I searched for it but couldn't find any answer for this.
For a light source is it simply the average of the full spectrum power?
And how do you compute it for an image? How can you tell the difference between a yellow object lighted by a brighter light and a white object lighted by a yellowish light? Or maybe you can accurately compute it in front of a white sheet?
Edit:
I am talking about a lighting Kelvin temperature (for example of a bulb) rather than the actual temperature of the light source.
