1. Brightness and viewing angle
The brightness of the display mainly depends on the luminous intensity of the LED and the density of the LED. In recent years, new technologies in LED substrate, epitaxy, chip and packaging have emerged in an endless stream, especially the stability and maturity of indium tin oxide (ITO) current spreading layer technology and process, which has greatly improved the luminous intensity of LED. At present, when the horizontal viewing angle is 110 degrees and the vertical viewing angle is 50 degrees, the luminous intensity of the international first-class brand low-power LEDs has reached 4000mcd for green tubes, 1500mcd for red tubes, and 1000mcd for blue tubes. When the pixel pitch is 20mm, the display brightness can reach more than 10000nit. The display can work 24/7 in any environment.
When it comes to the viewing angle of the display screen, there is a phenomenon worth thinking about: LED display screens, especially outdoor display screens, people basically observe from the bottom up, but judging from the product form of the existing LED display screens, there are Half of the luminous flux disappeared into the vast sky.
2. Uniformity and clarity
With the development of LED display technology to this day, the uniformity has become the most important indicator to measure the quality of the display. It is often said that LED display is “a bit of brilliance, a piece of brilliance”, which is a metaphor for the serious unevenness between pixels and modules. The more professional terms are “dust effect” and “mosaic phenomenon”. The root causes of the uneven phenomenon mainly include: inconsistency of various performance parameters of LEDs; insufficient assembly accuracy of the display screen during production and installation; insufficient consistency of electrical parameters of other electronic components; non-standard design of modules and PCBs, etc. Among them, “the inconsistency of LED performance parameters” is the main reason. The inconsistency of these performance parameters mainly includes: inconsistent light intensity, inconsistent optical axis, inconsistent color coordinates, inconsistent light intensity distribution curves of each primary color, and inconsistent attenuation characteristics. How to solve the inconsistency of LED performance parameters, there are currently two main technical approaches in the industry: one is to improve the consistency of LED performance through further subdivision of LED specification parameters; the other is to improve the display screen through subsequent corrections Uniformity. Subsequent calibration has also developed from the early module calibration and module calibration to today’s point-by-point calibration. The correction technology has developed from simple light intensity correction to light intensity color coordinate correction. However, we believe that follow-up corrections are not a panacea. Among them, inconsistent optical axis, inconsistent light intensity distribution curve, inconsistent attenuation characteristics, poor assembly accuracy, and irregular design cannot be eliminated through subsequent corrections, and even such subsequent corrections will cause differences in optical axis, attenuation, and assembly accuracy. The inconsistency got worse. Therefore, through practice, our conclusion is: follow-up correction is only to cure the appearance, and LED parameter subdivision is the root cause, which is the mainstream of the LED display industry in the future. When it comes to the relationship between display uniformity and clarity, there is often a misunderstanding in the industry, that is, resolution replaces clarity. In fact, the clarity of the display screen is the human eye’s subjective experience of multiple factors such as the resolution, uniformity (signal-to-noise ratio), brightness, and contrast of the display screen. Simply reducing the physical pixel pitch to improve resolution while ignoring uniformity is beyond doubt for improving clarity. Just imagine a display with severe “dust effect” and “mosaic phenomenon”. Even if its physical pixel pitch is smaller and its resolution is higher, it is impossible to get a good image clarity. Therefore, in a sense, the main reason restricting the improvement of the clarity of LED displays is “uniformity” rather than “physical pixel spacing”.
3. Display pixels are out of control
There are many reasons for the out-of-control display pixels, the most important of which is “LED failure”. The main cause of LED failure can be divided into two aspects: one is the poor quality of the LED itself; the other is improper use. Through the analysis, we conclude the corresponding relationship between the LED failure mode and the above two main causes. We mentioned above that many LED failures are usually not found in the routine inspection and testing of LEDs. In addition to being improperly used by electrostatic discharge, high current (causing excessive junction temperature), external strong force, etc., many LED failures are caused by high temperature, low temperature, rapid temperature change or other harsh conditions, due to LED chips, epoxy resin, brackets, etc. , Inner leads, solid crystal glue, PPA cups and other materials have different thermal expansion coefficients, resulting in different internal stresses. Therefore, the quality inspection of LEDs is a very complicated task.
4. Lifespan
The factors that affect the life of the LED display include internal and external factors. The internal factors include the performance of peripheral components, the performance of LED light-emitting devices, and the fatigue resistance of the product; the internal factors include the working environment of the LED display and so on.
