Fig. 6. Illustration of a hollow sphere having a total surface area of 12.57 (4π) square feet. The solid angle subtended by lines A, B, C, and D is 1 steradian. The source of the illumination is 1 foot from the inner wall of the sphere; thus, there is a luminous flux of 1 lumen falling on each square foot. The luminous intensity, therefore, is 1 candela. The illumination at any point on the sphere is 1 foot-candle (one lumen per square foot). It is assumed that the light source is a uniform point source of 1 candela located at the exact center of the sphere and that the surface of this sphere has zero reflectance. (Modified from Kaufman JE, Christensen JF: I.E.S. Lighting Handbook, 5th ed. New York, Illuminating Engineering Society, 1972)