Which unit is used to measure luminous intensity?

Luminous intensity is specifically measured in candelas. One candela is defined based on the light emitted by a source in a particular direction, and it quantifies how bright that light appears to the human eye. This unit is fundamental in photometry, the science of measuring visible light as perceived by human sight.

While lux measures illuminance, which is the amount of light falling on a surface, and watt measures power in terms of energy per time (applicable to all forms of energy, including light), neither of these units relates specifically to the intensity of light emitted from a source in a specific direction. Joules represent energy, not light intensity. Thus, candela is the correct and relevant unit for measuring luminous intensity, as it captures the quality of light specifically in terms of human visual perception.