Spectrographs

Spectrographs 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 often has a 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.

As the used detectors usually have the form of some detector array, spectrographs are also called array spectrometers.

A spectroradiometer is a special type of spectrograph, an instrument that has been calibrated to measure absolute radiometric quantities like spectral radiance or irradiance, which is not true for all spectrographs.

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

The optical design of spectrographs can vary significantly:

• Czerny–Turner: A widely used configuration employing two concave mirrors (collimating and focusing) and a planar diffraction grating. It is flexible and minimizes spherical aberrations but must be corrected for astigmatism if spatial resolution along the slit height (imaging spectrograph) is required.
• Concave holographic gratings: These combine dispersion and focusing functions in a single optical element. This design reduces the component count, improving robustness and throughput, and often exhibits lower stray light, making it suitable for compact OEM modules.
• Echelle spectrographs: Use an echelle grating (operating at high diffraction orders) often combined with a cross-disperser (prism or grating) to separate overlapping orders. This allows covering a wide spectral range with very high spectral resolution on a 2D detector array.
• Transmission gratings: Utilized often in combination with lenses rather than mirrors. Volume phase holographic (VPH) gratings in transmission can offer very high efficiency and robust alignment.

Modern spectrographs rely heavily on the performance of the integrated digital detector array. The choice of detector material determines the spectral sensitivity range: silicon for UV to near-infrared (approx. 200–1100 nm) and InGaAs for short-wave infrared (approx. 900–1700 nm or up to 2.5 μm).

Key detector features include:

• Cooling: Thermoelectric cooling (TEC) reduces dark current, which is critical for long exposure times and low-light detection.
• Back-thinning: For silicon CCDs, back-thinned technology significantly enhances quantum efficiency in the UV region.
• Readout speed: CMOS arrays often allow for faster readout speeds compared to CCDs, which is beneficial for time-resolved spectroscopy.

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.