View source for Optical spectrometer

{{Short description|Instrument to measure the properties of visible light}}
{{hatnote group|
{{Redirect-distinguish|Spectrograph|Spectrogram}}
{{Broader|Photometry (optics)}}
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{{more footnotes needed|date=December 2010}}

[[Image:Spectrometer.svg|thumb|right|300px|Grating spectrometer schematic]]
[[File:Simple_grating_spectrometer_inside.jpg|thumb|right|300px|Internal structure of a grating spectrometer: Light comes from left side and diffracts on the upper middle reflective grating. The wavelength of light is then selected by the slit on the upper right corner.]]

An '''optical spectrometer''' ('''spectrophotometer''', '''spectrograph''' or '''spectroscope''') is an instrument used to measure properties of [[light]] over a specific portion of the [[electromagnetic spectrum]], typically used in [[spectroscopic analysis]] to identify materials.<ref>{{cite journal |first1=L. R. P. |last1=Butler |first2=K. |last2=Laqua |year=1995 |title=Nomenclature, symbols, units and their usage in spectrochemical analysis-IX. Instrumentation for the spectral dispersion and isolation of optical radiation (IUPAC Recommendations 1995) |journal=Pure Appl. Chem. |volume=67 |issue=10 |pages=1725–1744 |doi=10.1351/pac199567101725 |s2cid=94991425 |url=http://iupac.org/publications/pac/67/10/1725/ |quote=A spectrometer is the general term for describing a combination of spectral apparatus with one or more detectors to measure the intensity of one or more spectral bands.|doi-access=free }}</ref> The variable measured is most often the [[irradiance]] of the light but could also, for instance, be the [[Polarization (waves)|polarization]] state. The independent variable is usually the [[wavelength]] of the light or a closely derived physical quantity, such as the corresponding [[wavenumber]] or the [[photon]] energy, in units of measurement such as centimeters, [[reciprocal centimeters]], or [[electron volt]]s, respectively.

A [[spectrometer]] is used in [[spectroscopy]] for producing [[spectral line]]s and measuring their [[wavelength]]s and intensities. Spectrometers may operate over a wide range of non-optical wavelengths, from [[gamma ray]]s and [[X-ray]]s into the [[far infrared]]. If the instrument is designed to measure the spectrum on an [[absolute scale]] rather than a relative one, then it is typically called a [[spectrophotometer]]. The majority of spectrophotometers are used in spectral regions near the visible spectrum.

A spectrometer that is calibrated for measurement of the incident optical power is called a [[spectroradiometer]].<ref>{{cite book |last1=Schneider |first1=T. |last2=Young |first2=R. |last3=Bergen |first3=T. |last4=Dam-Hansen |first4=C |last5=Goodman |first5=T. |last6=Jordan |first6=W. |last7=Lee |first7=D.-H |last8=Okura |first8=T. |last9=Sperfeld |first9=P. |last10=Thorseth |first10=A |last11=Zong |first11=Y. |title=CIE 250:2022 Spectroradiometric Measurement of Optical Radiation Sources |date=2022 |publisher=CIE - International Commission on Illumination |location=Vienna |isbn=978-3-902842-23-7 |url=https://orbit.dtu.dk/en/publications/cie-2502022-spectroradiometric-measurement-of-optical-radiation-s}}</ref>

In general, any particular instrument will operate over a small portion of this total range because of the different techniques used to measure different portions of the spectrum. Below optical frequencies (that is, at [[microwave]] and [[radio]] frequencies), the [[spectrum analyzer]] is a closely related electronic device.

Spectrometers are used in many fields.  For example, they are used in astronomy to analyze the radiation from objects and deduce their chemical composition.  The spectrometer uses a prism or a grating to spread the light into a spectrum.  This allows astronomers to detect many of the chemical elements by their characteristic spectral lines.  These lines are named for the elements which cause them, such as the [[hydrogen alpha]], beta, and gamma lines.  A glowing object will show bright spectral lines.  Dark lines are made by absorption, for example by light passing through a gas cloud, and these absorption lines can also identify chemical compounds.  Much of our knowledge of the chemical makeup of the universe comes from spectra.

