The luminous flux is an artificial quantity, 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, 1W = 683 lumens, that is to say, the power of 1W is all converted into light with a wavelength of 555nm, that 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.
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Lumen refers to the total luminous flux produced by a candle (cd, Candeira candle, luminous intensity unit, equivalent to the luminous intensity of ordinary candle) at a solid angle (the angle represented by a spherical cone corresponding to a spherical cap with a radius of 1 m/m2, corresponding to the central angle of the middle section of 65). Luminous flux reflects the intensity of visual reaction produced by light source radiation in unit time, and the unit is lumen, also called brightness.