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The spectral resolution of a spectrograph, or, more generally, of a frequency spectrum, is a measure of its ability to resolve features in the electromagnetic spectrum. It is usually denoted by Δλ{displaystyle Delta lambda }, and is closely related to the resolving power of the spectrograph, defined as

R=λΔλ{displaystyle R={lambda over Delta lambda }},

where Δλ{displaystyle Delta lambda } is the smallest difference in wavelengths that can be distinguished at a wavelength of λ{displaystyle lambda }. For example, the Space Telescope Imaging Spectrograph (STIS) can distinguish features 0.17 nm apart at a wavelength of 1000 nm, giving it a resolution of 0.17 nm and a resolving power of about 5,900. An example of a high resolution spectrograph is the Cryogenic High-Resolution IR Echelle Spectrograph (CRIRES) installed at ESO's Very Large Telescope, which has a spectral resolving power of up to 100,000.^[1]

Doppler effect

The spectral resolution can also be expressed in terms of physical quantities, such as velocity; then it describes the difference between velocities Δv{displaystyle Delta v} that can be distinguished through the Doppler effect. Then, the resolution is Δv{displaystyle Delta v} and the resolving power is

R=cΔv{displaystyle R={c over Delta v}}

where c{displaystyle c} is the speed of light. The STIS example above then has a spectral resolution of 51 km/s.

IUPAC definition

IUPAC defines resolution in optical spectroscopy as the minimum wavenumber, wavelength or frequency difference between two lines in a spectrum that can be distinguished.^[2] Resolving power, R, is given by the transition wavenumber, wavelength or frequency, divided by the resolution.^[3]

See also

• Angular resolution


• Resolution (mass spectrometry)

References

Further reading

• Kim Quijano, J., et al. (2003), STIS Instrument Handbook, Version 7.0, (Baltimore: STScI)


• Frank L. Pedrotti, S.J. (2007), Introduction to optics, 3rd version, (San Francisco)