Luminous flux (F, Flux), unit lumens, ie lm.
Definition: The amount of light emitted by a light source per unit time is called the luminous flux of the light source.
Explanation: Again, this amount is for the light source, it is the size of the total amount of light emitted by the light source, and is equivalent to the optical power. The greater the luminous flux of the light source, the more light is emitted
For isotropic light (ie, the light of the source is emitted at the same density in all directions), then F = 4πI. That is, if the I of the light source is 1 cd, the total luminous flux is 4π = 12.56 lm. Compared with the mechanical unit, the luminous flux is equivalent to the pressure, and the luminous intensity is equivalent to the pressure. If the spot to be illuminated looks brighter, we must not only increase the luminous flux, but also increase the means of convergence. In fact, it is to reduce the area so that we can get more intensity.
It is important to know that luminous flux is also artificial. It may not be the same for other animals, and it is not completely natural, because this definition is based entirely on the human eye's response to light.
The human eye feels differently for different colors of light, and this feeling determines the conversion relationship between luminous flux and optical power. For the most sensitive 555 nm yellow-green light of the human eye, 1W = 683 lm, that is, the power of 1W is all converted into light with a wavelength of 555 nm, which is 683 lumens. This is the maximum light conversion efficiency and is also the calibration value because the human eye is most sensitive to 555 nm light. For other colors of light, such as 650 nm red, 1 W of light is only equivalent to 73 lumens, which is because the human eye is not sensitive to red light. For white light, it depends on the situation, because the light of many different spectral structures is white. For example, white light from LEDs, white light on television, and daylight vary widely, with different spectra.
As for the luminous efficiency of the electric light source, it is another related topic, which is to say how much luminous flux can be converted into 1W electric power. If all is converted to 555nm light, it is 683 lumens per watt. But if half of it is converted to 555nm light and the other half becomes heat loss, the efficiency is 341.5 lumens per watt. Incandescent lamps can reach 1W = 20 lm, and the rest are heat or infrared. To measure the luminous flux of an irregular illuminator, it is more professional and complicated to use the integrating sphere.