question between lumen to watt


Newly Enlightened
Mar 17, 2005
please i am trying to solve the problem about candle and lumen transfer to watt or watt/m2 . such as a lamp have 1000cd, or 12000lumens, how to know its intensity like solar energy in w/m2


Flashlight Enthusiast
Apr 15, 2003
I do not think there is an easy transform. Luminous intensity -- the candela, cd -- is a base unit in the SI unit sysem, just like mass, time, or temperature. It can't really be converted to anything else directly.

Intensity to flux -- the lumen, lm -- is easy. Just integrate intensity over the solid ange in steridians, sr, to get lm.

But Watts is energy [edit: W is the time rate of energy; the Joule is the SI base unit for energy] -- Joules per second, J/s -- not luminous intensity. In order to go from cd to J you have to know something of the physics of light and the wavelength you are measuring. Shorter wavelengths have more energy. I don't know how they compare in luminous intensity.

I have never done such a calculation, so I don't know the details of how to do it. I would consult a text on optical physics.

Hope this helps.