chopper for the incident light sources at frequencies f1 and f2 and generates a reference signal at a frequency f1 + f2 which can be used to drive a subsequent instrument, such as a lock-in amplifier.