Abstract
Likening a perspective-view looking at a cubic light source from a calculating point to a flat surface source, the illuminance is readily obtainable by a simple procedure using a “modified contour integration formula”, even if a shape of the light source is complex such as a toroidal source. However, the external contour takes on a curious property of tentatively-named “abnormal continuation” when it is in a nearest state being positioned at which an toroidal source is looked on its slender side, therefore the correct value of illuminance is unobtainable resulted from a careless execution of the afore-mentioned definite integral due to the disorder of calculated value. Now, analyzing the cause of such “abnormal continuation” phenomenon is made, and the means of solving it will be presented hereinafter.
