Strehl ratio ($S=I_{\mathrm{peak}}/I_{\mathrm{ideal},\mathrm{peak}}$)

The Strehl ratio compares the peak of an observed point spread function with the ideal diffraction-limited system peak for the same pupil and total flux. It measures how strongly wavefront errors redistribute light away from the central peak.

For small residual phase errors, $S\simeq e^{-{\sigma }_{\varphi }^2}$, where ${\sigma }_{\varphi }=2\pi h_{\mathrm{rms}}/\lambda$. The variance is taken after removing irrelevant constant phase. This approximation to the Strehl ratio is most useful near the diffraction limit of a telescope.