==Spectroscopes==
{{Infobox laboratory equipment
|name    = Spectroscope
|image    = Spektrometr.jpg
|alt     = <!-- See Wikipedia:Alternative text for images -->
|caption   =
|acronym   =
|other_names = Spectrograph
|uses    =
|related   = [[Mass spectrograph]]
}}
[[Image:Optical spectrometers.png|thumb|right|Comparison of different diffraction based spectrometers: Reflection optics, refraction optics, fiber/integrated optics {{Citation needed|date=October 2013}}]]
Spectroscopes are often used in [[astronomy]] and some branches of [[chemistry]]. Early spectroscopes were simply [[Triangular prism (optics)|prisms]] with graduations marking wavelengths of light. Modern spectroscopes generally use a [[diffraction grating]], a movable [[Diffraction#Single-slit diffraction|slit]], and some kind of [[photodetector]], all automated and controlled by a [[computer]]. Recent advances have seen increasing reliance of computational algorithms in a range of miniaturised spectrometers without diffraction gratings, for example, through the use of quantum dot-based filter arrays on to a CCD chip<ref>{{Cite journal|last1=Bao|first1=Jie|last2=Bawendi|first2=Moungi G.|date=2015-07-01|title=A colloidal quantum dot spectrometer|url=https://www.nature.com/articles/nature14576|journal=Nature|language=en|volume=523|issue=7558|pages=67–70|doi=10.1038/nature14576|pmid=26135449|bibcode=2015Natur.523...67B|s2cid=4457991|issn=1476-4687|url-access=subscription}}</ref> or a series of photodetectors realised on a single nanostructure.<ref>{{Cite journal|last1=Yang|first1=Zongyin|last2=Albrow-Owen|first2=Tom|last3=Cui|first3=Hanxiao|last4=Alexander-Webber|first4=Jack|last5=Gu|first5=Fuxing|last6=Wang|first6=Xiaomu|last7=Wu|first7=Tien-Chun|last8=Zhuge|first8=Minghua|last9=Williams|first9=Calum|last10=Wang|first10=Pan|last11=Zayats|first11=Anatoly V.|date=2019-09-06|title=Single-nanowire spectrometers|journal=Science|volume=365|issue=6457|pages=1017–1020|doi=10.1126/science.aax8814|pmid=31488686|bibcode=2019Sci...365.1017Y|s2cid=201845940|doi-access=free}}</ref>

[[Joseph von Fraunhofer]] developed the first modern spectroscope by combining a prism, diffraction slit and [[refracting telescope|telescope]] in a manner that increased the spectral resolution and was reproducible in other laboratories. Fraunhofer also went on to invent the first diffraction spectroscope.<ref name="Brand 37">{{cite book |title=Lines of Light: The Sources of Dispersive Spectroscopy, 1800–1930 |last=Brand |first=John C. D. |publisher=Gordon and Breach Publishers |year=1995 |isbn=978-2884491624 |pages=37–42 }}</ref> [[Gustav Robert Kirchhoff]] and [[Robert Bunsen]] discovered the application of spectroscopes to chemical analysis and used this approach to discover [[caesium]] and [[rubidium]].<ref>{{cite journal|title=The discovery of the elements. XIII. Some spectroscopic discoveries|pages=1413–1434|last=Weeks|first=Mary Elvira|author-link=Mary Elvira Weeks|doi=10.1021/ed009p1413|journal=[[Journal of Chemical Education]]|volume=9|issue=8|year=1932|bibcode=1932JChEd...9.1413W}}</ref><ref>{{cite web|title=Robert Bunsen|url=http://www.infoplease.com/biography/var/robertbunsen.html|work=infoplease|publisher=[[Pearson Education]]|year=2007|access-date=2011-11-21}}</ref> Kirchhoff and Bunsen's analysis also enabled a chemical explanation of [[Astronomical spectroscopy#Stars and their properties|stellar spectra]], including [[Fraunhofer lines]].<ref name="Brand 63">{{harvnb|Brand|1995|p=63}}</ref>

When a material is heated to [[incandescence]] it emits [[light]] that is characteristic of the atomic makeup of the material.
Particular light frequencies give rise to sharply defined bands on the scale which can be thought of as fingerprints. For example, the element [[sodium]] has a very characteristic double yellow band known as the Sodium D-lines at 588.9950 and 589.5924 nanometers, the color of which will be familiar to anyone who has seen a low pressure [[sodium vapor lamp]].

