Semiconductor Light Emitting Diode (LED) is a new type of lighting device that produces light by injecting current into the electron and hole radiation complex. With the advantages of energy-saving, long life, effortless control, and maintenance-free, white LEDs are widely used in automotive headlights, backlight display modules, high-speed rail ship searchlights, medical and industrial lighting, and other fields. Depending on the chip, there are three main technologies that can realize white LED light: multi-chip white LEDs, UV-excited white LEDs, and blue-excited white LED methods.
The following will briefly introduce the principles of these three ways to achieve white light and their advantages and disadvantages.
1. Blue-excited white LED light
Blue light excitation white LED refers to the use of a blue LED chip and phosphor combination to produce white light (PC-LED), this method is currently the most widely commercialized and successful method.
The principle of this method is as follows: the blue light from the blue LED chip is irradiated into the fluorescent material, part of which is absorbed by the fluorescent material and excited to produce yellow, green, or red light, and the other part of the blue light penetrates through the fluorescent material, and the penetrated blue light is mixed with the light excited by the phosphor to form white light.
There are two key parts of the method to produce white light: one is the excitation spectrum of the fluorescent material needs to match the emission spectrum of the blue LED chip, which is commonly used internationally is GaN blue diode as the laser light source, the excitation wavelength of the laser diode is between 460-470nm, so the excitation spectrum of the phosphor is required to be between 460-470nm to obtain the highest The other is that the emission spectrum of the fluorescent material should match the white light of the blue diode, which can be seen from the CIE coordinate chart, the emission spectrum of the fluorescent material should be around 570nm.
Based on the above requirements, the current practical application of high-performance YAG: Ce yellow phosphor as a light conversion fluorescent material. YAG: Ce yellow phosphor in blue light excitation absorbs part of the blue light and emits yellow light, unabsorbed blue light and the emission of yellow light combined to produce white light, through the regulation of the ratio of blue light and yellow light, you can get the different color temperature of white light.
Since the circuit design of white LED using blue light excitation is simple and the phosphor preparation process is quite mature, which greatly reduces the manufacturing cost, so the solution is based on blue LED chips to generate blue light to excite yellow phosphor to produce white light has been the most widely used commercially, but the white light produced by this method lacks the red part of the spectrum, so the color rendering of this white LED is low (<70).
2. UV-excited white LED light
UV-excited white LEDs are white light (UV-LED) obtained by using near-ultraviolet light from a UV chip as the excitation source to excite red, green, and blue composite fluorescent materials. However, the more colors are used to mix the fluorescent material the better the color rendering of white light obtained, but this increases the complexity of the system, so the composite fluorescent material is usually mixed using only red, green, and blue phosphors.
Since all the white light comes from the light emitted by the phosphor, and the excitation spectrum of the phosphor matches the emission spectrum of the UV chip, so the luminous efficiency of the white light generated by this method is higher, and the quality of white light is good, and the cost is lower, but because they use of ultraviolet or near-ultraviolet down-conversion involves a large Stokes shift, which will inevitably lead to an increase in energy loss during the conversion process. In addition, due to the use of UV light source as the laser light source, the method is also prone to the problem of UV pollution.
3. Multi-chip white LED light
Multi-chip white LEDs (MC-LEDs) are LEDs with red, blue and green LED chips packaged together to achieve white light, which has two significant features: high luminous efficiency and flexible regulation. As the multi-chip white LED is directly electroluminescent through each LED chip, there is no Stokes displacement energy loss and non-radiative compound loss in the phosphor, so its energy loss is small and the luminous efficiency is high.
In addition, the method uses three primary colors of light emitted by the LED chip to mix to achieve white light, so you can adjust the size of the current of the LED chip to control the luminous color and color temperature, and other luminous characteristics, in practical applications with a certain degree of flexibility. But this adjustment device is very complex because different wavelengths of LED chips on different voltage changes in response to the intensity are different, the impact of temperature changes in the work also requires additional circuitry to adjust the control, which will lead to the chip circuit design becomes too complex and costly.
Moreover, the LED chip emits a narrower spectrum of light, mixed into a poor color rendering of white light, which requires the use of more LED chips to form a light-emitting array to improve the color rendering index of white light. In addition, because the aging decay rate of different chips is not consistent, long-term work is prone to the problem of white light color coordinate shift.
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