Which SI base unit measures luminous intensity?

Which SI base unit measures luminous intensity?

• A. Mole
• B. Meter
• C. Candela
• D. Kelvin

Answer: (C)

Luminous intensity describes how bright a light source appears in a specific direction. The unit that measures this quantity in the SI system is the candela. Candela is one of the seven SI base units, alongside meter (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), and mole (amount of substance). The candela is defined so that a source emitting a precise amount of luminous energy per solid angle in a given direction has a defined brightness; specifically, the standard uses monochromatic light at 540 THz with radiant intensity of 1/683 watt per steradian to define 1 candela. Other options measure different physical quantities (length, temperature, amount of substance), not luminous intensity.

Luminous intensity describes how bright a light source appears in a specific direction. The unit that measures this quantity in the SI system is the candela. Candela is one of the seven SI base units, alongside meter (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), and mole (amount of substance). The candela is defined so that a source emitting a precise amount of luminous energy per solid angle in a given direction has a defined brightness; specifically, the standard uses monochromatic light at 540 THz with radiant intensity of 1/683 watt per steradian to define 1 candela. Other options measure different physical quantities (length, temperature, amount of substance), not luminous intensity.