Five factors determining the quality of LED display

LED (Light Emitting Diode) is as important to the display screen as the engine of the car and the compressor of the air conditioner. Selecting an LED with excellent performance is the basic condition for completing a high-performance LED display. However, even with the same side dishes, the same seasonings, and the same stove, chefs of different levels may cook different dishes. Therefore, whether LEDs can be used well is the touchstone for testing display manufacturers. It is generally believed that the following five key performance indicators of the display are closely related to the LED quality parameters: brightness and viewing angle, uniformity and clarity, pixel runaway rate, life, energy consumption and energy efficiency.

1. Brightness and viewing angle

The brightness of the display mainly depends on the LED light intensity and LED density

The brightness of the display mainly depends on the luminous intensity and LED density of the LED. In recent years, LEDs have emerged with new technologies in substrates, epitaxy, chips, and packaging. In particular, the stability and maturity of indium tin oxide (ITO) current spreading layer technology and technology have greatly improved the luminous intensity of LEDs. At present, when the horizontal viewing angle is 110 degrees and the vertical viewing angle is 50 degrees, the luminous intensity of the green tube has reached 4000mcd, the red tube is 1500mcd, and the blue tube is 1000mcd. When the pixel pitch is 20mm, The brightness of the display can reach more than 10000nit. The display can work around the clock in any environment.

When it comes to the viewing angle of the display screen, there is a phenomenon worthy of our consideration: LED display screens, especially outdoor display screens, people's viewing angles are basically from the bottom up, and from the existing LED display product form, there are Half of the luminous flux disappears into the vast sky. In today's energy shortage, do we have a more reasonable solution? Worth pondering.

2. Uniformity and clarity

The inconsistency of LED performance parameters is the main reason for uniformity

With the development of LED display technology to this day, uniformity has become the most important indicator for measuring the quality of display screens. It is often said that the LED display is "a little bit splendid and brilliant", which is an image metaphor for the serious unevenness between pixels and modules. A professional statement is "dust effect" and "mosaic phenomenon".

The main causes of unevenness are: the inconsistency of LED performance parameters; the lack of assembly accuracy of the display during production and installation; the consistency of the electrical parameters of other electronic components; the irregular design of modules and PCBs.

Among them, "the inconsistency of LED performance parameters" is the main reason. The inconsistency of these performance parameters mainly includes: inconsistency of light intensity, inconsistency of optical axis, inconsistency of color coordinates, inconsistency of distribution curve of light intensity of each primary color, and inconsistency of attenuation characteristics. How to solve the inconsistency of LED performance parameters, there are currently two main technical approaches in the industry: one is to further subdivide the LED specification parameters to improve the consistency of LED performance; the second is to improve the display through subsequent correction Uniformity. Subsequent corrections have also evolved from early module corrections and module corrections to point-by-point corrections today. The correction technology is developed from simple light intensity correction to light intensity + color coordinate correction.

However, we believe that subsequent corrections are not omnipotent. Among them, inconsistent optical axes, inconsistent light intensity distribution curves, inconsistent attenuation characteristics, poor assembly accuracy, and design irregularities cannot be eliminated by subsequent corrections, and even such subsequent corrections will cause optical axis, attenuation, and assembly accuracy. The inconsistency worsens.

Therefore, through practice, our conclusion is that the subsequent correction is only a cure, and the LED parameter subdivision is the root cause, and it is the future mainstream of the LED display industry.

When it comes to the relationship between display uniformity and resolution, there is often a misunderstanding in the industry that resolution is used instead of resolution. In fact, the definition of the display screen is the subjective experience of the human eye on the display screen resolution, uniformity (signal-to-noise ratio), brightness, contrast and other factors. Simply reducing the physical pixel pitch to increase the resolution, while ignoring the uniformity, has no doubt about improving the clarity. Imagine a display screen with serious "dust effect" and "mosaic phenomenon". Even if the physical pixel pitch is small and the resolution is high, it is impossible to get a good image clarity.

Therefore, in a sense, the main reason for restricting the improvement of the clarity of LED display screens is "uniformity" rather than "physical pixel pitch".

3. The display pixels are out of control

The main reason for the pixel out of control of the display is LED failure

There are many reasons for the pixel out of control of the display screen, and the main reason is "LED failure".

The main causes of LED failure can be divided into two aspects: one is the poor quality of LED itself; the other is the improper use. Through analysis, we summarized the correspondence between the LED failure mode and the above two main causes.

As we mentioned above, the failure of many LEDs is usually not found in the routine inspection and testing of LEDs. In addition to improper use due to electrostatic discharge, high current (causing excessive junction temperature), external strength, etc., many LED failures are due to high temperature, low temperature, rapid temperature change or other harsh conditions, due to LED chips, epoxy resin, brackets , Inner lead, die bonding glue, PPA cup body and other materials thermal expansion coefficient difference, caused by the difference in its internal stress and generated, therefore, LED quality inspection is a very complicated task.

