where is the root-mean-square deviation of the wavefront measured in wavelengths. The Strehl ratio is the ratio of peak focal intensities in the aberrated and ideal point spread functions (Born and Wolf 1999).
References
Born, M. and Wolf, E. Principles of Optics: Electromagnetic Theory of Propagation, Interference, and Diffraction of Light, 7th ed. Cambridge, England: Cambridge University Press, 1999.
