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Candela (cd)

Quantity: Luminous intensity · Category: SI base

The candela is the SI base unit of luminous intensity — how bright a light source appears to the human eye in a given direction. It is defined by fixing the luminous efficacy of 540 THz radiation at exactly 683 lumens per watt.

Relation to SI

Base unit — defined by K_cd = 683 lm/W at 540×10¹² Hz (exact)

Examples

  • An ordinary candle shines with about 1 candela.
  • A car headlight beam can exceed 10,000 candela in its brightest direction.
  • Screen brightness is measured in candela per square metre (nits).