Furthermore, for GaN-based LEDs, electrostatic discharge is the biggest cause of its failure. The mechanism of LED failure caused by electrostatic discharge is very complicated. Equipment, tools, utensils and human bodies may carry static electricity and discharge it. This static electricity ranges from a few hundred volts to a high tens of thousands of volts. . The failure of the blue-green tube in the production, installation and use of the display screen is often caused by the breakdown of the LED-PN junction by electrostatic discharge. The International Electrostatic Association strictly stipulates the standard electrostatic discharge mode, which is mainly divided into the human body discharge mode (HBM) and the machine discharge mode (MM). The electrostatic discharge sensitivity (ESDS) of devices in China is divided into three levels (human body model): level 1 is 0 ~ 1999V; level 2 is 2000 ~ 3999V; level 3 is above 4000V. Under normal circumstances, the electrostatic discharge sensitivity of LEDs is between several hundred volts and tens of thousands of volts in human body mode, and only tens to 500 volts in machine mode. Due to the complicated production process of LED display screens, electrostatic discharge is extremely difficult to prevent. Therefore, LED electrostatic discharge sensitivity should be selected to be level 2 or above (human body model), and electrostatic protection must be throughout the entire production process.

4. Life

The life of the LED determines the life of the display

The life of the LED display is determined by many factors, but the end of life caused by many factors can be continuously extended through the replacement of parts (such as switching power supply). The LED is unlikely to be replaced in large quantities, so once the LED's end of life, it means the end of the display's life. To a certain extent, the life of the LED determines the life of the display. The life time of LED is usually the time when the luminous intensity decays to 50% of the initial value. As a semiconductor material, LED is often said to have a life span of 100,000 hours, but that is an evaluation under ideal conditions. However, it cannot be achieved in actual use. We have a simple experimental method and calculation formula to measure the life of the LED: place the LED under the same conditions as the actual working environment to work for 1000 hours, and measure the initial value and final value of the light intensity, and then can be derived through the formula LED lifetime. We selected a well-known brand blue tube to work for 1000 hours in an environment with an ambient temperature of 50 ℃ and a current of 20mA. The measured final value is 0.88 × initial value. According to the formula, we can calculate the life of the blue tube in this environment as 5422 hours.

We say that the LED life determines the life of the display, but it does not mean that the LED life is equal to the life of the display. Because the display screen is not working, every LED is working at full load at all times. When the display screen is playing a video program normally, the life span of the display screen should be 6 to 10 times that of the LED. When the LED The service life can be longer under the condition of small current. Therefore, the life span of the display screen using the brand LED can reach about 50,000 hours.

How to make LED life longer? In general, we can start from two aspects: device manufacturing and device application. From the aspect of device manufacturing: choosing high-quality epitaxial materials; increasing the chip area, reducing the current density; balancing the current density; reducing the thermal resistance; choosing the packaging material with excellent performance and strong UV resistance can make the LED life longer .

From the aspect of device application: heat dissipation is a central task from module design to project implementation and even future system maintenance; reducing LED operating current; properly configuring LEDs, so that the primary color LEDs are simultaneously decayed, etc. can extend the life of LEDs.

5. Energy consumption and energy efficiency

Increasing LED light efficiency and reducing display energy consumption is the development direction

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 power expansion, power equipment, and heat dissipation equipment. Investment; three is to save electricity costs and reduce operating costs; four is to reduce the temperature rise of the display; five is to delay the LED attenuation rate; six is ​​to improve system reliability; seven is to extend the life of the display; eight is to reduce the temperature drift of the photoelectric parameters of the display , To stabilize the image effect.

The luminous efficiency (ie external quantum efficiency) of an LED is determined by the internal quantum efficiency and the escape rate of the LED. Nowadays, the internal quantum efficiency of LEDs has reached more than 90%, but due to the low escape rate, the external quantum efficiency has become the bottleneck for improving LED light efficiency. In order to break through this bottleneck restricting the development of the industry, many novel solutions have been proposed and theoretically verified. Most of them have entered the experimental stage, and some have been successful, and have laid a solid foundation for the final industrialization.

LED is favored by people as a green and energy-saving light source, and will surely lead the future of display technology as a mainstream media.

In short, device manufacturing and device application are a complementary unity. The advancement of device technology brings prosperity to the application market, and the demand of the application market is the eternal power of device technology progress. Let us upstream and downstream companies work together to create a new future for LED display technology.

Chaff Cutter

Chaff Cutter,Chaff Cutter Machine,Mini Chaff Cutter,Hand Chaff Cutter

Hunan Furui Mechanical and Electrical Equipment Manufacturing Co., Ltd. , https://www.thresher.nl

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