Abstract: There is provided spectral discrimination apparatus for use in a scanning optical microscope, the spectral discrimination apparatus comprising dispersive means (31) and frequency selective means including a rotatable disc (36) or discs. The discs are formed with apertures or spiral slots which, on rotation of the disc or discs, controls the frequency of light transmitted by the apparatus. The apparatus includes a detector (39) for receiving light from the frequency selective means. The, or each, rotatable disc (36) is positioned at an aperture plane after the dispersive means (31), although this is not essential if the rotatable disc has its outer periphery shaped to provide a cam surface engaged by a cam follower. There is also provided a method of spectral discrimination in a scanning optical microscope, comprising dispersing the light and passing the light through frequency selective means in which the form of rotating discs controls the frequency of transmitted light.