Compute the 2mm pupil LSF from Geisler (1984, JOSA A, 1, pp. 775-782),
Figure 1. This is also used in Banks, Geisler & Bennett (1987,Vision
Research, 27, 1915-1924).
They give the parameters of a sum of Gaussians LSF as a function of
passed distance. Distance passed in minutes of arc.
Return is normalized to a maximum of 1.