Optical Spectrometer: a general class of instruments that collect, spectrally disperse, and reimage an optical signal. The output signal is a series of monochromatic images corresponding to wavelengths present in the light imaged at the entrance slit.

Subclasses of spectrometers include the following:

Monochromator: manually tuned, presenting one wavelength or bandpass at a time from its exit slit.

Scanning monochromator: a motorised monochromator to sequentially scan a range of wavelengths.

Polychromator: provides fixed wavelengths selected at multiple exit slits.

Spectrograph: presents a range of wavelengths at the exit focal plane for detection by multichannel detector or photographic film. Many modern spectrographs have two exits, one with an exit slit, so that one instrument can serve as a spectrograph as well as a scanning monochromator.

Imaging spectrograph: has special corrective optics that maintain better image quality and resolution along the length of the slit (perpendicular to the wavelength dispersion axis) as well as along the dispersion axis in the exit focal plane.

A spectrometer is an apparatus designed to measure the distribution of radiation of a source in a particular wavelength region. Its principal components are a monochromator and a radiant power detector such as a photoemissive cell or a photomultiplier tube. Radiant power enters the entrance slit of the monochromator. The monochromator selects a narrow spectral band of radiant power and transmits it through the exit slit to the photosensitive surface of the detector.

A spectrometer consists of the following elements:

  1. An entrance slit or aperture stop.
  2. A collimating element to make the rays parallel which pass though one point of the entrance slit or field-stop. This collimator may be a lens, a mirror or an integral part of the dispersing element, as in a concave grating spectrometer.
  3. A dispersing element, usually a grating which spreads the light intensity in space as a function of wavelength.
  4. A focusing element to form an image of the entrance slit or field-stop at some convenient focal plane. The image is formed at the exit slit of a monochromator and at the detector focal plane of a spectrograph.
  5. An exit at the focal plane which transmits the light from the image that the focusing system has formed. Usually, this consists of a long narrow slit but there does not need to be a real aperture. The exit field-stop could be, and sometimes is, defined by the detector. In fact, the multichannel system can be designed so that the sensitive area of the detector forms the field-stop.

The monochromator, also known as a monochromatic illuminator, is an instrument designed for isolating a narrow portion of the spectrum.

The two principle applications of this type of instrument are:

  • Used as a filter: the monochromator will select a narrow portion of the spectrum (the bandpass) of a given source, for example to irradiate a sample.
  • Used in analysis: with a photosensitive detector behind the exit slit, the monochromator will sequentially select for the detector to record the different components (spectrum) of any source or sample emitting light.