Editing Monochromator (section)

==Techniques==
A monochromator can use either the phenomenon of [[optical dispersion]] in a [[Prism (optics)|prism]], or that of [[diffraction]] using a [[diffraction grating]], to spatially separate the colors of light. It usually has a mechanism for directing the selected color to an exit slit. Usually the grating or the prism is used in a reflective mode. A reflective prism is made by making a right triangle prism (typically, half of an equilateral prism) with one side mirrored. The light enters through the hypotenuse face and is reflected back through it, being refracted twice at the same surface. The total refraction, and the total dispersion, is the same as would occur if an equilateral prism were used in transmission mode.

===Collimation===
The dispersion or diffraction is only controllable if the light is [[collimated]], that is if all the rays of light are parallel, or practically so. A source, like the sun, which is very far away, provides collimated light. [[Isaac Newton|Newton]] used sunlight in his [[Isaac Newton#Optics|famous experiments]]. In a practical monochromator, however, the light source is close by, and an optical system in the monochromator converts the diverging light of the source to collimated light. Although some monochromator designs do use focusing gratings that do not need separate collimators, most use collimating mirrors. Reflective optics are preferred because they do not introduce dispersive effects of their own.

===Geometrical design of a prism or grating monochromator===

There are grating/prism configurations that offer different tradeoffs between simplicity and spectral accuracy.
* Czerny–Turner (discussed below)
* [[Paschen-Runge mounting|Paschen-Runge]]
* [[Eagle mounting|Eagle]]
* [[Wadsworth constant deviation system|Wadsworth]]
* [[Ebert-Fasti mounting|Ebert-Fasti]]
* [[Littrow mounting|Littrow]]
* [[Pfund mounting|Pfund]]

[[Image:Czerny-Turner Monochromator.svg|thumb|200px|Diagram of a Czerny–Turner monochromator]]
[[File:Parabolic Reflective Diffraction Grating.jpg|thumb|right|200px|A combined reflecting-focusing diffraction grating]]
[[Image:Monochromator.jpg|thumb|right|200px|A Littrow grating monochromator. This is similar to the Czerny–Turner but uses a common collimator/refocusing mirror.]]


In the common [[Marianus Czerny|Czerny]]–Turner design,<ref>{{cite journal|last=Czerny|first=M.|author2=Turner, A. F.|author-link1=Marianus Czerny|title=Über den astigmatismus bei spiegelspektrometern.|journal=Zeitschrift für Physik|year=1930|volume=61|issue=11–12|pages=792–797|doi=10.1007/BF01340206|bibcode = 1930ZPhy...61..792C |s2cid=126259668}}</ref> the broad-band illumination source ('''A''') is aimed at an entrance slit ('''B'''). 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. The slit is placed at the effective focus of a curved mirror (the [[Collimated light|collimator]], '''C''') so that the light from the slit reflected from the mirror is collimated (focused at infinity). The collimated light is [[diffraction|diffracted]] from the [[diffraction grating|grating]] ('''D''') and then is collected by another mirror ('''E'''), which refocuses the light, now dispersed, on the exit slit ('''F'''). In a prism monochromator, a reflective [[Littrow prism]] takes the place of the diffraction grating, in which case the light is [[refraction|refracted]] by the prism.

At the exit slit, the colors of the light are spread out (in the visible this shows the colors of the rainbow). Because each color arrives at a separate point in the exit-slit plane, there are a series of images of the entrance slit focused on the plane. Because the entrance slit is finite in width, parts of nearby images overlap. The light leaving the exit slit ('''F''') contains the entire image of the entrance slit of the selected color plus parts of the entrance slit images of nearby colors. A rotation of the dispersing element causes the band of colors to move relative to the exit slit, so that the desired entrance slit image is centered on the exit slit. The range of colors leaving the exit slit is a function of the width of the slits. The entrance and exit slit widths are adjusted together.

===Stray light===
The ideal transfer function of such a monochromator is a triangular shape. The peak of the triangle is at the nominal wavelength selected, so that the image of the selected wavelength completely fills the exit slit. The intensity of the nearby colors then decreases linearly on either side of this peak until some cutoff value is reached, where the intensity stops decreasing. This is called the ''[[stray light]]'' level. The cutoff level is typically about one thousandth of the peak value, or 0.1%.

