PURPOSE: To easily calculate perpendicular surface illuminance and axial light intensity without using a trigonometrical function chart, etc., by arranging the logarithmic scale of a specific function of the ratio of horizontal distances and heights on an arc on one of a 1st and a 2nd disk and also arranging the logarithmic scale of perpendicular surface illuminance corresponding to it on the arc on the other disk.

CONSTITUTION: While a point to be irradiated is irradiated by a light source with axial light intensity I from a point of height H and a horizontal distance X from the point, the perpendicular surface illuminance E is calculated. For the purpose, this calculator is equipped with the 1st disk 1 and 2nd disk 2 which mutually rotate around the same axis. The logarithmic scale of the function H2 of the height H is arranged on the arc on the 1st disk 1. The logarithmic scale 2a of the axial light intensity I corresponding to it is arranged on the arc on the 2nd disk 2. Further, the logarithmic scale 2b of the function m×(m2+1)-3/2 of the radio (m=X/H) of the horizontal distance X and height H is arranged on one of the arcs on the 1st and 2nd disks. The logarithmic scale of the perpendicular surface illuminance E corresponding to it is arranged on the arc on the other disk.