Automatic control of the spacing of Fabry-Perot interferometers.

The automatic control of the spacing of Fabry-Perot interferometers to transmit at a given wavelength is discussed, and an interferometer is described in which the plates are maintained parallel to within the accuracy of the plates themselves and at a preset spacing. The variation in mean spacing over test periods of several hours has not exceeded +/- 0.7 nm despite temperature fluctuations and other uncontrolled environmental conditions.