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Brightness has a unit of its own. The candela measures light as a human eye would judge it.

Physics Facts, sourced to NIST.

What it means

The candela is the odd one out among the base units, because it has a person built into it. A watt of green light looks far brighter to us than a watt of deep red, so the definition weights the raw power by the sensitivity of typical human vision. It is the only unit in the international system whose value depends on the biology of the people using it.

The claim comes from NIST, and the page it comes from was read on 2026-08-27 to confirm it still says this.

This is not the kind of claim that goes out of date, which is why it was set as a puzzle rather than a fact with a shelf life.

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