We know that the light source radiates electromagnetic waves (light waves) of different wavelengths to the surrounding space, and the propagation process of light is the propagation process of energy. The radiant energy passing through an area in unit time is called the radiant flux passing through the area, which objectively reflects the amount of radiated energy from the light source. The radiant power it emits can be expressed in units of "W" (Watt) (i.e. radiant flux).
This radiation flux includes electromagnetic waves of various wavelengths, such as infrared, ultraviolet, visible light, etc. For visible light, the human eye has different sensitivities to visible light of different wavelengths, that is, light waves of different wavelengths but with the same radiant flux have different brightness perceptions on the human eye. For example, yellow and green light are the brightest, while red and purple light make people feel much darker.
This characteristic of the human eye is called spectral visual efficiency (Vλ). Due to this characteristic of the human eye, we cannot directly use the radiant flux of the light source to express the size of visible light energy. Instead, we must use the light perception of the human eye, that is, the "strength" and "weakness" of the light visual effect of the radiant flux. "How much" means how much light the light source emits, which is expressed as luminous flux in photometry.
The unit of luminous flux is called "lumen" (lm). The 16th International Conference on Weights and Measures in 1979 resolved that monochromatic radiation with a power of 1 watt and a wavelength of 555nm has a luminous flux of 683lm.
