Autocorrelation Length is a measure that helps to describe the autocorrelation character of a surface. It is described as the shortest horizontal distance in which the autocorrelation function decays to 0.2 in any direction.
For surfaces with predominate lay, Sal is found in a direction perpendicular to the lay of the surface. If a surface is dominated by low frequency components (long wavelength components), Sal will be large. If a surface is dominated by high frequency components (short wavelength components), Sal will be small.