What unit is used to measure luminous flux?

• A. Lumens
• B. Candela
• C. Lux
• D. Watts

Answer: (A)

Luminous flux represents the total amount of light emitted by a source in all directions per unit time, as perceived by the human eye. The unit for this quantity is the lumen, which captures how bright the light appears overall. This differs from luminous intensity, which measures light in a specific direction and is given in candelas. It also differs from illuminance, which describes how much light lands on a surface per unit area and is measured in lux. Watts measure power, not perceived brightness, so they don't quantify luminous flux. Lumens incorporate the eye’s sensitivity to different wavelengths, making them the appropriate measure of how bright a light source seems.