What unit is commonly used to measure light intensity?

The unit commonly used to measure light intensity is the candela. The candela is one of the seven base units in the International System of Units (SI) and is specifically defined as the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 terahertz and that has a radiant intensity of 1/683 watt per steradian. This definition emphasizes the relationship between light intensity and its perceptible effects on the human eye.

Lux, while it is related to light measurement, actually measures illuminance, which represents how much luminous flux is spread over a certain area. Joules are units of energy, not directly related to light intensity. Decibels measure sound intensity or sound pressure level, not light. Therefore, the candela is the most appropriate answer as it directly refers to the measurement of light intensity itself.