With the application of LED displays to all kinds of fields, more and more people are getting familiar with and beginning to use them. How can we judge the quality of LED displays to choose the most suitable product with a high cost performance for ourselves? On the basis of the knowledge of their basic technological standards, we can indentify their quality from the aspects of their function and performance.
First, from the aspect of their function, the basic function of LED displays is to show texts, images and videos to a large amount of people. The most important thing is to make a real show of the materials. Therefore, high fidelity is necessary. We can improve the fidelity through adjusting the following technological standards.
The degree of brightness brings people a direct sense. Once we see the LED display, we can sense it. Do not consider that the higher the brightness, the better the showing effect. The showing effect depends on the using environment and viewing distance. For indoor displays, it is usually about 1000nit. For outdoor ones, it is usually larger than 6000nit. For semi-outdoor ones, it is usually between 2500nit and 5000nit. High resolution ones require even lower brightness, usually about 100nit to 300nit. In a word, the most suitable brightness is the one that not only makes audiences see images clearly in different environments but also does not hurt their eyes while watching for a long time.
When the image colors are not the same as or different with those of the real materials, the picture will lose its fidelity. This is related to the color temperature of white balance. If people watch LED displays directly, the suitable color temperature of white balance is beteen 6500K to 8000K. If the LED displays are transmitted by TV stations, the color temperature needs to be adjusted to about 5500K. In this way, the pictures on LED displays will be real after transmitted by video cameras.
As for the display equipments, the larger the viewing angle, the better the showing effect, and of course the larger the viewing scope. The viewing angles can be divided into the horizontal one and the vertical one. Up to now, for indoor full-color LED displays, the horizontal one and the vertical one can be larger than 140° and 100° respectively. For outdoor full-color LED displays, the horizontal one and the vertical one can be larger than 100° and 90° respectively. On the basis of seeing pictures clearly and completely within the scope of viewing angle, colors can not be different while watching from different viewing angles. This is mainly related to the arrangement of the RGB chips during the encapsulation of LED lamps. Up to now, there are two ways of arrangement:
When the sense of layers on the screen is not good and there is the phenomenon of losing pixels compared with materials, this means that the gray level is low and the screen can not show the materials clearly in a real way. The professional testing way is to use gray scale strips, through which people can see the gray level clearly. Up to now, enterprises can realize 16bit gray scale. In the future, it will reach 18bit. The following pictures show gray scale strips used for testing:
If flicker appears when people watch LED displays and if dark lines appear when we use cameras to make a shot, it means the refresh rate is low. The refresh rate larger than 300Hz will not influence people’s viewing effect. However, if the showing effect is good while using cameras or video cameras, the refresh rate will be above 2000Hz. The following pictures show the comparison between high and low refresh rate:
When we watch pictures on LED displays, sometimes we will feel they are a little bit white and gray, like gauze covering on them. This means their contrast is low. Usually, the contrast can not be lower than 1000:1. The contrast is mainly related to the encapsulation of LED lamps and the design of module masks. The contrast of LED lamps on black holders is much higher than that of LED lamps with white holders. The following pictures show the effect comparison:
The appearance of spots and mosaic on LED displays mainly related to the brightness and color uniformity of LED lamps. We can check the brightness and color uniformity of LED displays by testing their RGB and white colors. The following pictures show the test and effect comparison:
After knowing the functions, a high-quality LED display should also have good performance. We can indentify its performance through the following methods.
（1）、Pixels out of control (under warranty)
Pixels out of control can be divided into pixels normally on and off. The following table shows the standards and classes in this industry:
With the development of high resolution LED displays, we can no longer apply these standards to judge them. The standard should be higher, with the rate of pixels out of control below 1X10-5
（2）、Levels of protection and safety
Levels of protection can be divided into waterproof levels and dustproof levels. The larger the IP number, the higher the levels of protection. The following table shows the standards in this industry:
afety can be represented by the following aspects: 1. Leakage current (when rated voltageX1.1, the earth leakage current should be ≤3.5mA; 2. Dielectric strength (under the condition of 1500V, 1min test without insulation breakdown).
（3）、Energy consumption and energy efficiency
By reducing the power consumption and improving the energy efficiency, we can lower the temperature and prolong the lifespan of LED displays.
If the temperature is 10℃ lower, the lifespan will double. The reduction of energy consumption and the improvement of energy efficiency can be
realized through the following aspects:
A、LED with low Vf value
B、Constant current driver IC with low Vds value
C、PCB with low impedance
D、Module working voltage: 4.2V 3.8V
E、Switching power supply PFC≥0.95 transfer efficiency (≥ 0.85)
Even though the effect of a LED screen is very good, if its reliability and stability are low, its quality is also very low. For the
improvement of reliability, we can focus on the following aspects:
A、Material selection: Material analysis
B、Thermal design: heat dissipation and distribution
C、Redundancy: switching power supply, controller and PCB circuit
D、Material tolerance: 20% margin
E、Simulation verification test for reliability: lifespan, authentication
After the above study, when we watch a LED screen, we will know how to check its quality and indentify its class. Therefore, we will finally choose a product most suitable to us with a high cost performance.