What measurement is used for luminous intensity?

Luminous intensity is quantified in candelas, which is the SI unit specifically defined for this purpose. Candelas measure the amount of light emitted by a source in a particular direction, which is crucial for understanding how lighting works in various environments. The definition is based on the light emitted in a specific spatial direction, standardized to the intensity of a light source that emits monochromatic radiation of frequency 540 x 10^12 Hz and has a radiant intensity of 1/683 watts per steradian.

In contrast, lux, foot-candles, and nits serve different purposes. Lux measures illuminance, or the amount of light that falls on a surface, while foot-candles are another unit of illuminance, representing the illumination produced by one candle at one foot away from the surface. Nits, on the other hand, measure luminance, which describes the amount of light that is emitted or reflected off a surface at a given angle. Understanding these distinctions helps clarify why candelas is specified for luminous intensity while the others pertain to different aspects of light measurement.