Patent US 5,481,408

The distance between the base point of the light source and the curvature start point of the virtual reflection mirror is preset. Then the distance between the curvature start point of the virtual reflection mirror and the start point of the object being illumined is set. The ray of light striking the entire object is determined by the fourth straight line. As the fourth straight line is progressively moved, the third straight line is also progressively moved from the curvature start point of the virtual reflection mirror to describe the locus of the curvature of the virtual reflection mirror. This locus is obtained as the light distribution data for the object.