In the original spectroscope design in the early 19th century, light entered a slit and a [[collimating lens]] transformed the light into a thin beam of parallel rays. The light then passed through a prism (in hand-held spectroscopes, usually an [[Amici prism]]) that [[refraction|refracted]] the beam into a spectrum because different wavelengths were refracted different amounts due to [[dispersion (optics)|dispersion]]. This image was then viewed through a tube with a scale that was transposed upon the spectral image, enabling its direct measurement.

With the development of [[photographic film]], the more accurate [[#Spectrographs|spectrograph]] was created. It was based on the same principle as the spectroscope, but it had a camera in place of the viewing tube. In recent years, the electronic circuits built around the [[photomultiplier]] tube have replaced the camera, allowing real-time spectrographic analysis with far greater accuracy. Arrays of photosensors are also used in place of film in spectrographic systems. Such spectral analysis, or spectroscopy, has become an important scientific tool for analyzing the composition of unknown material and for studying astronomical phenomena and testing astronomical theories.

In modern spectrographs in the UV, visible, and near-IR spectral ranges, the spectrum is generally given in the form of photon number per unit wavelength (nm or μm), wavenumber (μm<sup>−1</sup>, cm<sup>−1</sup>), frequency (THz), or energy (eV), with the units indicated by the [[abscissa]]. In the mid- to far-IR, spectra are typically expressed in units of Watts per unit wavelength (μm) or wavenumber (cm<sup>−1</sup>). In many cases, the spectrum is displayed with the units left implied (such as "digital counts" per spectral channel).

[[Image:Units visible spectrum.png|thumbnail|left|550px|A comparison of the four abscissa types typically used for visible spectrometers.]]
[[Image:Units IR spectrum.png|thumb|left|550px|A comparison of the four abscissa types typically used for infrared spectrometers.]]

=== In Gemology ===
[[Gemology|Gemologists]] frequently use spectroscopes to determine the absorption spectra of gemstones, thereby allowing them to make inferences about what kind of gem they are examining.<ref>{{Cite web|title=Spectroscope - The Gemology Project|url=http://gemologyproject.com/wiki/index.php?title=Spectroscope#Use_of_the_spectroscope|access-date=2022-01-04|website=gemologyproject.com}}</ref> A gemologist may compare the absorption spectrum they observe with a catalogue of spectra for various gems to help narrow down the exact identity of the gem.{{clear}}

==Spectrographs==
[[Image:simple spectroscope.jpg|right|thumb|200px|A very simple spectroscope based on a prism]]
[[File:The KMOS spectrograph before shipping to Chile.jpg|thumb|The [[K-band Multi-Object Spectrograph|KMOS]] spectrograph.<ref>{{cite news|title=Powerful New VLT Instrument Arrives in Chile|url=http://www.eso.org/public/announcements/ann12071/|access-date=11 October 2012|newspaper=ESO Announcement}}</ref>]]
[[File:Solar Spectrograph 2, Ondřejov Astronomical.jpg|thumb|Horizontal Solar Spectrograph at the Czech Astronomical Institute in Ondřejov, Czech Republic]]

A spectrograph is an instrument that separates light into its wavelengths and records the data.<ref name=spie>{{cite web| url = http://spie.org/x32350.xml| title = Spectrometer, Spectroscope, and SpectrographExcerpt from Field Guide to Spectroscopy}}</ref> A spectrograph typically has a multi-channel detector system or camera that detects and records the spectrum of light.<ref name=spie/><ref>{{GoldBookRef |title=spectrograph |file=S05836 }}</ref>

The term was first used in 1876 by [[Henry Draper|Dr. Henry Draper]] when he invented the earliest version of this device, and which he used to take several photographs of the spectrum of [[Vega]]. This earliest version of the spectrograph was cumbersome to use and difficult to manage. Jackson S. Richardson and Johnathon D. Hogan popularized the machine.<ref>{{citation | title = Memoir of Henry Draper, 1837-1882 | author = George Barker | page = 103 | url = http://www.nasonline.org/publications/biographical-memoirs/memoir-pdfs/draper-henry.pdf}}</ref>

There are several kinds of machines referred to as ''spectrographs'', depending on the precise nature of the waves. The first spectrographs used [[photographic paper]] as the detector. The plant pigment [[phytochrome]] was discovered using a spectrograph that used living plants as the detector. More recent spectrographs use electronic detectors, such as [[Charge-coupled device|CCD]]s which can be used for both visible and [[ultraviolet|UV]] light. The exact choice of detector depends on the wavelengths of light to be recorded.

