A candela is the SI base unit of which quantity?

• A. Luminous intensity
• B. Luminous flux
• C. Illuminance
• D. Luminance

Answer: (A)

Candela measures luminous intensity, which is the brightness of a light source in a specific direction per unit solid angle. It focuses on how much light is emitted in that direction, not the total amount of light produced or how bright a surface appears. Luminous flux (lumen) is the total light emitted in all directions, illuminance (lux) describes how much light lands on a surface, and luminance (candela per square meter) describes the brightness of a surface as seen by the eye. Since candela is defined as the base unit for directional light output, it is the correct unit for luminous intensity.