For Acoustic uses in spectrographs of sound waves, see below.

A spectrometer is an optical instrument for measuring properties of light over some portion of the electromagnetic spectrum. The measured variable is often the light intensity but could also be the polarization state, for instance. The independent variable is often the wavelength of the light, usually expressed as some fraction of a meter, but it is sometimes expressed as some unit directly proportional to the photon energy, such as wavenumber or electron volts, which has a reciprocal relationship to wavelength. A spectrometer is used in spectroscopy for producing spectral lines and measuring their wavelengths and intensities. Spectrometer is a term that applied to instruments that operate over a very wide range of wavelengths, from gamma rays and x rays into the far infrared. 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. The radio electronics spectrum analyzer is a closely related device.

Spectroscopes

Spectrometers known as 'spectroscopes' are used in spectroscopic analysis to identify materials. Spectroscopes are used often in astronomy and some branches of chemistry. Early spectroscopes were simply a prism with graduations marking wavelengths. Modern spectroscopes 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 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. 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 that refracted the beam into a spectrum because different wavelengths were refracted different amounts because of dispersion. This image is then viewed through a tube with a scale that was transposed upon the spectral image, enabling its direct measurement. Particular light frequencies give rise to sharply defined bands on the scale which can be thought of as fingerprints. The element sodium has a very characteristic double yellow band known as the Sodium D-lines at 588.9950 and 589.5924 nanometers.

With the development of photographic film, the more accurate 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.

See also:

• mass spectrometer
• Monochromator
• Colorimeter

Spectrographs

A spectrograph is an instrument that transforms an incoming time-domain waveform into a frequency-domain 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.

Optical uses

In optics, a spectrograph separates incoming light according to its wavelength and records the resulting spectrum in some detector. It is a type of spectrometer and superseded the spectroscope for scientific applications.

In astronomy, spectrographs are widely used. These are installed at the focus of a telescope which may be either in a ground-based observatory or in a spacecraft.

The first spectrographs used photographic paper as the detector. The star spectral classification and discovery of the main sequence, Hubble's law and the Hubble sequence were all made with spectrographs that used photographic paper.

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.

Acoustic uses

In acoustics, a spectrograph converts a sound wave into a sound spectrogram. The first acoustic spectrograph was developed during World War II at Bell Telephone Laboratories, and was widely used in speech science, acoustic phonetics and audiology research, before eventually being superseded by digital signal processing techniques.