Monochromator
Although there are a number of useful ways to select a narrow band of wavelengths (which, in the visible range, is perceived as a pure color), there are not as many other ways to easily select any wavelength band from a wide range.
In hard X-ray and neutron optics, crystal monochromators are used to define wave conditions on the instruments.
The amount of light energy available for use depends on the intensity of the source in the space defined by the slit (width × height) and the acceptance angle of the optical system.
A monochromator's adjustment range might cover the visible spectrum and some part of both or either of the nearby ultraviolet (UV) and infrared (IR) spectra, although monochromators are built for a great variety of optical ranges, and to a great many designs.
It is common for two monochromators to be connected in series, with their mechanical systems operating in tandem so that they both select the same color.
Achieving low stray light is a large part of the art of making a practical monochromator.
A master grating consists of a hard, optically flat, surface that has a large number of parallel and closely spaced grooves.
Sometimes broadband preselector filters are inserted in the optical path to limit the width of the diffraction orders so they do not overlap.
The construction of high-quality ruling engines was a large undertaking (as well as exceedingly difficult, in past decades), and good gratings were very expensive.
The slope of the triangular groove in a ruled grating is typically adjusted to enhance the brightness of a particular diffraction order.
Ruled gratings have imperfections that produce faint "ghost" diffraction orders that may raise the stray light level of a monochromator.
Prism monochromators are favored in some instruments that are principally designed to work in the far UV region.
Using a longer focal length optical system also unfortunately decreases the amount of light that can be accepted from the source.
The most common optical system uses spherical collimators and thus contains optical aberrations that curve the field where the slit images come to focus, so that slits are sometimes curved instead of simply straight, to approximate the curvature of the image.
This allows taller slits to be used, gathering more light, while still achieving high spectral resolution.
Some designs take another approach and use toroidal collimating mirrors to correct the curvature instead, allowing higher straight slits without sacrificing resolution.
A spectrophotometer built with a high quality double monochromator can produce light of sufficient purity and intensity that the instrument can measure a narrow band of optical attenuation of about one million fold (6 AU, Absorbance Units).
Monochromators are used in many optical measuring instruments and in other applications where tunable monochromatic light is wanted.
If an imaging device replaces the exit slit, the result is the basic configuration of a spectrograph.
Such an instrument can record a spectral function without mechanical scanning, although there may be tradeoffs in terms of resolution or sensitivity for instance.
In the UV, visible and near IR, absorbance and reflectance spectrophotometers usually illuminate the sample with monochromatic light.
Monochromators are also used in optical instruments that measure other phenomena besides simple absorption or reflection, wherever the color of the light is a significant variable.
Coordination of the imager, calibrated detector, and monochromator allows one to calculate the carriers (electrons or holes) generated for a photon of a given wavelength, QE.
