What is a foot candle?

• A. The total lumens emitted by the lamp.
• B. Lumens per square meter.
• C. Illuminance measured in foot-candles.
• D. The amount of light produced by a lamp (lumens) divided by the area that is illuminated.

Answer: (D)

Foot-candle is a unit of illuminance, telling you how much light reaches a surface. It’s defined as lumens per unit area on that surface, specifically lumens per square foot. So if a lamp emits a total of 1000 lumens and those lumens land evenly on 100 square feet, the illuminance is 1000 divided by 100, which equals 10 foot-candles. That direct ratio—lumens divided by the illuminated area—is what foot-candles measure.

The other ideas don’t fit as well: total lumens describes how much light a lamp emits overall (not how much lands on a surface), while lumens per square meter is lux (the metric version), and saying illuminance is measured in foot-candles is true but doesn’t express the calculation; D gives the exact relationship that defines the unit.