In this experiment, we wondered if illumination affected the dominant angle detected. To check, a photograph was taken in which the photographer generated a shadow by standing in front of the morning sun. OrientationJ was then run on selected segments across the image to see any discontinuities correlating with the caste shadow were evident. Looking at the image below from left to right, a steady increase in the acuity of the dominant angle was accompanied by a steady decrease of coherency; the central shadow did not seem to interrupt this continuity. Our conclusion: differences in illumination do not affect our results.
Check it our for yourself, this directory contains the raw image.