Spectrographs

Author: the photonics expert Dr. Rüdiger Paschotta

Spectrographs, also called spectroradiometers, are optical instruments which belong to the class of spectrometers. A spectrograph contains a fixed diffraction grating or some other kind of polychromator (a device which can spatially separate different wavelength components of light) and some kind of multi-channel photodetector (e.g. a photodiode array) for measuring the spectral light intensities. (Early versions of spectrographs used photographic plates for recording spectra.) That way one can measure the optical spectrum of a light source. In contrast, some other kinds of spectrometers use a rotating grating and/or a moving detector. Compared with those, a spectrograph has a tentatively simpler setup and can acquire spectra faster, but may not reach the same performance e.g. in terms of spectral resolution or width of the covered spectral region.

The operation principles of spectrographs are explained in the article on spectrometers.

Applications of Spectrographs

Some typical applications of spectrographs are:

• Stellar and solar spectrographs are used for analyzing in detail the radiation from stars. For example, one can measure the locations and strengths of certain absorption lines (Fraunhofer lines) for measuring chemical compositions and relative velocities.
• With a laboratory spectrograph, one may spectrally analyze fluorescence light e.g. from gas discharges or from active optical fibers.
• In spectral phase interferometry, one often requires a spectrograph for measuring the positions of minima and maxima in optical spectra. An intensity calibration is often not required.
• Spectrographs are also used for other methods of pulse characterization, for example for frequency-resolved optical gating.

More to Learn

Encyclopedia articles:

• spectrometers
• polychromators
• optical spectrum
• laser beam characterization