A monochromator provided with a plurality of entrance windows for beams of white light emitted by respective lamps which are alternately operated. The internal optics of the monochromator consist of a dispersing system disposed so that each of the beams of white light is intercepted and converted into dispersed light which is composed of discrete monochromatic light beams of different wavelengths. The monochromator has an exit window with a slit of predetermined dimensions so as to transmit only one wavelength at a time.
