How is illuminance expressed?

Illuminance refers to the amount of light that falls on a specific surface area and is typically expressed in terms of illumination intensity per unit area. The correct way to express illuminance is in lux and foot-candles.

Lux is a metric unit that defines illuminance as one lumen per square meter, providing a standard for measuring light intensity in a given area. Foot-candles, which are used primarily in the United States, represent illuminance in terms of lumens per square foot. This means both lux and foot-candles are directly connected to the impact of light on a surface, making them appropriate units for describing illuminance.

Other units mentioned in the options, such as lumens per watt and candelas, do not measure illuminance. Lumens per watt is a measure of luminous efficacy, which indicates how well a light source converts electrical power into visible light, while candelas measure luminous intensity in a specific direction rather than the illumination over a surface area. Thus, the focus on lux and foot-candles accurately captures the definition and measurement of illuminance.