![Simple_grating_spectrometer_inside](https://upload.wikimedia.org/wikipedia/commons/6/61/Simple_grating_spectrometer_inside.jpg)
An optical spectrometer (spectrophotometer, spectrograph or spectroscope) is an instrument used to measure properties of
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
over a specific portion of the
electromagnetic spectrum
The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.
The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from ...
, typically used in
spectroscopic analysis
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wav ...
to identify materials. The variable measured is most often the light's
intensity
Intensity may refer to:
In colloquial use
*Strength (disambiguation)
*Amplitude
* Level (disambiguation)
* Magnitude (disambiguation)
In physical sciences
Physics
*Intensity (physics), power per unit area (W/m2)
*Field strength of electric, ma ...
but could also, for instance, be the
polarization state. The independent variable is usually the
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
of the light or a unit directly proportional to the
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
energy, such as
reciprocal centimeters Reciprocal length or inverse length is a quantity or measurement used in several branches of science and mathematics. As the reciprocal of length, common units used for this measurement include the reciprocal metre or inverse metre (symbol: m&min ...
or
electron volt
In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacuum. ...
s, which has a reciprocal relationship to wavelength.
A
spectrometer
A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the ...
is used in
spectroscopy
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
for producing
spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...
s and measuring their
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
s and intensities. Spectrometers may operate over a wide range of non-optical wavelengths, from
gamma ray
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...
s and
X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
s into the
far infrared
Far infrared (FIR) is a region in the infrared spectrum of electromagnetic radiation. Far infrared is often defined as any radiation with a wavelength of 15 micrometers (μm) to 1 mm (corresponding to a range of about 20 THz to ...
. If the instrument is designed to measure the spectrum on an
absolute scale
There is no single definition of an absolute scale. In statistics and measurement theory, it is simply a ratio scale in which the unit of measurement is fixed, and values are obtained by counting. According to another definition it is a system of ...
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.
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
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ran ...
and
radio
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
frequencies), the
spectrum analyzer
A spectrum analyzer measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. The primary use is to measure the power of the spectrum of known and unknown signals. The input signal that most co ...
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
H-alpha (Hα) is a specific deep-red visible spectral line in the Balmer series with a wavelength of 656.28 nm in air and 656.46 nm in vacuum; it occurs when a hydrogen electron falls from its third to second lowest energy level. H-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
Spectroscopes are often used in
astronomy
Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...
and some branches of
chemistry
Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...
. Early spectroscopes were simply
prisms with graduations marking wavelengths of light. Modern spectroscopes generally use a
diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure that diffracts light into several beams travelling in different directions (i.e., different diffraction angles). The emerging coloration is a form of structura ...
, a movable
slit, and some kind of
photodetector
Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation. There is a wide variety of photodetectors which may be classified by mechanism of detection, such as Photoelectric effect, photoelectric or photoc ...
, all automated and controlled by a
computer
A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as C ...
. 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 or a series of photodetectors realised on a single nanostructure.
Joseph von Fraunhofer
Joseph Ritter von Fraunhofer (; ; 6 March 1787 – 7 June 1826) was a German physicist and optical lens manufacturer. He made optical glass, an achromatic telescope, and objective lenses. He also invented the spectroscope and developed diffract ...
developed the first modern spectroscope by combining a prism, diffraction slit and
telescope
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...
in a manner that increased the spectral resolution and was reproducible in other laboratories. Fraunhofer also went on to invent the first diffraction spectroscope.
Gustav Robert Kirchhoff
Gustav Robert Kirchhoff (; 12 March 1824 – 17 October 1887) was a German physicist who contributed to the fundamental understanding of electrical circuits, spectroscopy, and the emission of black-body radiation by heated objects.
He coine ...
and
Robert Bunsen
Robert Wilhelm Eberhard Bunsen (;
30 March 1811
– 16 August 1899) was a German chemist. He investigated emission spectra of heated elements, and discovered caesium (in 1860) and rubidium (in 1861) with the physicist Gustav Kirchhoff. The Bu ...
discovered the application of spectroscopes to chemical analysis and used this approach to discover
caesium
Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...
and
rubidium
Rubidium is the chemical element with the symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have a density higher ...
. Kirchhoff and Bunsen's analysis also enabled a chemical explanation of
stellar spectra
Astronomical spectroscopy is the study of astronomy using the techniques of spectroscopy to measure the spectrum of electromagnetic radiation, including visible light, ultraviolet, X-ray, infrared and radio waves that radiate from stars and othe ...
