## ABSTRACT

Figure A .l shows the illum ination geom etry th a t is the basis for deriving the 50% -heuristic. I t shows a spherical object w ith radius r which is viewed and illum inated under perspective projection . The following analysis assumes th a t the sphere is illum inated by a point light source a t some distance d from the object center. I t further assumes th a t the cam era is positioned a t the same distance d from the object center and th a t an a rb itra ry phase angle g exists between the viewing and the illum ination direction.