Luminous flux is also an artificial quantity, which may be different from other animals and not completely natural, because this definition is completely based on the human eye's reaction to light.
Human eyes have different feelings for different colors of light, which determines the conversion relationship between luminous flux and optical power. For the yellow-green light of 555nm, which is most sensitive to human eyes, the power of 1W = 683 lumens, and 1W is all converted into light with a wavelength of 555nm, which is 683 lumens. This is the maximum light conversion efficiency and a calibration value, because human eyes are most sensitive to light at 555nm.
For other colors of light, such as 650nm red light, 1W light is only equivalent to 73 lumens, because human eyes are not sensitive to red light. For white light, it depends, because many different spectral structures of light are white. For example, the white light of LED, white light on TV and sunlight are very different and have different spectra.
Extended data
Lumen is a physical unit to describe luminous flux, which is physically interpreted as the total luminous flux produced by a candle (cd, candle, which is equivalent to the luminous intensity of an ordinary candle) at a solid angle (the angle represented by a spherical cone with a radius of 1m and the central angle of the middle section of 65).
Considering that the solid angle of the whole sphere is 4π, the total luminous flux emitted by 1 candle light is 4π (about 12.56) lumens. The total solid angle of a ball of any size is 4π, so the total luminous flux of candlelight is fixed at 4π, which does not change with the change of distance, so it is wrong to say that the total luminous flux emitted by 1 candlelight is 1 lumen at about 0.28m around it.
The light energy emitted by the light source radiates in all directions, and the light energy passing through a certain area in unit time is called the radiant energy flux passing through the area. The sensitivity of eyes to different colors of light varies with frequency. Even if the radiant energy flux of different colors of light is equal, it cannot produce the same brightness visually. Among all colors of light, yellow and green light can cause the feeling of brightness most.
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