, including
Fraunhofer lines
In physics and optics, the Fraunhofer lines are a set of spectral absorption lines named after the German physicist Joseph von Fraunhofer (1787–1826). The lines were originally observed as dark features (absorption lines) in the optical spectru ...
.
When a material is heated to
incandescence
Incandescence is the emission of electromagnetic radiation (including visible light) from a hot body as a result of its high temperature. The term derives from the Latin verb ''incandescere,'' to glow white. A common use of incandescence is ...
it emits
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
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
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable iso ...
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
A sodium-vapor lamp is a gas-discharge lamp that uses sodium in an excited state to produce light at a characteristic wavelength near 589 nm.
Two varieties of such lamps exist: low pressure and high pressure. Low-pressure sodium lamps are ...
.
In the original spectroscope design in the early 19th century, light entered a slit and a
collimating lens
A collimator is a device which narrows a beam of particles or waves. To narrow can mean either to cause the directions of motion to become more aligned in a specific direction (i.e., make collimated light or parallel rays), or to cause the spati ...
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
An Amici prism, named for the astronomer Giovanni Battista Amici, is a type of compound dispersive prism used in spectrometers. The Amici prism consists of two triangular prisms in contact, with the first typically being made from a medium-di ...
) that
refracted
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
the beam into a spectrum because different wavelengths were refracted different amounts due to
dispersion
Dispersion may refer to:
Economics and finance
*Dispersion (finance), a measure for the statistical distribution of portfolio returns
*Price dispersion, a variation in prices across sellers of the same item
*Wage dispersion, the amount of variatio ...
. 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
Photographic film is a strip or sheet of transparent film base coated on one side with a gelatin photographic emulsion, emulsion containing microscopically small light-sensitive silver halide crystals. The sizes and other characteristics of th ...
, the more accurate
spectrograph
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 mate ...
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 A photomultiplier is a device that converts incident photons into an electrical signal.
Kinds of photomultiplier include:
* Photomultiplier tube, a vacuum tube converting incident photons into an electric signal. Photomultiplier tubes (PMTs for sho ...
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 the
abscissa
In common usage, the abscissa refers to the (''x'') coordinate and the ordinate refers to the (''y'') coordinate of a standard two-dimensional graph.
The distance of a point from the y-axis, scaled with the x-axis, is called abscissa or x coo ...
. 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).
In 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. A gemologist may compare the absoprtion spectrum they observe with a catalogue of spectra for various gems to help narrow down the exact identity of the gem.
Spectrographs
![Solar Spectrograph 2, Ondřejov Astronomical](https://upload.wikimedia.org/wikipedia/commons/7/74/Solar_Spectrograph_2%2C_Ond%C5%99ejov_Astronomical.jpg)
A spectrograph is an instrument that separates light by its wavelengths and records this data.
A spectrograph typically has a multi-channel detector system or camera that detects and records the spectrum of light.
[
The term was first used in 1876 by 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
Vega is the brightest star in the northern constellation of Lyra. It has the Bayer designation α Lyrae, which is Latinised to Alpha Lyrae and abbreviated Alpha Lyr or α Lyr. This star is relatively close at only from the Sun, an ...
. This earliest version of the spectrograph was cumbersome to use and difficult to manage.
There are several kinds of machines referred to as ''spectrographs'', depending on the precise nature of the waves. The first spectrographs used photographic paper
Photographic paper is a paper coated with a light-sensitive chemical formula, like photographic film, used for making photographic prints. When photographic paper is exposed to light, it captures a latent image that is then developed to form a v ...
as the detector. The plant pigment phytochrome
Phytochromes are a class of photoreceptor in plants, bacteria and fungi used to detect light. They are sensitive to light in the red and far-red region of the visible spectrum and can be classed as either Type I, which are activated by far-re ...
was discovered using a spectrograph that used living plants as the detector. More recent spectrographs use electronic detectors, such as CCDs which can be used for both visible and UV light. The exact choice of detector depends on the wavelengths of light to be recorded.
A spectrograph is sometimes called polychromator A polychromator is an optical device that is used to disperse light into different directions to isolate parts of the spectrum of the light. A prism or diffraction grating can be used to disperse the light. Unlike a monochromator, it outputs multip ...
, as an analogy to monochromator
A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input. The name is from the Greek roots ''mono-'', "si ...
.
Stellar and solar spectrograph
The star spectral classification
In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...
and discovery of the main sequence
In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...
