- #1

JohnnyGui

- 796

- 51

I have a small confusion regarding the formula that calculates the luminous flux of a light source, which is given by:

(See this Wiki)

The ##\bar y (λ)## is the so-called luminosity function that corrects for the sensitivity of the human eye for a particular chosen wavelength.

The ##Φ_{e,\lambda}## is the "objective" radiant flux (in watts) per nanometer which is the total power per nanometer, regardless of the sensitivity of the human eye (hence "objective"). You can see from the formula that if a light source emits more than 1 wavelength, an integration has to be done with the limits being the endpoints of the emitting wavelength range, so that it gives the total luminous flux of that light source.

Now, I understand that this formula gives the luminous flux ##Φ_V## in

*lumen*because of the multiplication with ##683.002 lm/W##. However, if we remove this constant, then this means that:

Will give a luminous flux in

*Watts*, right? This means that you can calculate the

*subjective*power according to the human eye's sensitivity in Watts. According to the definition of 1 candela, if we now plug in a wavelength of 555nm of 1 Watt in the formula, we'd get 1/683 Watts out of this formula. This means that the subjective power of 1 candle according to the human eye is 1/683 Watts.

If that's the case, then I don't get why this Wiki says the following:

**"**

*"The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1⁄683 watt per steradian*Notice that it says that 1 candela is the

*radiant*intensity of 1/683 watt, which is the overall objective power (per steradian in this case) and not the

*luminous*intensity (per steradian) as I concluded, which is the subjective power according to the human eye.

What am I concluding wrong here?