Time-delay-induced mean-square error in adaptive optics

For atmospheric turbulence-induced wave-front distortion, the mean-square change in the measured wave-front distortion for two measurements made at times separated by an interval Δt is shown to be equal to (Δt/τG)5/3, where τG is called the Greenwood time constant. It is shown that τG is related to the Greenwood frequency fG by the equation τGfG = 0.134.