lumen

n. A unit of subjective light intensity or the illumination equal to the flux of 1 candela of luminous intensity into a solid angle of 1 steradian. This is a scientific standard of the intensity of light that the eye actually perceives in a given unit of visual space. It differs from measures of radiant light, which are not adjusted to the perceptive capacities of the human eye but measure total illumination within a given visual space which could be very bright but unseen by a human eye if the wavelength of light were outside that seen by humans.