Summer is coming soon, Shenzhen has entered the high temperature weather, for users of large led screen, we must pay attention to the temperature change of the large led screen, because the high temperature has the following four effects on the led display:
(1) If the operating temperature of the LED display exceeds the load temperature of the chip, the luminous efficiency of the LED display will be rapidly reduced, resulting in significant light attenuation and damage;
(2) LED display screens are mostly encapsulated with transparent epoxy resin. If the junction temperature exceeds the solid phase transition temperature (usually 125°C), the packaging material will change to a rubbery shape and the thermal expansion coefficient will suddenly rise, resulting in open circuit and failure of the LED display .
The lifespan of the led display is manifested by its light decay, that is, after a long time, the brightness will become lower and lower until it goes out. It is usually defined that the time for the luminous flux of the led display to decay by 30 is its lifetime.
The causes of the light decay of LED display screens are as follows:
(1) The defects existing in the LED display chip material will rapidly multiply and multiply at higher temperatures until they invade the light-emitting area, forming a large number of non-radiative recombination centers, which seriously reduces the luminous efficiency of the LED display. In addition, under high temperature conditions, micro-defects in the material and fast-expanding impurities from the interface and the electric board will also be introduced into the light-emitting region, forming a large number of deep energy levels, which will also accelerate the light decay of the LED display device.
(2) The transparent epoxy resin will be denatured and yellowed at high temperature, which affects its light transmission performance. The higher the working temperature, the faster this process will be carried out. This is another main reason for the decay of LED display screens.
(3) The optical decay of the phosphor is also a major cause of the decay of the LED display screen, because the attenuation of the phosphor at high temperatures is very serious.
Therefore, high temperature is the main source of the LED display light decay and shorten the life of the LED display.
Different brands of LED displays have different light decay, and usually LED display manufacturers will give a standard set of light decay curves. The luminous flux attenuation of the LED display screen caused by high temperature is unrecoverable. The luminous flux of the LED display screen before the unrecoverable light attenuation does not occur is called the "initial luminous flux" of the LED display screen.
The reasons why temperature affects the light effect of LED display screen include the following aspects:
(1) As the temperature rises, the radiative recombination probability of electrons and holes in the potential well decreases, resulting in non-radiative recombination (heat generation), thereby reducing the internal quantum efficiency of the LED display screen.
(2) As the temperature increases, the concentration of electrons and holes will increase, the band gap will decrease, and the electron mobility will decrease.
(3) The increase in temperature causes the blue peak of the chip to shift to the long wave direction, which makes the emission wavelength of the chip and the excitation wavelength of the phosphor mismatch, and it also causes a reduction in the extraction efficiency of the external light of the custom LED display.
(4) As the temperature rises, the phosphor quantum efficiency decreases, the light output decreases, and the external light extraction efficiency of the LED display decreases.
(5) The performance of silica gel is greatly affected by the ambient temperature. As the temperature increases, the thermal stress inside the silica gel increases, causing the refractive index of the silica gel to decrease, thereby affecting the light efficiency of the LED display screen.
The emission wavelength of the LED display screen can generally be divided into a peak wavelength and a dominant wavelength. The peak wavelength is the wavelength of light intensity, and the dominant wavelength can be determined by the X and Y chromaticity coordinates, reflecting the colors perceived by the human eye. Obviously, the change in the luminous wavelength of the LED display screen caused by the junction temperature will directly cause different perceptions of the human eye to the luminous color of the LED display screen. For a led display device, the forbidden band width of the light-emitting area material directly determines the wavelength or color of the device. As the temperature increases, the forbidden band width of the material will decrease, resulting in a longer emission wavelength of the device and a red-shifted color.
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