A spectrograph is sometimes called [[polychromator]], as an analogy to [[monochromator]].

===Stellar and solar spectrograph=== 
The star [[stellar classification|spectral classification]] and discovery of the [[main sequence]], [[Hubble's law]] and the [[Galaxy morphological classification|Hubble sequence]] were all made with spectrographs that used photographic paper. [[James Webb Space Telescope]] contains both a near-infrared spectrograph ([[NIRSpec (Near-Infrared Spectrograph)|NIRSpec]]) and a mid-infrared spectrograph ([[MIRI (Mid-Infrared Instrument)|MIRI]]).

===Echelle spectrograph===
An [[echelle grating|echelle]]-based spectrograph uses two [[diffraction grating]]s, rotated 90 degrees with respect to each other and placed close to one another. Therefore, an entrance point and not a slit is used and a CCD-chip records the spectrum. Both gratings have a wide spacing, and one is [[Blazed grating|blazed]] so that only the first order is visible and the other is blazed with many higher orders visible, so a very fine spectrum is presented to the CCD.

===Slitless spectrograph===
In conventional spectrographs, a slit is inserted into the beam to limit the image extent in the dispersion direction.  A [[slitless spectrograph]] omits the slit; this results in images that [[convolution|convolve]] the image information with spectral information along the direction of dispersion.  If the field is not sufficiently sparse, then spectra from different sources in the image field will overlap.  The trade is that slitless spectrographs can produce [[spectral imaging|spectral images]] much more quickly than scanning a conventional spectrograph.  That is useful in applications such as [[solar physics]] where time evolution is important.

== See also ==
{{div col|colwidth=20em}}
* [[Circular dichroism]]
* [[Cosmic Origins Spectrograph]]
* [[Monochromator#Czerny-Turner monochromator|Czerny-Turner monochromator]]
* [[Imaging spectrometer]]
* [[List of astronomical instruments]]
* [[List of light sources]]
* [[Long-slit spectroscopy]]
* [[Prism spectrometer]]
* [[Scanning mobility particle sizer]]
* [[Spectrogram]]
* [[Spectrometer]]
* [[Spectroradiometer]]
* [[Spectroscopy]]
*[[Virtually imaged phased array]]
{{div col end}}

== References ==
{{Reflist}}

== Bibliography ==
* J. F. James and R. S. Sternberg (1969), ''The Design of Optical Spectrometers'' (Chapman and Hall Ltd)
* James, John (2007), ''Spectrograph Design Fundamentals'' (Cambridge University Press) {{ISBN|0-521-86463-1}}
* Browning, John (1882), ''[https://archive.org/details/howtoworkwithspe00browrich How to work with the spectroscope : a manual of practical manipulation with spectroscopes of all kinds]''
* {{cite book |last=Palmer |first=Christopher |title=Diffraction Grating Handbook |edition=8th |publisher=MKS Newport |date=2020 |url=https://www.newport.com/b/richardson-gratings}}

== External links ==
{{Wiktionary}}
{{Commons category|Spectrographs}}
*[https://web.archive.org/web/20130126044903/http://outreach.atnf.csiro.au/education/senior/astrophysics/spectrographs.html Spectrograph for astronomical Spectra]
*[http://digitalcollections.ucsc.edu/cdm/search/collection/p265101coll10/searchterm/spectrograph/order/title Photographs of spectrographs used in the Lick Observatory from the Lick Observatory Records Digital Archive, UC Santa Cruz Library's Digital Collections] {{Webarchive|url=https://web.archive.org/web/20190425143833/http://digitalcollections.ucsc.edu/cdm/search/collection/p265101coll10/searchterm/spectrograph/order/title |date=2019-04-25 }}

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[[Category:Spectrometers| ]]
[[Category:Spectrographs| ]]
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