  The phase structure function accumulated by two parallel rays after
transmission through a layer of turbulent air is best known by a
proportionality to the 5/3rd power of the lateral distance in the aperture,
derived from an isotropic Kolmogorov spectrum of the refractive index. For a
von-Karman spectrum of the refractive index, a dependence involving a modified
Bessel function of the ratio of the distance over the outer scale is also
known.
  A further standard proposition is a proportionality to the path length
through the atmospheric layer. The manuscript modifies this factor through a
refined calculation of an integral representation of the structure function.
The correction establishes a sub-linearity as the lateral distance grows in
proportion to the layer thickness; it is more important for large than for
small outer scales.
