1) Physical quantity 
In photometry, luminous energy is the perceived energy of light. This is sometimes called the quantity of light. Luminous energy is not the same as radiant energy, the corresponding objective physical quantity. This is because the human eye can only see light in the visible spectrum and has different sensitivities to light of different wavelengths...
[n] - the energy associated with visible light
noun the energy associated with visible light
light (def. 2a).