The structure function (SF) represents the average height difference squared as a function of separation. The linear SF
has been used in astronomy and captures data of all spatial frequencies. However, it does not capture anisotropy on the
surface. The one-quadrant area SF has been introduced to represent surface roughness information, but it is inadequate
when surfaces are rotationally varying, as is frequently the case for optical surfaces. The recently introduced twoquadrant
representation of the area SF can be calculated for any aperture shape, for all spatial content and isotropies.
This paper describes the physical interpretation of the two-quadrant area SF and the comparison to area power spectral
density (PSD) and area autocorrelation function (ACF) for a range of optical surfaces.