Spectrometer

The Variable measured is generally the intensity of the light but could be also, for example, the state of polarization. The measured quantity is usually the Wavelength of the light, normally expressed like a fraction of one meter, but sometimes expressed like a certain unit directly proportional to the energy of Photon, such as the Fréquence or the electronvolt, which is inversely proportional to the wavelength. In practice the wavelengths are observed in the form of spectral lines.

Generally an apparatus will function only on one small portion of the spectrum because of the variety of the techniques employed to measure each band of the spectrum. Below the optical frequencies (i.e., the Microwave S, the waves radio, and at the sound frequencies), one employs a closely dependant electronic device, the analyzer of spectrum.

Spectroscopes

Spectrometers known under the name of spectroscopes are used in the spectroscopic analysis to identify materials. The spectroscopes are often used in Astronomie and in some branches of the Chimie. The first spectroscopes simply consisted of a prism with reference marks marking the wavelengths of the light. The modern spectroscopes, such as monochromators, generally employ a Diffraction pattern, a mobile slit, and a photoelectric detector. The whole is automated and ordered by a computer. The spectroscope was invented by Gustav Kirchhoff and Robert Wilhelm Bunsen.

When a matter is carried to Incandescence, it emits a light which is characteristic of the atomic components of this matter. The light emitted by an excited atom is made up various very specific wavelengths which one can regard as the digital fingerprint of the atom. For example, the Sodium has a double known yellow band very characteristic under the name of sodium D-lines with 588.9950 and 589.5924 Nanomètre S: this Couleur is well-known those which already observed a Lamp vapor sodium to low pressure.

The spectroscopes of the beginning of the XIXe century, the light entered by a slit and a lens of diffraction transformed the light into ends luminous rays parallel. The light crossed then a prism (in portable spectroscopes, usually a Prisme of Amici) which refracted the beam of light in a spectrum. This image was then looked in a tube with a scale which made it possible to measure the transposed spectral image.

With the development of the photographic Film, a more precise spectrograph was invented. It was based on the same principle that the spectroscope, but comprised a Camera instead of the tube of viewing. These last years, of the electronic circuits assembled around the tube of Photomultiplicateur replaced the camera, allowing the spectographic analysis in real-time with a precision much higher. Lines of Photodétecteur S are also used in the place of film in spectographic systems. Such a spectral analysis, or spectroscopy, became an important scientific tool to analyze the composition of an unknown matter, to study astronomical phenomena and to confront the astronomical theories.

Spectrographs

A spectrograph is an instrument which transforms a wave entering into a frequency spectrum, or generally a sequence of such a spectrum. There are several kinds of apparatuses indicated under the name of spectrographs, according to the precise nature of the waves.

Use in optics

In optics, the spectrograph separates the light entering according to its wavelength and records the resulting spectrum in a certain detector. It is this type of spectrometer which replaced the spectroscope in the scientific applications.

In astronomy, the spectrographs are of an everyday usage. One assembles them in the center of a Télescope which can be a terrestrial telescope of Observatoire or a telescope embarked in a Spaceship.

The Mars Exploration Rovers (SEA) comprised each one Minis-TES - a thermal Spectromètre miniature of emission (i.e. an infra-red spectrometer).

The first spectrographs employed photographic paper like detector. The classification of the spectrum of the star S and the discovery of the principal sequence, by the Law of Hubble and the sequence of Hubble all were carried out with the spectrographs which used photographic paper. The Phytochrome, a dye resulting from the Plant S.A. discovered using a spectrograph which used alive plants as detector.

The more recent spectrographs use electronic detectors, such as the Capteur CCC which can be employed as well for the visible light UV as that. The precise choice of the detector depends on the wavelengths of the light to measure.

Next the Space telescope James Webb will as well contain a spectrograph close relation-infra-red (NIRSpec) and a spectrometer semi-infra-red (MIRI).

A Echelle spectrograph employs two diffraction patterns, turned each of 90 degree S and placed one close to the other. Consequently one collects the light by an entrance point, and not by a slit, and a second sensor CCC records the spectrum.

Normally, one would have to expect to read the spectrum on the diagonal, but when the two networks have a sufficient step and that one is configured so that only the first order is distinguished, while second is configured to break up several of the higher orders, one obtains a quite separate spectrum on a small ordinary sensor CCC. The use of a small sensor also has the advantage that the Collimateur does not need to be corrected for the coma or the Astigmatisme, because the spherical aberration can be regarded as null.

Acoustic use in

In the field of acoustics, a spectrograph converts a Sound wave into a sound Specter. The first acoustic spectrograph was developed during the Second world war by the laboratories of telephony of Beautiful, and was usually used in science of the Parole, Phonétique, acoustics and research on Audiologie, to be, thereafter, replaced by digital techniques of Treatment of the signal.

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