If my understanding of candelas is correct, I can move onto lumens. But the Earth is 1.5 x 10^8 metres from the sun.Therefore, compared to a surface 1 metre from a candle, the Sun provides 683,000 times more power at the surface of the Earth. Let's say the power arriving on the Earth's surface from the Sun is 1kW per square metre.On the surface of an imaginary sphere, 1 metre from a candle, the power arriving at each square metre will be about 1/683 Watts.Our eyes interpret that power as brightness, but really, it's Watts. A source that emits monochromatic radiation of frequency 540×10^12 hertz and that has a radiant intensity in that direction of 1/683 Watts per steradian, has an intensity of 1 candela.Īn ordinary candle has a luminous intensity of about 1 candela.Ĭandelas, and therefore all related light measurements, at the most fundamental level are measuring Watts – the power of the light.
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