Physics Talk

Illuminance describes the amount of light that arrives at each unit of surface area from a light source. In the same way that the butter gun works, the illuminance near a single point source of light can be calculated using the inverse square law:

where E is illuminance in footcandles (fc),

I is luminous intensity of the light source in candela (cd), and

D is distance in feet (ft.).

The footcandle (fc) is the unit of illuminance widely used in the United States. One footcandle is the illuminance at a distance of 1 ft. from a 1 cd source.

Example:

Demonstrate that the above equation shows that the illuminance, E₁, 1 ft. away from a 100 cd point source of light is 100 fc. Calculate the illuminance E₂ at 2 ft. and E₃ at 3 ft. from the source.