- Candela ("cd")
Candela (cd) is the SI (International System of Units) base unit of luminous intensity. It measures the amount of visible light emitted in a specific direction. Unlike lumens, which measure total light output, candela accounts for directional intensity, making it critical for emergency lighting systems and exit sign systems and visual alarm devices (VADs).
Although candela is a linear unit, human perception of brightness is logarithmic. This means that increasing the candela value does not result in a directly proportional increase in perceived brightness—a tenfold increase in candela does not appear ten times brighter to the human eye.