While I and my friends were talking about some topics in optics, we come to realize that we do not know how to measure the the intensity of light. Does anybody have a good idea? I don't want to hear 'Oh, You use blah blah to measure the Intensity of light' but rather what's being directly measured. Since the Intensity of light relates to Power and the Radius. One should measured the radius. How should one measure the power given off by the light source?