Five key technologies for the quality LED display screens Published：2018-09-04
When we use LED devices on a daily basis, we often need to evaluate the quality of LED displays. How can we test the quality of the devices? Let's discuss this issue.
1, brightness and viewing angle
The brightness of the display depends mainly on the luminous intensity of the LED and the density of the LED screens. In recent years, new technologies for LEDs in substrates, epitaxy, chips and packages have emerged, especially the stability and maturity of indium tin oxide (ITO) current spreading layer technology and technology, which has greatly improved the luminous intensity of LEDs. At present, the world-class brand low-power LED has a horizontal viewing angle of 110 degrees and a vertical viewing angle of 50 degrees. The luminous intensity of the green tube has reached 4000mcd, the red tube has reached 1500mcd, and the blue tube has reached 1000mcd. When the pixel pitch is 20mm, the display brightness can reach 10000nit or more. The display can work 24/7 in any environment.
When it comes to the viewing angle of the display, there is a phenomenon worthy of our consideration: LED display, especially the outdoor display, people's observation angle is basically from the bottom up, and from the product form of the existing LED display, there are Half of the luminous flux disappeared into the sky.
2, uniformity and clarity
With the development of LED display technology to today, uniformity has become the most important indicator to measure the pros and cons of the led video display. It is often said that the LED display "a little bit brilliant, a piece of brilliant" is an image metaphor for serious unevenness between pixels and modules. The professional point is "dust effect" and "mosaic phenomenon".
The root causes of the inhomogeneity are: the inconsistency of LED performance parameters; the lack of assembly accuracy in the production and installation process; the consistency of electrical parameters of other electronic components is not sufficient; the module and PCB design are not standardized.
Among them, "the inconsistency of LED performance parameters" is the main cause. The inconsistencies of these performance parameters mainly include: inconsistent light intensity, inconsistent optical axis, inconsistent color coordinates, inconsistent light intensity distribution curves of various primary colors, and inconsistent attenuation characteristics.
How to solve the inconsistency of LED performance parameters, there are two main technical approaches in the industry: First, through the further subdivision of LED specifications, improve the consistency of LED performance; Second, improve the display through subsequent correction Uniformity. Subsequent corrections have evolved from early module corrections and module corrections to today's point-by-point corrections. The correction technique is developed from simple light intensity correction to light intensity color coordinate correction.
However, we believe that subsequent corrections are not omnipotent. Among them, the inconsistent optical axes, inconsistent light intensity distribution curves, inconsistent attenuation characteristics, poor assembly accuracy, and irregular design are not eliminated by subsequent corrections. Even such subsequent corrections will result in optical axes, attenuation, and assembly accuracy. The inconsistency is getting worse.
Therefore, through the practice of our conclusions: the follow-up correction is only the treatment of the table, and the LED parameter subdivision is the root cause, is the future mainstream of the LED display industry.
When it comes to the relationship between display uniformity and sharpness, there is often a misunderstanding in the industry that replaces sharpness with resolution. In fact, the display resolution is a subjective feeling of the human eye on the display resolution, uniformity (signal-to-noise ratio), brightness, contrast and other factors. Simply narrowing the physical pixel pitch to improve the resolution, while ignoring the uniformity, there is no doubt about improving the definition. Imagine a display with a serious "dust effect" and "mosaic phenomenon". Even if its physical pixel pitch is small and the resolution is high, it is impossible to get a good image sharpness.
Therefore, in a sense, the main reason for restricting the improvement of the definition of LED display screens is "uniformity" rather than "physical pixel pitch".
3, the display pixel is out of control
There are many reasons why the display pixels are out of control. The most important reason 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 the improper use method. Through analysis we sum up the LED failure mode and the correspondence between the above two main causes.
As mentioned above, the failure of many LEDs is usually not found in the routine inspection of LEDs. In addition to being improperly used due to electrostatic discharge, high current (causing excessive junction temperature), external strength, etc., many LED failures are caused by high temperature, low temperature, rapid temperature change or other harsh conditions, due to LED chips, epoxy resin, and brackets. The difference in thermal expansion coefficient of materials such as inner leads, solid crystal adhesives, and PPA cups is caused by the difference in internal stress. Therefore, quality inspection of LEDs is a very complicated task.
