In photometry, luminous energy is the perceived energy of light. This is sometimes also called the quantity of light.
Luminous energy is not the same as the 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 within the spectrum. When adapted for bright conditions (photopic vision), the eye is most sensitive to light at a wavelength of 555 nm. Light with the same power at longer or shorter wavelengths has a lower luminous energy.