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Anoptical spectrometer (spectrophotometer,spectrograph orspectroscope) is an instrument used to measure properties oflight over a specific portion of theelectromagnetic spectrum, typically used inspectroscopic analysis to identify materials.[1] The variable measured is most often theirradiance of the light but could also, for instance, be thepolarization state. The independent variable is usually thewavelength of the light or a closely derived physical quantity, such as the correspondingwavenumber or thephoton energy, in units of measurement such as centimeters,reciprocal centimeters, orelectron volts, respectively.
Aspectrometer is used inspectroscopy for producingspectral lines and measuring theirwavelengths and intensities. Spectrometers may operate over a wide range of non-optical wavelengths, fromgamma rays andX-rays into thefar infrared. If the instrument is designed to measure the spectrum on anabsolute scale rather than a relative one, then it is typically called aspectrophotometer. 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 aspectroradiometer.[2]
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, atmicrowave andradio frequencies), thespectrum 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 thehydrogen 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.
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| Other names | Spectrograph |
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| Related items | Mass spectrograph |
Spectroscopes are often used inastronomy and some branches ofchemistry. Early spectroscopes were simplyprisms with graduations marking wavelengths of light. Modern spectroscopes generally use adiffraction grating, a movableslit, and some kind ofphotodetector, all automated and controlled by acomputer. 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[3] or a series of photodetectors realised on a single nanostructure.[4]
Joseph von Fraunhofer developed the first modern spectroscope by combining a prism, diffraction slit andtelescope in a manner that increased the spectral resolution and was reproducible in other laboratories. Fraunhofer also went on to invent the first diffraction spectroscope.[5]Gustav Robert Kirchhoff andRobert Bunsen discovered the application of spectroscopes to chemical analysis and used this approach to discovercaesium andrubidium.[6][7] Kirchhoff and Bunsen's analysis also enabled a chemical explanation ofstellar spectra, includingFraunhofer lines.[8]
When a material is heated toincandescence it emitslight 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 elementsodium 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 pressuresodium vapor lamp.
In the original spectroscope design in the early 19th century, light entered a slit and acollimating lens transformed the light into a thin beam of parallel rays. The light then passed through a prism (in hand-held spectroscopes, usually anAmici prism) thatrefracted the beam into a spectrum because different wavelengths were refracted different amounts due todispersion. 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 ofphotographic film, the more accuratespectrograph 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 thephotomultiplier 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−1, cm−1), frequency (THz), or energy (eV), with the units indicated by theabscissa. In the mid- to far-IR, spectra are typically expressed in units of Watts per unit wavelength (μm) or wavenumber (cm−1). In many cases, the spectrum is displayed with the units left implied (such as "digital counts" per spectral channel).
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.[9] 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.
A spectrograph is an instrument that separates light into its wavelengths and records the data.[11] A spectrograph typically has a multi-channel detector system or camera that detects and records the spectrum of light.[11][12]
The term was first used in 1876 byDr. Henry Draper when he invented the earliest version of this device, and which he used to take several photographs of the spectrum ofVega. This earliest version of the spectrograph was cumbersome to use and difficult to manage. Jackson S. Richardson and Johnathon D. Hogan popularized the machine.[13]
There are several kinds of machines referred to asspectrographs, depending on the precise nature of the waves. The first spectrographs usedphotographic paper as the detector. The plant pigmentphytochrome was discovered using a spectrograph that used living plants as the detector. More recent spectrographs use electronic detectors, such asCCDs which can be used for both visible andUV light. The exact choice of detector depends on the wavelengths of light to be recorded.
A spectrograph is sometimes calledpolychromator, as an analogy tomonochromator.
The starspectral classification and discovery of themain sequence,Hubble's law and theHubble sequence were all made with spectrographs that used photographic paper.James Webb Space Telescope contains both a near-infrared spectrograph (NIRSpec) and a mid-infrared spectrograph (MIRI).
Anechelle-based spectrograph uses twodiffraction gratings, 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 isblazed 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.
In conventional spectrographs, a slit is inserted into the beam to limit the image extent in the dispersion direction. Aslitless spectrograph omits the slit; this results in images thatconvolve 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 producespectral images much more quickly than scanning a conventional spectrograph. That is useful in applications such assolar physics where time evolution is important.
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.
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