There are internal and external factors affecting the life of LED display. The internal factors include the performance of peripheral components, the performance of LED light-emitting devices, and the fatigue resistance of products. The internal environment has the working environment of LED display.
1). Impact of peripheral components
In addition to LED lighting devices, LED display screens use many other peripheral components, including circuit boards, plastic housings, switching power supplies, connectors, chassis, etc., any problem with any component, may lead to the life of the display reduce. Therefore, the longest life of the display is determined by the life of the critical component of the shortest life. For example, LED, switching power supply, and metal casing 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, then we can only get a piece of 3 years. Life display.
2). The impact of LED lighting device performance
LED lighting devices are the most critical and most relevant components of the display. For LEDs, the following indicators are mainly: attenuation characteristics, water vapor permeability, and UV resistance. If the LED display manufacturer evaluates the performance of the LED device, it will be applied to the display, which will lead to a large number of quality accidents, which seriously affect the life of the LED display.
3). Anti-fatigue performance of the product
The fatigue resistance of LED display products depends on the production process. The anti-fatigue performance of the module made by the poor three-proof treatment process is difficult to guarantee. When the temperature and humidity change, the protective surface of the circuit board will be cracked, resulting in a decrease in the protective 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, over-furnace welding process, three-proof process, and waterproof sealing process. The effectiveness of the process is related to material selection and ratio, parameter control and quality of the operator. For the vast 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). The impact of the working environment
The operating conditions of the display vary widely depending on the application. From the environmental point of view, the indoor temperature difference is small, no rain, snow and ultraviolet light; the outdoor temperature difference can reach up to 70 degrees, plus wind and sun and rain. A harsh environment can exacerbate the aging of the display, which is an important factor affecting the life of the display.
The life of an LED display is determined by a number of factors, but the end of life caused by many factors can be extended by the replacement of components (such as switching power supplies). LEDs are not likely to be replaced in large quantities, so once the LED life is over, it means the end of the display life.
We say that LED lifetime determines the life of the display, but it does not mean that LED lifetime is equal to display life. Since the display does not work at full load every time while the display is working, the display screen should have a lifetime of 6 to 10 times the life of the LED when the video is normally played. Working at a low current can last longer. Therefore, the display life of the brand LED is up to 50,000 hours.
How to make LEDs have a longer lifespan? In general, we can start from two aspects of device manufacturing and device application. In terms of device manufacturing: choose high-quality epitaxial materials; increase chip area, reduce current density; balance current density; reduce thermal resistance; select packaging materials with excellent performance and strong UV resistance, etc. .
From the aspect of device application: the heat dissipation is a central work from module design to engineering implementation and even future system maintenance; reducing LED working current; correctly configuring LEDs, making each primary color LED synchronously attenuate, etc. can extend the service life of LED.
Whether the LED lamp is stable, the quality is good or not, and the heat dissipation of the lamp body itself is important. At present, the heat dissipation of the high-brightness LED full-color screen on the market usually adopts natural heat dissipation, and the effect is not satisfactory. The heat dissipation is not ideal, and the life of the lamp itself will 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 meanings: first, energy conservation, emission reduction, and environmental protection; second, reducing power capacity, power equipment, and heat dissipation equipment. Input; save electricity costs and reduce operating costs; fourth, reduce display temperature rise; fifth, delay LED decay speed; sixth, improve system reliability; seventh, extend display life; , stabilize the image effect.
The luminous efficiency of an LED (ie, external quantum efficiency) is determined by the quantum efficiency and escape rate of the LED. Nowadays, the internal quantum efficiency of LEDs has been as high as 90% or more, but because of the low escape rate, external quantum efficiency has become a bottleneck for improving LED light efficiency. In order to break through the bottleneck that restricts the development of the industry, many novel solutions have been proposed and verified by theory. Most of them have entered the experimental stage, some have been successful, and laid a solid foundation for the final industrialization.
As a green, energy-saving light source, LED is favored by people, and it will also serve as a mainstream media to lead the future of display technology.
In short, device manufacturing and device application itself are a complementary unit. The advancement of device technology has brought prosperity to the application market, and the demand of the application market is the eternal driving force for device technology advancement. Only the upstream and downstream enterprises can work together to create a new future for LED display technology.