Aperture averaging of the two-wavelength intensity covariance function in atmospheric turbulence.

The influence of aperture averaging on the two-wavelength intensity covariance function was experimentally determined for visible (0.63 microm) and infrared (1.06 microm) collinear, approximately spherical beams which propagated through the earth's turbulent atmosphere. Range varied from 1300 to 3250 m, and due to the prevailing atmospheric conditions, most measurements were made in the strong turbulence regimes. Results show that (1) the covariance function monotonically decreases as the receiver aperture size increases; (2) the correlation coefficient attains high values > or = 0.7) even for a relatively small aperture size of 5 mm; (3) while the single wavelength probability distribution of the intensity is approximately lognormal, the experimental two-wavelength conditional probabilities are higher than those predicted by the lognormal model.