(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.