Which SI unit measures luminous intensity?

• A. Candela (cd)
• B. Ampere (A)
• C. Kelvin (K)
• D. Mole (mol)

Answer: (A)

Luminous intensity describes how bright a light source appears in a specific direction, taking into account human eye brightness. The SI unit for this quantity is the candela, which is one of the base units and is defined by a standard light source’s radiant intensity at a given wavelength. This makes candela the appropriate unit for expressing how much light is emitted in a particular direction.

Ampere measures electric current, Kelvin measures temperature, and a mole measures amount of substance, so they don’t pertain to light output in a direction. Related concepts like lumen (luminous flux) and lux (illuminance) describe total light flow and illumination, not intensity in a direction.