Also known as monochromatic illuminator. An instrument for isolating narrow portions of the spectrum. The spectrum of any light source is formed by a prism or grating, and an exit slit placed in the spectrum selects a narrow band of wavelengths for emission. By moving the spectrum of a source internally past the slit, the color of the emitted light can be varied at will. As most monochromators emit several percentages of unwanted light -- either white or of the wrong wavelength -- along with the desired wavelength, two monochromators often are used in tandem, both being set to transmit the same wavelength. In this way the percentage of unwanted light can be reduced drastically.