The so-called lumens simply refers to the brightness of a candle-lit light outside one meter. An ordinary 40-watt incandescent light bulb has a luminous efficiency of about 10 lumens per watt, so it can emit 400 lumens of light. The 40 watt incandescent lamp has a luminous flux of 340 lumens at 220 volts. Luminous flux is the ability to describe the visual response of light source radiation per unit time. The unit is lumens, also known as brightness. The projector indicates that the unit of luminous flux is ANSI lumens. ANSI lumens is the standard for measurement of projector luminous flux by the American National Standards Institute. It measures the illuminance at each of the nine cross points of the screen “Tian†shape, multiplies the area, and then asks for nine. The average of the points is the ANSI lumens of the projector. The higher the lumen value is, the brighter it is, and the higher the brightness is, the lighter does not need to be turned off when projecting. ANSI stands for American National Standards Institute.
Detailed introduction
Similarly, this quantity is the size of the light source, which is equivalent to the light power. The greater the luminous flux of the light source, the more light emitted
For isotropic light (ie, the light of the light source is emitted at the same density in all directions), then F=4Ï€I. That is, if the I of the light source is 1 cd, the total luminous flux is 4Ï€=12.56 lm. Compared with mechanical units, luminous flux corresponds to pressure, and luminous intensity corresponds to pressure. To be brighter at the point of exposure, we must not only increase the luminous flux, but also increase the means of convergence. In fact, it is to reduce the area, so as to obtain greater intensity.
It should be noted that luminous flux is artificial, and 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 different to different colors of light, and this feeling determines the conversion relationship between luminous flux and optical power. For the yellow-green light at 555 nm, which is most sensitive to human eyes, 1 W = 683 lm, that is, the power of 1 W is all converted to light with a wavelength of 555 nm, which is 683 lumens. This is the maximum light conversion efficiency, which is also the scaling value, because the human eye is most sensitive to light at 555 nm. 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 insensitive to red light. For white light, it depends on the situation, because the light of many different spectral structures is white. For example, the white light of the LED, the white light on the TV, and the sunlight are very different, and the spectrum is different.
As for the luminous efficiency of electric light sources, it is another related topic, which is how much luminous flux can be converted into electric power of 1W. If you convert all the light to 555 nm, that's 683 lumens per watt. However, if half of the light is converted to 555 nm and the other half is lost as heat, the efficiency is 341.5 lumens per watt. Incandescent lamps can reach 1W = 20lm is very good, and the rest become heat or infrared. Measuring the luminous flux of an irregular luminous body requires an integrating sphere, which is more professional and complicated.
Common luminous efficiency (lumens/watts)
Incandescent, 15
White LED, 80-90
Fluorescent lamp, 50
Sun, 94
Sodium lamp, 120
Energy-saving lamp, 60-80