Which unit is used to express illuminance in the context of lighting measurements?

• A. Lux.
• B. Foot candle.
• C. Lumen.
• D. Candela.

Answer: (B)

Illuminance is the amount of light hitting a surface per unit area. In practical lighting practice, that per-area measure is expressed in foot-candles when using the US customary system, while the SI equivalent is lux. The other terms describe different quantities: a lumen is the total light emitted by a source (luminous flux), and a candela is light intensity in a given direction. So foot-candle is the unit used to express illuminance in certain measurement contexts, with 1 foot-candle ≈ 10.764 lux. This explains why it fits as the right choice in many lighting measurement contexts.