Three-Dimensional Unit Sphere
The unit sphere, located on the front wall of the lighting classroom, is a sphere with a one-foot radius, which defines the relationship between lumens, candela, and footcandles. Imagine a candle flame centered within a sphere that is one foot in radius. That point source, of uniform intensity equal to one candela, produces an amount of light flowing through one square foot of the sphere's area that is defined as one lumen. The amount of light falling on that one square foot of the sphere is equal to one footcandle. Since the area of the sphere is equal to 4π, then the amount of light flowing through the sphere is 4π lumens, or about 12.57 lumens. In other words, a one candela source produces 12.57 lumens and an illuminance of one footcandle at a distance of one foot. he source and the surface.