Setting aside it’s basically a theological question, not a technical one -
Watts. When you buy a lightbulb you check how many watts it ‘has’. The lightbulb’s brightness derives from that.
‘Candles’ is not a SI unit and shouldn’t be used. The SI unit is candela. It is defined as
‘The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×10^12 hertz and that has a radiant intensity in that direction of 1⁄683 watt per steradian.’
And look - there are the watts again!
The definition describes how to produce a light source that (by definition) emits one candela. Such a source could then be used to calibrate instruments designed to measure luminous intensity.
If we want to discuss ‘brighness’, let’s first agree on whether we look at Luminous energy, Luminous flux, Luminous intensity, Luminance, Illuminance, Luminous emittance, Luminous exposure, Luminous energy density, Luminous efficacy or Luminous efficiency.
Or, to keep things simple: how many watts should the lightbulb have?
(edited for The Devil’s typos)
