How many candlepower in a watt?

Answer 1

A watt is a unit of power; candlepower is measured in candelas (sometimes called candles), and is a unit of luminous intensity. A candela is a lumen per steradian. A lumen is a unit of luminous flux, which is radiant power tempered by the response of the human eye. Thus, one watt of radiant power at the wavelength of peak response of the eye for bright light (about 555 nanometer) equals 683 lumen. That's green light; light of other colors will equal less.

A steradian is a unit of solid angle; there are 4*pi steradians in a sphere. So clearly, we cannot convert watts to candlepower, just as we can't convert pounds to inches. But we CAN convert if we make a few assumptions. Candlepower depends on the direction from the light source. For a given light source, reflectors and lenses will greatly affect the numbers. So let's assume an isotropic source (one that radiates equally in all directions). One candlepower would then equal 4*pi lumen. If the wavelength of the light is 555 nm, then 4*pi/683 (or about 0.0184 watt) would be the equivalent power. In the case of "white" light, that number would be about 0.14 or so (depending on the color temperature or "whiteness").

Please bear in mind that the power referred to above is the radiant power, not the power applied to the light source. In the case of incandescent lamps, much applied power is converted to heat and infrared radiation.

Answer 2

21

Answer 3

345