Luminous flux, or luminous power, is the measure of the perceived power of light. It differs from the measure of the total power of light emitted, termed ‘radiant flux’, in that the former takes into account the varying sensitivity of the human eye to different wavelengths of light. In other words, it is photometrically weighted radiant flux (power). The SI unit of luminous flux is the lumen (lm). One lumen is defined as the luminous flux of light produced by a light source that emits 1 cd of luminous intensity over a solid angle of 1 sr. In other systems of units, luminous flux may have units of power. ‘Lumen’ is the unit of luminous flux, defined as flux emitted through unit solid angle (one steradian) from a directional unit point source of 1 cd. To avoid reference to candela, and to use as photometric base unit, the lumen may be defined as the luminous flux of monochromatic radiant energy whose radiant flux is 1/683 W and whose frequency is 540 × 1012 Hz (closely equal to a wavelength of 555 nm). As with luminous intensity, the luminous flux of light with other wavelengths can be calculated using the CIE photometric curve.
From: Asim Kumar Roy Choudhury, in Principles of Colour and Appearance Measurement, 2014