2023-12-30

White LED lamp beads of several light loss

White LED lamp beads have a wide range of applications in life, but the light attenuation of white LED lamp beads is also one of the most serious LED lamp beads, below, Blue Jin photoelectric will introduce several kinds of light loss of white LED lamp beads.

The first reason is the production of white light and the improvement of color rendering index

Blue chip and phosphor are the main production mode of commercial white LED. Figure 1 shows the spectrum of white LED made by combining blue chip and YAG: Ce3+ phosphor. From the spectrum diagram of white light, it can be seen that the photoluminescence emission spectrum of phosphor is distributed in the entire visible band, and the peak value is near the peak value of human visual curve. Therefore, the luminous efficiency (lumen efficiency) of the white LED produced by this method is high, and the color rendering index can reach more than 70, which is close to the traditional fluorescent lamp, and can meet the ordinary lighting needs.
In order to improve the color rendering index of the white LED produced by the blue chip +YAG phosphor method, it can be made up by introducing silicon-based nitrogen oxide cyan LED phosphor with emission peak at 490nm, violet LED chip and silicon-based nitrogen oxide red LED phosphor with emission peak above 630nm.
Under normal circumstances, the appropriate addition of red phosphor can increase the color rendering index to more than 80, and then the introduction of cyan phosphor can get a color rendering index higher than 90, on this basis, the increase of violet chip can get the full spectrum of white LED, the color rendering index can be close to 100 daylight.

The second reason is the energy loss of photoluminescence

A key physical process in the production of white light by the blue LED chip + phosphor method is photoluminescence, that is, the phosphor will turn blue light to other wavelengths of light. This process inevitably involves a loss of energy. This loss has three components:
First, the quantum efficiency of phosphor excitation from low energy level to high energy level is lost, and the number of particles transiting to high energy level is less than the number of blue photons absorbed.
Second, when the phosphor transitions from high energy level to low energy level, there is a non-radiative transition, which causes the loss of the quantum efficiency of radioluminescence, and the number of visible photons emitted is less than the number of photons that transition to low energy level.
Third, the photon energy of a single blue light is higher than the long-wavelength photon energy emitted after the phosphor conversion, and the corresponding radiation flux is smaller in the case of the same number of photons.
The first and two kinds of energy loss, the loss is the number of photons, improve the phosphor formula and preparation process, improve the quantum efficiency of the phosphor excitation and emission process, can reduce the energy loss of these two parts.
The third kind of energy loss is the different energy of the photon itself, which is determined by the physical nature of the photon, and changing the process can not reduce the energy loss of this part. In today's white leds, the loss of the above three parts accounts for about 20%-30% of the blue light energy.

The third reason is to improve the luminous efficiency loss of the visible finger

The white LED made of blue LED chip +YAG phosphor has a high luminous efficiency, on the one hand, thanks to the early commercialization of YAG phosphor, high process and technical maturity, the quantum efficiency of phosphor excitation and emission is relatively high, on the other hand, the emission wavelength peak of YAG phosphor is near the peak of the human visual function, and the lumen efficiency is high. The emission peak of cyan and red phosphor is far away from the peak of the visual function, and the lumen efficiency is low. After the incorporation of cyan and red phosphor, the color rendering index is increased, and the luminous efficiency must be accepted.
Under normal circumstances, the color rendering index is increased from 70 to 80, the luminous efficiency will decrease by 10-15%, the color rendering index is increased from 80 to 90, and the luminous efficiency will decrease by about 10%.

Cause four, Fresnel loss at the interface

Photon from the LED chip active layer (PN junction) is emitted into the air, need to pass through the chip and packaging adhesive, packaging adhesive and air two interfaces, because the two sides of the interface material refractive index difference, light through this interface, a part of the light will be reflected back, reflected back a large part of the light will be absorbed and lost. The loss caused by this kind of interface reflected light is called Fresnel loss. The size of Fresnel loss is related to the size of the emission rate and the refractive index difference of the optical medium on both sides of the interface, and the quantitative analysis is very complicated. In general, the greater the refractive index difference, the more serious the Fresnel reflection.

Reason five, total reflection loss

When the light is transmitted from the optically dense medium to the optically sparse medium, when the incidence Angle of the light is greater than a certain critical value qc, total reflection will occur at the interface, which is called the total reflection Angle.
LED chip manufacturing material is a high refractive index of semiconductor material, refractive index is greater than the refractive index of packaging adhesive and air, so in the LED chip and packaging adhesive interface, packaging adhesive and air interface are only less than a certain Angle of light can pass through, this part of the light formed a full reflection Angle for half Angle width of the cone, It is often graphically referred to as the "escape cone" of light.
The main materials of blue chip are GaN and sapphire, the typical refractive indices are 2.45 and 1.78 respectively. The typical refractive indices of epoxy resin and silica gel are 1.42 and 1.51 respectively. The refractive index of air is approximately 1.
When GaN enters silica gel, the critical Angle of total reflection is 38.050. When GaN enters the air, the critical Angle of total reflection is 24.090. The critical Angle of total reflection is 58.030 when incident from sapphire to silica gel (corresponding to inverted package). When GaN enters the air, the critical Angle of total reflection is 34.180. It can be seen that from the Angle of increasing the critical Angle of total reflection, inversion technology is also conducive to the improvement of chip light extraction efficiency.

The above is about the introduction of several kinds of light loss of white LED lamp beads, have you learned? Want to know more about LED lamp beads, welcome to pay attention to Blue Jin Optoelectronics, we will regularly push LED articles for you.