Definition
The SI base unit of luminous intensity, representing the amount of visible light a source emits in a particular direction. One candela is approximately the light output of a single common wax candle viewed from the side.
Plain English
A unit used to measure how bright a light source appears when looking at it from a given direction. The higher the candela value, the brighter the light looks from that angle.
Context Anchor
Seen in discussions of aircraft lights, airport lights, runway lighting, beacons, and lighting equipment specifications.
Derivation
From the Latin candela, meaning 'candle.' The unit was originally defined by the brightness of a standard candle flame, which is why the name stuck even after the modern scientific definition replaced the actual candle reference.
Why Pilots Care
Determines whether runway and aircraft lights are bright enough to be seen at required distances during night or low-visibility operations.
Analogy
A flashlight can look very bright when pointed at you and much dimmer from the side. Candela describes that directional brightness.
Intuition Check
Candela does not mean the physical size of a light or how much electrical power it uses. It means how intense the visible light is in a specific direction.
Example Sentence 1
The aircraft's anti-collision strobe light is rated at 400 candela, well above the minimum required for night visibility.
Example Sentence 2
The anti-collision light on the aircraft is certified to a minimum candela rating to ensure it is visible to other traffic.