Question

Lux is the SI unit of:

• A. Intensity of illumination
• B. Luminous efficiency
• C. Luminous flux
• D. Luminous intensity

Hint:

Lux equals lumen per square metre, think about which quantity that ratio describes.

Answer 1

The Correct Option is A

Start from the defining formula for illuminance, $E = F/A$, where $F$ is luminous flux in lumen and $A$ is the area in square metre that the flux falls on.

1. Dividing lumen by square metre gives a combined unit, and this combined unit has the special SI name lux.
2. Luminous intensity is a different quantity altogether, and it carries the unit candela, not lux.
3. Luminous flux itself is measured directly in lumen, not lux.
4. Luminous efficiency compares lumen output to watt input, giving lumen per watt, again not lux.

Only illuminance, that is intensity of illumination, matches the unit lux exactly, so option A is correct.