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

{\displaystyle \Delta \lambda }

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

R
=

λ

Δ
λ

,

{\displaystyle R={\frac {\lambda }{\Delta \lambda }},}

where

Δ
λ

{\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 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 installed at ESO's Very Large Telescope, which has a spectral resolving power of up to 100,000.