Luminous flux
As mentioned earlier, when describing "the light energy emitted by a visible light source per unit time", it is not appropriate to use only "power" to measure, and the spectral response of the human eye to light of different wavelengths must be introduced. The introduction of "luminous flux" just solves this problem. The International Commission on Illumination (CIE) selected 555nm, the most sensitive wavelength of the human eye under photopic vision, as the reference wavelength for the conversion of "power" to "luminous flux", and stipulated that the 555nm wavelength monochromatic light with a power of 1W corresponds to a luminous flux of 683 lumens. Lumen is the unit of luminous flux, and the English abbreviation is lumen or lm. As for why it is "683", it is related to the history of describing lighting with candlelight in the early days, which is not shown here.
For the luminous flux of other wavelengths of light, the corresponding luminous flux at 555nm is multiplied by the photopic spectral efficiency. The luminous flux of a broad-spectrum light source also needs to consider the spectral distribution of the light source itself. CIE also defines luminous flux under scotopic vision, but it is not commonly used.
