With the continuous development of LED technology, LED is made possible as an illumination source. Since the light source has strict requirements on the color rendering index CRI in the lighting fixture, the current white LED technology is not mature in color rendering, making it become Restricting LED as a major obstacle to lighting sources.
White light is a composite light, and it is easy to decompose the light of various colors contained in white light by a prism. LED is a kind of light source with comprehensive chromatogram. This wide color gamut has laid a theoretical foundation for the high color rendering of LED lighting products.
Light is a substance with wave-particle two-phase properties. This material contains the characteristics of the wave and contains the characteristics of the particle, which makes it have strong composite characteristics. Therefore, in terms of the nature of light and the characteristics of LED, LEDs are becoming a high-quality illumination source.
At present, white LED technology mainly adopts the manufacturing process of blue-chip chip coating phosphor. This technology is developed by Nichia Corporation and follows the principle of fluorescent light in a certain sense. This technology is lacking in red and green light components. In order to make up for this defect, phosphor technology has become the main direction of the industry.
The blue chip is coated with phosphor to make white light technology. This technology can produce white light with better effect in principle, but the difference in the adaptability to the environment during use makes the difference between theory and actual product. The difference is largely due to the effect of the LED heat build-up on the phosphor.
We all have the experience that high-power white LEDs gradually reach equilibrium with the generation of heat and heat loss as the working time progresses. The higher the temperature of this equilibrium state, the more the influence on the white LEDs using the same type of phosphors. Large, the following figure shows the effect of temperature on the luminescent properties of the phosphor;
As can be seen from the above figure, as the LED temperature continues to rise, the phosphor activity will be greatly reduced (Fahrenheit / Celsius conversion formula: Celsius = 5 / 9 * (Fahrenheit - 32), at 100 degrees Fahrenheit phosphor The activity is preferred, but the activity of the phosphor is substantially lost at around 250 degrees Fahrenheit (i.e., 120 degrees Celsius).
While affecting the activity of the phosphor, the temperature also has a certain influence on the wavelength of the light wave excited by the phosphor. The following figure shows the effect of temperature on the wavelength of the phosphor.
It can be seen from the above figure that as the LED temperature rises, the phosphor activity is gradually deactivated, and the wavelength of the light emitted by the phosphor also shifts. This shift is a red shift, that is, the phosphor emission wavelength. Offset towards the red wavelength; this phenomenon seems to indicate that as the temperature rises, the color rendering of the white LED should be better, but actually lower, seemingly a contradiction? The essence of this phenomenon is that the phosphor shifts to the red wavelength, which is smaller than the deactivation amount of the phosphor. That is, the red light excited by the phosphor at high temperature is not enough to compensate for the red light lost by the mixture, so it is expressed as The increase in temperature of the LED's color rendering index is still declining, which is consistent with the decrease in the color rendering index of white LEDs as temperature increases.
From the above analysis, it can be concluded that the primary means of increasing the color rendering index of the phosphor white LED is to reduce the temperature of the LED, and secondly to improve the ability of the phosphor to adapt to high temperatures. The third is to use color compensation technology to compensate the color rendering index with red or amber LEDs.
In terms of reducing the LED junction temperature, the most effective means is to design a LED package with high heat dissipation performance. This is one of the directions that China's LED industry can make significant progress in LED intellectual property in the near future, because LED is a low voltage. The electronic products are more demanding in terms of electrical safety, so there are many materials for heat-conducting substrates that can be used, and our industry currently has few studies on this, resulting in repeated violations of foreign intellectual property disputes.
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