physical model for irregularity fringe tolerance

[optics_jz]

One tolerancing parameters describing deviations of the surface
contour from the ideal form is (besides power fringes) the parameter
"irregularity fringes".
What exactly are irregularity fringes (as used in OSLO tolerancing
analysis)? On what physical model is it based?

I interpret it as follows:
"power fringes" are systematic surface contour deviations that can be
described as a tolerance for the radius of curvature.

"irregularity fringes" are high-spatial-frequency surface contour
deviations that are stochastic in nature and can be described by a
functional dependence of sagitta deviation (delta_sag) on spatial
frequence (f_sp), i.e. the power spectrum: delt_sag=function(f_spat)
Browsing the literature, I found expressions like delt_sag=A/f_spat^2
(A: constant).

Is this interpretation correct? If not, what is the theory behind
"irregularity fringes" in OSLO?

I appreciate your help.

Regards,
Joachim Zeller