Understanding luminous flux from the perspective of currency exchange

Last year, a company's LED light efficiency made a major breakthrough, reaching 303lm/W. Many industry insiders praised it, and various “brick homes” also began to make bold predictions for the next step, 400lm/W, 500lm/W, 1000lm/ W...

Amount, dear, do you really understand the luminous flux?

Luminous flux is the most commonly used physical quantity in the lighting industry. The unit is lumens, but it can also be understood from the perspective of currency exchange.

1. What is the spectrum?

More than 300 years ago, the famous Newton had experimented with prism dispersion. He shot a beam of sunlight through the prism and presented a colorful ribbon on the other end of the prism. Experiments proved that white light is made of light of various colors. Mixed, as shown in Figure 1 below.

Figure 1: Prism dispersion experiment and spectrum

If we understand the ribbon in Figure 1 as a ruler and mark it, we can use the length (wavelength) to describe the color of the light. For example, 380nm is purple, 580nm is yellow, and 700nm is red. Here nm is called Nano (1 nanometer is to divide 1 meter into 1 billion). If we mark the content of the various colors of light in the sun (absolute spectral intensity), we can draw the spectrum.

It can be seen from Fig. 1 that the yellow-green light content in sunlight is the highest and the violet light is the least. We know that light is an energy that can be described by power.

The calculation formula of optical power is as follows:

Use points? In fact, the integral is the meaning of summation. The formula calculates the area of ​​the spectrum. In other words, the area of ​​the absolute spectrum is the optical power (the unit of optical power is watts). Since light is an energy, it can be described by optical power. Directly using optical power to describe the amount of light is not finished. Why use luminous flux?

2. What is luminous flux?

For example, if you have two banknotes with a face value of 100 yuan in Figure 2 for you to choose, which one would you choose?

Figure 2: RMB and USD

There is no doubt that you will definitely choose the following $100 dollar. Although the face value is 100 yuan, the value of the two banknotes is different. We know that 100 yuan can be exchanged for 645 yuan. Therefore, in order to compare the value of different currencies in international trade, we need to use the exchange rate.

Light is an energy. Experiments show that under the same conditions, the human eye has different brightness and dark perceptions (values) of different colors of light. In order to describe the human eye's perception of light and darkness of different colors of light, we need to introduce luminous flux. The physical quantity, the class of the same currency is shown in Table 1 below.

Table 1 Currency and light analogy

The V(λ) in Table 1 is called the visual function and is obtained according to the experiment. The International Lighting Society CIE has a form for inquiry, just like you check the exchange rate. Of course, it will not change like the exchange rate every day, V ( The λ) curve is shown by the black dotted line in Figure 3.

Figure 3. V(λ) curve

For example, as shown in Fig. 3, in order to obtain a luminous flux of 683 lm, the optical power required for each of the ABCD monochromatic lights in the figure is as shown in Table 2.

Table 2. Optical power and luminous flux

As can be seen from Table 2, in order to obtain a luminous flux of 683 lm, 555 nm of yellow-green light requires only 1 W of optical power, and 455 nm of blue light requires 20.8 W of optical power. Think of different lights as different currencies and make an analogy, as shown in Table 3 below.

Table 3 Currency and light analogy

Compare the above sentence: In order to buy (obtain) 683 ml (lm) of water (light flux), RMB (yellow-green light) 1 yuan (W) and yen (blue light) 20.8 yuan (W) are required.

From the V(λ) curve in Figure 3, we can see that the human eye is most sensitive to yellow-green light at 555 nm and is not sensitive to red and blue light. Under the same optical power, the human eye will think that the yellow-green light is very bright, but the red and blue light It will feel darker. Lighting is mainly perceived by the human eye, and our human eyes are not "uniform" to the perception of light and dark without color light, so we need to convert it into luminous flux, just as countries in international trade generally convert their national currency into US dollars.

If it is expressed by optical power, this will happen. It is 1W (1 yuan) of light (currency), but the brightness and dark perception (value) is different. 1 yuan (1W) RMB (yellow green light) and 1 yuan. (1W) The value of the yen (Blu-ray) (light perception) is definitely different.

PS: The V(λ) in this paper is a visual visual view function.

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