Which unit is used to express luminous intensity in the International System of Units?

Luminous intensity is a measure of the perceived power of light emitted by a source in a particular direction. In the International System of Units (SI), the unit specifically designated to express luminous intensity is the candela.

The candela is defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10^12 Hz and that has a radiant intensity in that direction of 1/683 watt per steradian. This definition emphasizes the relationship between light perception and physical measures of energy.

Other units provided, such as joules and watts, relate to energy and power, not directly to the perception of light. Joules measure energy, while watts quantify power, which is energy per unit time. Lux, on the other hand, measures illuminance and is defined as one lumen per square meter, which takes into account the area over which the light is spread rather than the intensity of the source itself. Hence, candela is the correct choice for expressing luminous intensity in the SI system.