Abstract: Apparatus for determining displacement of an object comprises a light source (1) for propagating polychromatic light along a path to a detector (7). Radiation modulation means such as a filter (5) is provided in predetermined fixed location with respect to the source (1) and the path of the light is adjusted in response to a displacement of the object such as to vary the distributed spectral content of the light reaching the detector (7). The detector comprises first and second photoresponsive elements (10, 11), of differing responsivity with respect to wavelength, and a microprocessor (14) receiving signals from the photoresponsive elements. The microprocessor calculates the color of the light incident on the detector (7) as represented by two or more parameters on the chromaticity (CIE) diagram, and interprets the color of the light incident on the detector (7) in terms of the displacement of the object.