Footcandles and Lumens

Module: Module 1 - Principles of Lighting

Activity: Overview

Units of light

The illumination 1 foot from a standard candle is called 1 foot-candle. The "amount of light" or light flux falling on 1 square foot of surface illuminated to 1 footcandle is called 1 lumen.

So lamps put out so many lumens of light. This can be spread out over an area.

For example, a 100 watt incandescent lamp puts out 1750 lumens (initially). If we take all this light and spread it out over a room 12 feet x 12 feet (i.e. 144 square feet), then each square foot is, on the average, receiving 1750/144 = 12.2 lumens.

This means that the room is lit (to an average of) 12.2 footcandles.

Of course, the illumination may be 30 footcandles directly under the lamp and 3 footcandles at the corners of the room. Just because you got the average footcandles right does not mean you did a good job on lighting design!

A typical office or conference room is lit to about 50 footcandles.

If the task involves more exacting detail you could light it up to 100 or even 200 footcandles. Outdoor parking lots are lit only to about 2 or 3 footcandles; home plate in a baseball game (in the major leagues) is lit to about 250 footcandles.