Editing Monochromator (section)

===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.