cd SI base
Measures: Luminous intensity — how bright, to human eyes
Candela
Brightness as human eyes perceive it — roughly one candle.
Luminous intensityHow it's defined
Defined by fixing the luminous efficacy of 540 THz light (green, where eyes are most sensitive) at exactly 683 lumens per watt. The only SI base unit built around human biology.
a common candle ≈ 1 cd — the historical anchor
Convert — honestly
This unit has no convertible siblings on the map yet.
Where you meet it
- Lumen ratings on light bulbs are built from it
- Phone screen brightness specs
Other ways to measure luminous intensity
On this map, it stands alone for its quantity.
Source
This page is an authored summary of the authoritative definition — checkable one click away.
BIPM — The International System of Units (SI Brochure, 9th ed.)