Abstract: A spectrum shifting optical switch in which an optical beam splitter couples input light from a light source to a manually movable spectral modulation element and couples output light from the spectral modulation element to a light detector. The spectral modulation element may be an optically resonant cavity having a manually movable diaphragm, thereby changing the resonant wavelength of the cavity. As a result, the wavelength spectrum of the input light is altered by the resonant wavelength of the cavity. The spectral modulation element may also be an optical filter through which the input light passes. The resulting spectral modulation of the input light is detected to determine the position of the resilient diaphargm or filter.