1) Optical device 
A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input. The name is from the Greek roots mono-, single, and chroma, colour, and the Latin suffix -ator, denoting an agent. == Uses == A device that can produce...
instrument that supplies light of one colour or light within a narrow range of wavelengths. Unwanted wavelengths (colours) are blocked by filters ... [4 related articles]
light goes in, hits a dispersing element (such as a diffraction grating or prism) and some portion of light is allowed out the other side.
an instrument for selecting narrow portions of the optical spectrum
<microscopy> A light-dispersing instrument which is used to obtain light of substantially one wavelength, or at least of a very narrow band of the spectrum. The light is apt to be weak because of its purity. The use of monochromatic light filters is usually adequate for microscopical work. ... (05 Aug 1998) ... 
A monochromator is a devise that uses a diffraction grating or prism to disperse light into a spectrum of its constituent wavelengths. The dispersive element is rotated such that only a narrow (monochromatic) band of light is permitted to exit the monochromator through a narrow aperture or slit.
Type: Term Pronunciation: mon′ō-krō′mā-tŏr, -tōr Definitions: 1. A prism or diffraction grating used in spectrophotometry to isolate a narrow spectral range.
A monochromator is a device that transmits the separated and narrowed portion of a wider range of optical wavelengths. Monochromators are used in a variety of optical measuring tools, including spectrometers and other imaging devices
a spectroscope with a slit that can be moved across the spectrum for viewing individual spectral bands. Also called