How do white light LEDs emit white light?

There are two main types of white light LED production technologies on the market:

the first is to use phosphors to convert the blue light or ultraviolet light generated by blue light LEDs or ultraviolet UV-LEDs into dichromatic or trichromatic white light, respectively. This component technology is called phosphor converted white light LED (Phosphor Converted-LED);

the second type is multi-chip white light LED, which forms white light by combining two (or more) LEDs of different colors. Currently, the most common white light LED products on the market are blue light LED chips with yellow light phosphors, which are mainly used in automotive lighting and mobile phone panels. Based on the current white light LED product market analysis, phosphor converted white light LED can be said to be the mainstream. (Red and green phosphors are mostly sulfide systems, which have poor luminous stability and large light decay.)

The figure below briefly summarizes and compares the various white light LED assembly principles and advantages and disadvantages. Among them, type (a) has the best color rendering, but the highest cost and has not yet been popularized; assembly method (b) has the advantages of the most mature technology and low cost, but the color shift and color rendering are not good, which must be improved with appropriate red and yellow phosphors. In addition, the most serious one is that Nichia Chemical’s patent restrictions are difficult to circumvent; and the white light LEDs produced by assembly methods (c) and (d) have excellent color rendering, small color shift, low cost and less serious patent limitations, so they have great development potential in the future.


The principle and advantages and disadvantages of using light-emitting diodes to generate white light

How to apply phosphor to LED lamps?

MRKramas et al. found that if phosphor is randomly placed on the LED chip, the uniformity of light emission is poor, as shown in Figure (a), so the change method is shown in Figure (b), where the phosphor is evenly applied on the LED surface. Figure (c) compares the CCT and Ra values ​​of the two, and found that the CCT value of the method in Figure (b) changes little.



What is CCT?

CCT is the abbreviation of correlated colour temperature. Color temperature refers to the change of a standard black body from dark red to light red to orange-yellow to white to blue-white to blue as the temperature rises. Using this characteristic of the black body, when the light source to be tested has the same light color as the black body at a certain temperature, the temperature of the black body is the color temperature of the light source to be tested. Light with high color temperature is colder, and light with low color temperature is warmer.


The impact of white light LED spectrum on human eyes

The human eye cannot tolerate blue light and UV light the most. Blue light is 10 times more effective in killing active cells in the human eye than green light, and UV light is 10 times more effective in killing active cells in the human eye than blue light. Long-term exposure to a large amount of low-wavelength blue light can kill a large number of active cells in the human eye. Even if you eat more foods that are good for the eyes, it will be ineffective.


Refer to the figure above. The formation of LED white light mainly relies on the 450-455nm wavelength of blue light to trigger the phosphor. The lower the wavelength, the stronger the triggering ability. Usually, the wavelength of LED is controlled within 5nm when it leaves the factory.

★The mechanism of blue light generation in LED is mainly that the blue light emitted by the blue light chip is mainly concentrated in 450-455nm, while the wavelength of blue light of fluorescent lamp is very wide, from 435-500nm;

★Yellow phosphor (or red, green phosphor) is used in LED. During the manufacturing process, the blue light may be different from the initial state due to phosphor attenuation (or small amount of phosphor, insufficient reflection), and a large amount of blue light may be emitted, which greatly increases the damage to human eyes;

★The blue light part of LED is guaranteed by the chip, while the other light colors are guaranteed by the phosphor. The chip has a strong guarantee ability, while the phosphor has a weak guarantee ability. The longer the white light LED is lit, the higher the proportion of blue light becomes.

Therefore, laptops, mobile phones, and TVs that use LED backlights at the beginning are more dazzling. After using them for a long time, you will feel dizzy and uncomfortable. Long-term use will cause eye damage. The longer you are exposed to LED blue light, the higher the chance of developing eye disease.