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
Δ
λ
, and is closely related to the **resolving power** of the spectrograph, defined as

R
=
λ
Δ
λ
,

where
Δ
λ
is the smallest difference in wavelengths that can be distinguished at a wavelength of
λ
. 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.

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

R
=
c
Δ
v

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

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