An improved means of finding the direction of a light source illuminating a sphere is presented. In particular, the vertical slant angle of a light source is computed using a new approach. The planar tilt angle of a source is known to be related to the average of the local gradient vectors of an image of a lit sphere. This research uncovers a straightforward link between the length of the average of the local normalized gradient vectors of an image and the slant angle of the corresponding light source. The new technique is far more effective when compared with existing approaches over a variety of test images.
Xue Dong Yang,
"Improved method of finding the illuminant direction of a sphere," Journal of Electronic Imaging 22(1), 013035 (27 March 2013). https://doi.org/10.1117/1.JEI.22.1.013035