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(mon-ŏ-kroh -mă-ter) An instrument in which one narrow band of wavelengths is isolated from a beam of light or other radiation. This is usually achieved by means of a narrow-band interference filter, or by a diffraction grating or prism, together with an exit slit through which the desired waveband may pass. Changes in the intensity of the monochromatic beam can then be investigated.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



(in optics), an instrument used to isolate narrow intervals of wavelengths (frequencies) of optical (visible, infrared, or ultraviolet) radiation; a spectral instrument. A monochromator consists of an entrance slit, illuminated by a radiation source; a collimator; a dispersion element; a focusing objective; and an exit slit. The dispersion element spatially separates rays of different wavelengths λ by directing them at various angles ϕ, and a spectrum (the aggregate of the images of the entrance slit in rays of all wavelengths emitted by the source) is formed in the focal plane of the objective. The required region of the spectrum is made to coincide with the exit slit by rotating the dispersion element; the spectral width (wavelength interval) δλ of the isolated region is changed by varying the width of the exit slit.

Dispersion prisms and diffraction gratings are used as the dispersion elements of monochromators. Their angular dispersion Δϕ/Δλ, together with the focal length of the objective, determines the linear dispersion of the monochromator Δl/Δλ (where Δϕ is the angular difference in the direction of the rays whose wavelengths differ by Δλ; Δl is the distance in the plane of the exit slit that separates the rays). Prisms are less expensive than gratings and have high dispersion in the ultraviolet region. However, their dispersion decreases sharply as λ increases; in addition, prisms made of different materials are needed for different regions of the spectrum. Gratings do not have these shortcomings.

In addition to dispersion, the quality of a monochromator is determined by its resolving power and luminosity. The resolving power of a monochromator, like that of any other spectral instrument, is equal to λ/(Δλ)*, where (Δλ)* is the smallest discernible difference in wavelengths in the output radiation of the monochromator. The luminosity indicates the part of the radiant energy emitted by the source in an isolated interval δλ that passes through the monochromator. The transmission depends on the geometric characteristics of the monochromator (particularly the dimensions of the slits and dispersion element) and on reflection and absorption losses in the monochromator’s optics.

The objectives of a monochromator (the collimator and focusing objectives) may be of the lens or mirror type. Mirror objectives are suitable in a much broader spectral range than lens objectives and do not require refocusing upon transition from one region of the spectrum to another. This is especially convenient in regions of the spectrum that are invisible to the eye (the ultraviolet and infrared). Because of this, mirror optics are usually used in monochromators for these fields.

Monochromators are the most important components of monochromatic light sources and spectrophotometers, which are used to measure the energy radiated by objects under study in various regions of the spectrum. In spectrophotometry it is particularly important to prevent the entry into the exit slit of the monochromator of scattered light with wavelengths far from the spectral region being isolated. Double monochromators, which are two monochromators structurally joined in such a way that the exit slit of the first is the entrance slit of the other, are often used for this purpose. The possibility of a significant increase in dispersion is another advantage of double monochromators.


Toporets, A. S. Monokhromatory. Moscow, 1955.
Peisakhson, I. V. Optika spektral’nykh priborov. Leningrad, 1970.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


A spectrograph in which a detector is replaced by a second slit, placed in the focal plane, to isolate a particular narrow band of wavelengths for refocusing on a detector or experimental object.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
The exit slit of the monochromator locates at 35 mm above the entrance slit, and the wavelength scanning of exit slit is achieved through prism rotation.
The beam shutter opening is 6.4 cm wide at the exit, opening to 9 cm on the end toward the source to fully illuminate the Soller collimators in the reactor beam, as well as to allow for horizontal focusing of the monochromator. The vertical opening is 16 cm.
A lens with 150-mm focal length is used as the output collimator (lens2), which focused a 10-times-magnified image of the output slit of the monochromator onto a plane close to the plane of the sample.
For the fixed direction of incident beam, the scanning of spectral component is done automatically, with the aid of monochromator, controlled by computer.
Tecan also introduced the Infinite 200 NanoQuant absorbance reader, available with either monochromators or filters, for the detecting DNA concentrations down to one ng per [micro]L, and the NanoQuant Plate for the simultaneous measurement of 16 samples.
One spectrometer was constructed from a film canister using an interchangeable LED as the combined light source and monochromator and a photoresistor as the detector.
The filtering is performed by a monochromator or interferometer.
2), with its double crystal monochromator (DCM) was funded by the Ontario Centre for Material Research (OCMR) in 1987.
Princeton Instruments has introduced the SpectraPro HRS-750, a 750mm focal length spectrograph and scanning monochromator. It features an astigmatism-corrected optical design, a mechanical scanning range of 0nm-1,500nm, and an exceptional resolution of 0.05nm or better.
The use of an advanced monochromator design helps to separate spectral signals In close proximity to one another and improves the dynamic range by reducing the influence of stray light.
Key words: laser; monochromator; optical fiber; optical power; spectral responsivity; uncertainty calculation.