What is the SI unit for luminous intensity?

The SI unit for luminous intensity is the candela, represented by the symbol "cd." Luminous intensity is defined as the amount of light emitted by a source in a particular direction per unit solid angle. Specifically, one candela corresponds to a source that emits monochromatic radiation of frequency 540 terahertz and has a radiant intensity of 1/683 watt per steradian. This unit measures how strong the light appears to a human observer in a given direction.

Understanding luminous intensity is important in various fields, including lighting and photography, where differentiating how light intensity is perceived by the human eye is key. Other options presented, such as "lm" (lumen), "W" (watt), and "lx" (lux), refer to different concepts related to light. Lumens measure the total amount of light emitted by a source, watts quantify energy (or power), and lux measures illuminance, which is the total luminous flux received per unit area. Knowing these distinctions is essential in studying photometry and its applications in real-world scenarios.