The average size of a speckle, if circular area of illumination or circular aperture is used in the imaging system. In an imaging system, it is assumed that the size of the speckle incident on the lens pupil is extremely small when compared to the diameter of the lens. So the lens surface is treated as a diffuse surface. The autocorrelation and power spectral density are calculated as such by applying the lens parameters. The autocorrelation is independent of any aberration present in the imaging system.
Aberrations only effect the phase of the intensity incident at the scattering spot.
Dainty, J. C., Second edition, Springer Verlag, 1984. 2. Eng ., Vol. Optics , Vol. Francon, M., Laser Speckle and Application to Optics , Academic Press, New York, 1979.11. Francon, M., Laser Speckle and Application to Optics , Academic Press, New York, 1979. 5.