===Spectral bandwidth===
[[Spectral bandwidth]] is defined as the width of the triangle at the points where the light has reached half the maximum value ([[full width at half maximum]], abbreviated as FWHM). A typical spectral bandwidth might be one nanometer; however, different values can be chosen to meet the need of analysis. A narrower bandwidth does improve the resolution, but it also decreases the signal-to-noise ratio.<ref>Keppy, N. K. and Allen M., Thermo Fisher Scientific, Madison, WI, USA, 2008</ref>

===Dispersion===
The dispersion of a monochromator is characterized as the width of the band of colors per unit of slit width, 1&nbsp;nm of spectrum per mm of slit width for instance. This factor is constant for a grating, but varies with wavelength for a prism. If a scanning prism monochromator is used in a constant bandwidth mode, the slit width must change as the wavelength changes. Dispersion depends on the focal length, the grating order and grating resolving power.

===Wavelength range===
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.

===Double monochromators===
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. This arrangement is not intended to improve the narrowness of the spectrum, but rather to lower the cutoff level. A double monochromator may have a cutoff about one millionth of the peak value, the product of the two cutoffs of the individual sections. The intensity of the light of other colors in the exit beam is referred to as the stray light level and is the most critical specification of a monochromator for many uses. Achieving low stray light is a large part of the art of making a practical monochromator.

===Diffraction gratings and blazed gratings===
Grating monochromators disperse ultraviolet, visible, and infrared radiation typically using replica gratings, which are manufactured from a master grating. A master grating consists of a hard, optically flat, surface that has a large number of parallel and closely spaced grooves. The construction of a master grating is a long, expensive process because the grooves must be of identical size, exactly parallel, and equally spaced over the length of the grating (3–10&nbsp;cm). A grating for the ultraviolet and visible region typically has 300–2000 grooves/mm, however 1200–1400 grooves/mm is most common. For the infrared region, gratings usually have 10–200 grooves/mm.<ref>{{cite book|last=Skoog|first=Douglas|title=Principles of Instrumental Analysis|url=https://archive.org/details/principlesinstru00dasc|url-access=limited|year=2007|publisher=Brooks/Cole|location=Belmont, CA|isbn=978-0-495-01201-6|pages=[https://archive.org/details/principlesinstru00dasc/page/n195 182]–183}}</ref> When a [[diffraction grating]] is used, care must be taken in the design of broadband monochromators because the diffraction pattern has overlapping orders. Sometimes broadband preselector filters are inserted in the optical path to limit the width of the diffraction orders so they do not overlap. Sometimes this is done by using a prism as one of the monochromators of a dual monochromator design.

The original high-resolution diffraction gratings were ruled. The construction of high-quality [[Dividing engine|ruling engine]]s 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. This is called blazing a grating. Ruled gratings have imperfections that produce faint "ghost" diffraction orders that may raise the stray light level of a monochromator. A later photolithographic technique allows gratings to be created from a holographic interference pattern. [[Holographic grating]]s have sinusoidal grooves and so are not as bright, but have lower scattered light levels than blazed gratings. Almost all the gratings actually used in monochromators are carefully made [[Diffraction grating#Fabrication|replicas]] of ruled or holographic master gratings.

===Prisms===
[[File:Prism monochromator,with light path illustrated,from the laboratory of AHNU,Mar 2017.jpg|thumb|right|200px|The internal structure of a Reflecting monochromator using a single prism.The yellow line indicates the path of light.]]
Prisms have higher dispersion in the [[UV]] region. Prism monochromators are favored in some instruments that are principally designed to work in the far UV region. Most monochromators use gratings, however. Some monochromators have several gratings that can be selected for use in different spectral regions. A double monochromator made by placing a prism and a grating monochromator in series typically does not need additional bandpass filters to isolate a single grating order.

===Focal length===
The narrowness of the band of colors that a monochromator can generate is related to the focal length of the monochromator collimators. Using a longer focal length optical system also unfortunately decreases the amount of light that can be accepted from the source. Very high resolution monochromators might have a focal length of 2 meters. Building such monochromators requires exceptional attention to mechanical and thermal stability. For many applications a monochromator of about 0.4 meters' focal length is considered to have excellent resolution. Many monochromators have a focal length less than 0.1 meters.

===Slit height===
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.

===Wavelength vs. energy===
Monochromators are often calibrated in units of wavelength. Uniform rotation of a grating produces a sinusoidal change in wavelength, which is approximately linear for small grating angles, so such an instrument is easy to build. Many of the underlying physical phenomena being studied are linear in energy though, and since wavelength and [[photon energy]] have a reciprocal relationship, spectral patterns that are simple and predictable when plotted as a function of energy are distorted when plotted as a function of wavelength. Some monochromators are calibrated in units of [[wavenumber|reciprocal centimeters]] or some other energy units, but the scale may not be linear.

===Dynamic range===
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).