View source for Spectrometer



{{otheruses4|the light measuring instrument|sound waves|Spectrogram}}
{{Infobox Laboratory equipment
|name        = Spectrometer
|image       = Spektrometr.jpg
|caption     =
|acronym     = 
|other_names = Spectroscope<br/>Spectrograph
|uses        = 
|related     = [[Mass spectrometer]]
}}

A '''spectrometer''' is an [[optical]] instrument used to measure properties of [[light]] over a specific portion of the [[electromagnetic spectrum]], typically used in [[spectroscopic analysis]] to identify materials. The variable measured is most often the light's [[intensity]] but could also, for instance, be the [[polarization]] state. The independent variable is usually the [[wavelength]] of the light, normally expressed as some fraction of a meter, but sometimes expressed as some unit directly proportional to the [[photon]] energy, such as [[wavenumber]] or [[electron volt]]s, which has a reciprocal relationship to wavelength. A spectrometer is used in [[spectroscopy]] for producing [[spectral line]]s and measuring their [[wavelength]]s and intensities.  Spectrometer is a term that is applied to instruments that operate over a very wide range of wavelengths, from [[gamma ray]]s and [[X-ray]]s into the [[far infrared]]. If the region of interest is restricted to near the [[visible spectrum]], the study is called [[spectrophotometry]].

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.

==Spectroscopes==
[[Image:Optical spectrometers.png|thumbnail|Comparison of different diffraction based spectrometers: Reflection optics, refraction optics, fiber optics]]
They are used often in [[astronomy]] and some branches of [[chemistry]].  Early spectroscopes were simply a [[Triangular prism (optics)|prism]] with graduations marking wavelengths of light. Modern spectroscopes, such as [[monochromator]]s, generally use a [[diffraction grating]], a movable slit, and some kind of [[photodetector]], all automated and controlled by a [[computer]].  The spectroscope was invented by both [[Gustav Robert Georg Kirchhoff]] and [[Robert Wilhelm Bunsen]].

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 colour 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 was 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. The wavelengths are measured with the spectrometer. 

==Spectrographs==
[[Image:Spectrometer schematic.gif|left|thumb|300 px|Grating spectrometer schematic]]
A '''spectrograph''' is an instrument that transforms an incoming time-domain [[waveform]] into a [[frequency spectrum]], or generally a sequence of such spectra. 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 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. 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.

The forthcoming [[James Webb Space Telescope]] will contain both a near-infrared spectrograph (NIRSpec) and a mid-infrared spectrometer (MIRI).

An [[echelle 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 2d CCD-chip records the spectrum. Usually one would guess to retrieve a spectrum on the diagonal, but when both grating have a wide spacing and one is blazed so that only the first order is visible and the other is blazed that a lot of higher orders are visible, one gets a very fine spectrum nicely folded onto a small common CCD-chip. The small chip also means that the collimating optics need not to be optimized for coma or astigmatism, but the [[spherical aberration]] can be set to zero.

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

== See also ==
* [[List of light sources]]
* [[Photometry (optics)]] Main Photometry/Radiometry article - explains technical terms
* [[Spectroscopy]]
* [[Mass spectrometer]]

== 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 0521864631
* Browning, John (1882), ''[http://www.archive.org/details/howtoworkwithspe00browrich How to work with the spectroscope : a manual of practical manipulation with spectroscopes of all kinds]''

== External links ==
* [http://www.jobinyvon.com/default.asp?DocID=612 The Optics of Spectroscopy Tutorial] 
* [http://astro.u-strasbg.fr/~koppen/spectro/designe.html How to design a spectroscope]   
* [http://www.cs.cmu.edu/~zhuxj/astro/html/spectrometer.html A CD spectrometer] Build from CD and cereal box - Spectrographs of common light sources
* [http://www-astronomy.mps.ohio-state.edu/~pogge/Ast161/Unit4/HandSpec/index.html Supplement: Build Yourself a Simple Hand-Held Spectrograph] Sample spectra
* [http://skolor.nacka.se/samskolan/eaae/summerschools/Spectro05Eng.html SPECTROSCOPY FOR THE SCHOOL] Build a simple spectroscope from a CD
* [http://www.exo.net/~pauld/activities/CDspectrometer/cdspectrometer.html CD spectrometer] CD + cardboard tube or cereal box
* [http://www.scitoys.com/scitoys/scitoys/light/cd_spectroscope/spectroscope.html Building a simple spectroscope]. Construction photos, razor-blade slit
* [http://mo-www.harvard.edu/Java/MiniSpectroscopy.html MiniSpectroscopy] displays a visual representation (a "spectroscope view") of a sample spectrum simultaneously with a graphical (intensity vs. wavelength) representation.

[[Category:Spectrometers| ]]
[[Category:Electronic test equipment]]
[[Category:Signal processing]]
[[Category:Measuring instruments]]
[[Category:Laboratory equipment]]

[[ca:Espectròmetre]]
[[cs:Spektrometr]]
[[da:Spektrometer]]
[[es:Espectrómetro]]
[[fr:Spectromètre]]
[[hr:Spektrometar]]
[[id:Spektrometer]]
[[it:Spettrometro]]
[[lt:Spektrometras]]
[[nl:Spectrometer]]
[[ja:分光器]]
[[pl:Spektrometr]]

{{WikiDoc Help Menu}}
{{WikiDoc Sources}}