monochromator


Also found in: Dictionary, Medical, Wikipedia.
Related to monochromator: Photomultiplier Tube

monochromator

(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.

Monochromator

 

(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.

REFERENCES

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

A. P. GAGARIN

monochromator

[¦män·ə¦krō‚mād·ər]
(spectroscopy)
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.
References in periodicals archive ?
Especially when it comes to Electron Energy Loss Spectroscopy (EELS), the new monochromator module significantly improves the resolution of fine structures of elemental absorption edges.
The output beam from the monochromator (transmitted through air) was directed alternately onto the DUT and the NIST transfer standard with a two-position mirror.
In contrast, a spectrofluorometer uses a pair of monochromators instead.
Up until the mid-1990s most of the laboratory powder diffractometers in use were divergent beam instruments with a narrow receiving slit, diffracted beam monochromator and a simple proportional/scintillation counter detector as shown in Fig.
While this is rather straightforward in theory, it required a major advance in monochromator technology to implement.
No optical elements are planned other than a liquid-nitrogen-cooled double-crystal monochromator with a choice of Si 111 or Si 311 crystals.
25 nm to longer wavelength when the monochromator is set to 13 nm.
A dedicated and highly-upgradeable aberration-corrected system, the microscope enables corrector and monochromator technology to enter into mainstream nanotechnology research and industrial markets.
For example, the use of predetermined "peak-to-area factor" as discussed by Bastin and Heijligers (5,6) or by measuring the peak area by summing the number of counts in each channel analyzed by stepping the monochromator across the wavelength domain containing the analyzed emission bands.
Tenders invited for Monochromator with light source
A spectrally tunable source using a conventional lamp and monochromator with a multi-element liquid crystal filter is currently under development (1).