1). Peripheral components influence
In addition to LED light-emitting devices, LED displays also use many other peripheral components, including circuit boards, plastic casings, switching power supplies, connectors, casings, etc. Any problem with any component may cause the life of the display reduce. Therefore, the longest life of the display is determined by the life of the key component with the shortest life. For example, LED, switching power supply, and metal shell are all selected according to the 8-year standard, and the protective process performance of the circuit board can only support its work for 3 years. After 3 years, it will be damaged due to rust, so we can only get one piece for 3 years. Lifetime display.
2). The influence of LED light emitting device performance
LED light-emitting devices are the most critical and life-span-related components of the display. For LEDs, the main indicators are: attenuation characteristics, water vapor penetration characteristics, and UV resistance. If the LED display manufacturer fails to evaluate the index performance of the LED device, it will be applied to the display screen, which will lead to a large number of quality accidents and seriously affect the life of the LED display screen.
3). The impact on the fatigue resistance of the product
The anti-fatigue performance of led display products depends on the production process. It is difficult to guarantee the anti-fatigue performance of the module produced by the poor three-proof treatment process. When the temperature and humidity change, cracks will appear on the protective surface of the circuit board, resulting in a decrease in protection performance. Therefore, the production process of the LED display is also a key factor in determining the life of the display. The production processes involved in the production of display screens include: component storage and pretreatment process, furnace welding process, three-proof treatment process, waterproof sealing process, etc. The effectiveness of the process is related to material selection and ratio, parameter control and operator quality. For most LED display manufacturers, the accumulation of experience is very important. A factory with many years of experience will be more effective in controlling the production process. .
4). Impact of working environment
Depending on the application, the working conditions of the display screen vary widely. In terms of environment, the indoor temperature difference is small, without the influence of rain, snow and ultraviolet rays; the outdoor temperature difference can reach up to 70 degrees, plus wind, sun and rain. The harsh environment will aggravate the aging of the display screen, and the working environment is an important factor affecting the life of the display screen. The life of the led display is determined by many factors, but the end of life caused by many factors can be continuously extended by replacing parts (such as switching power supplies). LEDs cannot be replaced in large quantities. Therefore, once the LED life ends, it means the end of the display life. We say that the life of the LED determines the life of the display, but it does not mean that the life of the LED is equal to the life of the display. Because not every LED is working at full capacity when the display screen is working, when the display screen is playing tv video wall programs normally, the life span of the display screen should be 6 to 10 times that of the LED life span. The service life can be longer under the condition of working with low current. Therefore, the lifespan of the display screen using this brand of LED can reach about 50,000 hours. How to make LED life longer? Generally, we can start from two aspects of device manufacturing and device application. From the aspect of device manufacturing: choose high-quality epitaxial materials; increase chip area, reduce current density; balance current density; reduce thermal resistance; choose packaging materials with excellent performance and strong UV resistance, etc. can make LED life longer . From the perspective of device application: taking heat dissipation as a central task from module design to engineering implementation and even future system maintenance; reducing the LED operating current; correctly configuring LEDs to make the LEDs of each primary color decay synchronously can prolong the service life of LEDs. Whether the LED light is stable, the quality is good or bad, and the heat dissipation of the light itself is very important. At present, the heat dissipation of high-brightness LED full-color screens on the market usually adopts natural heat dissipation, and the effect is not ideal. If the heat dissipation is not ideal, the life of the lamp itself will also be affected.
5. Energy consumption and energy efficiency
Improving LED light efficiency and reducing display energy consumption is an important development direction of LED display technology. It has the following positive significance: first, energy saving, emission reduction, and environmental protection; second, reducing the cost of power expansion, power equipment and cooling equipment. The third is to save electricity and reduce operating costs; the fourth is to reduce the temperature rise of the display screen; the fifth is to delay the LED attenuation speed; the sixth is to improve the reliability of the system; , to stabilize the image effect. The luminous efficiency of LED (that is, external quantum efficiency) is determined by the internal quantum efficiency and escape rate of LED. Today, the internal quantum efficiency of LEDs has reached over 90%, but due to the low escape rate, the external quantum efficiency has become a bottleneck for improving the light efficiency of LEDs. In order to break through this bottleneck restricting the development of the industry, many novel solutions have been proposed and theoretically verified, most of which have entered the experimental stage, and some have been successful, laying a solid foundation for the final industrialization. As a green and energy-saving light source, LED is favored by people, and it will also be used as a mainstream media to lead the future of display technology. In short, device manufacturing and device application itself are a complementary unity. The progress of device technology brings prosperity to the application market, and the demand of the application market is the eternal driving force for the progress of device technology. Only with the joint efforts of upstream and downstream enterprises can we create a new future for LED display technology.