, Hubble's law
Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving ...
and the Hubble sequence
The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926. It is often colloquially known as the Hubble tuning-fork diagram because the shape in which it is traditionally represented resembles a tun ...
were all made with spectrographs that used photographic paper. The forthcoming James Webb Space Telescope
The James Webb Space Telescope (JWST) is a space telescope which conducts infrared astronomy. As the largest optical telescope in space, its high resolution and sensitivity allow it to view objects too old, distant, or faint for the Hubble Spa ...
will contain both a near-infrared spectrograph (NIRSpec
The NIRSpec (Near-Infrared Spectrograph) is one of the four scientific instruments flown on the James Webb Space Telescope (JWST). The JWST is the follow-on mission to the Hubble Space Telescope (HST) and is developed to receive more information ...
) and a mid-infrared spectrograph (MIRI
)
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).
Echelle spectrograph
An Echelle spectrograph
An echelle grating (from French ''échelle'', meaning "ladder") is a type of diffraction grating characterised by a relatively low groove density, but a groove shape which is optimized for use at high incidence angles and therefore in high diffract ...
uses two diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure that diffracts light into several beams travelling in different directions (i.e., different diffraction angles). The emerging coloration is a form of structura ...
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 gratings have a wide spacing and one is blazed
Trail blazing or way marking is the practice of marking paths in outdoor recreational areas with signs or markings that follow each other at certain, though not necessarily exactly defined, distances and mark the direction of the trail.
A bla ...
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
In optics, spherical aberration (SA) is a type of optical aberration, aberration found in optical systems that have elements with spherical surfaces. Lens (optics), Lenses and curved mirrors are prime examples, because this shape is easier to man ...
can be set to zero.
See also
* Circular dichroism
* Cosmic Origins Spectrograph
The Cosmic Origins Spectrograph (COS) is a science instrument that was installed on the Hubble Space Telescope during Servicing Mission 4 (STS-125) in May 2009. It is designed for ultraviolet (90–320 nm) spectroscopy of faint point sources w ...
* Czerny-Turner monochromator
* Imaging spectrometer
An imaging spectrometer is an instrument used in hyperspectral imaging and imaging spectroscopy to acquire a spectrally-resolved image of an object or scene, often referred to as a datacube due to the three-dimensional representation of the data. T ...
* List of astronomical instruments
Astronomical instruments include:
*Alidade
*Armillary sphere
*Astrarium
*Astrolabe
*Astronomical clock
*the Antikythera mechanism, an astronomical clock
*Blink comparator
*Bolometer
*the Canterbury Astrolabe Quadrant
*Celatone
*Celestial sphere
*C ...
* List of light sources
This is a list of sources of light, the visible part of the electromagnetic spectrum. Light sources produce photons from another energy source, such as heat, chemical reactions, or conversion of mass or a different frequency of electromagnetic ener ...
* Long-slit spectroscopy
In astronomy, long-slit spectroscopy involves observing a celestial object using a spectrograph in which the entrance aperture is an elongated, narrow slit. Light entering the slit is then refracted using a prism, diffraction grating, or grism. T ...
* Prism spectrometer
A prism spectrometer is an optical spectrometer which uses a dispersive prism as its dispersive element. The prism refracts light into its different colors (wavelengths). The dispersion occurs because the angle of refraction is dependent on the ...
* Scanning mobility particle sizer A scanning mobility particle sizer (SMPS) is an analytical instrument that measures the size and number concentration of aerosol particles with diameters from 2.5 nm to 1000 nm. They employ a continuous, fast-scanning technique to provide ...
* Spectrogram
A spectrogram is a visual representation of the spectrum of frequencies of a signal as it varies with time.
When applied to an audio signal, spectrograms are sometimes called sonographs, voiceprints, or voicegrams. When the data are represen ...
* Spectrometer
A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the ...
* Spectroradiometer A spectroradiometer is a light measurement tool that is able to measure both the wavelength and amplitude of the light emitted from a light source. Spectrometers discriminate the wavelength based on the position the light hits at the detector array ...
* Spectroscopy
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
*Virtually imaged phased array
A virtually imaged phased array (VIPA) is an angular Dispersion (optics), dispersive device that, like a Prism (optics), prism or a diffraction grating, splits light into its Electromagnetic spectrum, spectral components. It works almost independ ...
References
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)
* Browning, John (1882),
How to work with the spectroscope : a manual of practical manipulation with spectroscopes of all kinds
'
*
External links
Spectrograph for astronomical Spectra
Photographs of spectrographs used in the Lick Observatory from the Lick Observatory Records Digital Archive, UC Santa Cruz Library's Digital Collections
{{Authority control
Electronic test equipment
Signal processing
Measuring instruments
Laboratory equipment
German inventions
Telescope types