Combining (8) and (10) we find the expression for the refraction index fluctuation structure function reported below

In this formula, the quantity C% is called the constant of refraction index fluctuations structure function and is a measurement of the strength of the atmospheric turbulence in the considered conditions.

3.2 The Phase Structure Function

To compute the phase perturbation experienced by a plane wave propagating through a medium with refraction index fluctuations expressed by (11), we will considered the geometry given in Fig. 7.

Even in this case, the phase fluctuations will be described using a structure function but the phase structure function is given as a two-dimensional function instead of a three-dimensional function (used for the refraction index structure function). In particular, the phase structure function measures the phase difference between two points at distance p when both points are located in a single plane perpendicular to the propagation direction of the considered plane wave. This plane is usually taken as containing the telescope

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