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How to Evaluate the Quality of the LED Screen Display?
In this issue, I have sorted out how to evaluate the quality of the LED screen display. Detailed introduction of key technical indicators of LED display. And how to do LED anti-static measures? I believe that it is helpful to you.
1. So how to evaluate the quality of the LED screen display?
The quality of a full-color display can be signed from the following aspects.
• Flatness
The surface flatness of the LED display screen should be within ±1mm to ensure that the displayed image is not distorted, and local protrusions or recesses will cause the viewing angle of the display screen to appear dead
• Brightness and viewing angle.
The brightness of the indoor full-color LED display screen must be above 800cd/m², and the brightness of the outdoor full-color LED display screen must be above 1500 cd/m² to ensure the normal operation of the display, otherwise, the displayed image will not be clear because the brightness is too low.
The brightness is mainly determined by the quality of the LEDs. The size of the viewing angle directly determines the number of display screen audiences, so the larger the better. The size of the viewing angle is mainly determined by the packaging method of the LED die.
• White balance effect
The white balance effect is one of the most important indicators of the display. Chromatically, when the ratio of the three primary colors of red, green, and blue is 1:4.6:0.16, it will show pure white. If there is a slight deviation in the actual ratio, there will be a deviation of the white balance and green phenomenon. The quality of the white balance is mainly determined by the control system of the display screen, and the led die also has an influence on the color reproduction.
• Color reproducibility
The reproducibility of color refers to the reproducibility of the display screen to the color, that is, the color displayed on the display screen must be highly consistent with the color of the playback source, so as to ensure the authenticity of the image.
• Whether there are mosaic or dead spots
Mosaic refers to the small squares that are always bright or often black appearing on the display screen, which is the phenomenon of module necrosis. The main reason is that the quality of the connectors used in the display screen is not good quality. The dead point refers to a single point that is always bright or often black appearing on the display screen. The amount of the dead point is mainly determined by the quality of the LED die.
• With or without color blocks
Color block refers to the obvious color difference between adjacent modules. The color transition is based on the module. The color block phenomenon is mainly caused by poor control systems, low gray levels, and low scanning frequency.
2. Key technical indicators of LED display
• Pixel runaway rate
The pixel runaway rate refers to the proportion of the smallest imaging unit (pixel) of the display that is not working properly (out of control). There are two modes of pixel out-of-control: one is the blind spot, that is, the blind spot, when it needs to be bright, called the blind spot. The second is the constant bright spot, which is always on when it needs to be off. It is called the constant bright spot. In general, the composition of pixels is 2R1G1B (2 red lights, 1 green light, and 1 blue light, the same applies below), 1R1G1B, 2R1G, 3R6G, etc., and the runaway is generally not the same as the red, The green and blue lights are all out of control at the same time, but as long as one of the lights is out of control, we think that this pixel is out of control. For simplicity, we count and calculate the out-of-control pixels according to the primary colors (ie, red, green, and blue) of the LED display, and take the maximum value as the pixel out-of-control rate of the display.
The ratio of the number of pixels out of control to the total number of pixels in the full screen is called the “out-of-control pixel rate of the whole screen”. In addition, in order to prevent out-of-control pixels from being concentrated in a certain area, we propose the “area out-of-control rate”, that is, the ratio of the number of out-of-control pixels to the total number of area pixels (that is, 10,000) in a 100×100 pixel area. This indicator quantifies the requirement of "out-of-control pixels are discretely distributed" in "General Specifications for LED Display Screens" SJ/T11141-2003, which is convenient and intuitive.
At present, domestic LED display screens will be aging (roasting machine) before leaving the factory. The LED lights of out-of-control pixels will be repaired and replaced. The "out-of-screen pixel out-of-control rate" is controlled within 1/104. It is no problem to control the "regional pixel runaway rate" within 3/104, and even some manufacturers' enterprise standards require that runaway pixel are not allowed before leaving the factory, but this will inevitably increase the manufacturer's manufacturing maintenance costs and extend shipments. time. In different applications, the actual requirements of the pixel runaway rate can be quite different. Generally speaking, LED display screens are used for video playback, and it is acceptable and achievable to control the index within 1/104. If it is used for simple character information release, it is reasonable to control the index within 12/104.
• Gray level
Grayscale is the so-called color scale or grayscale, which refers to the brightness of light and dark. For digital display technology, grayscale is the determining factor for the number of colors displayed. Generally speaking, the higher the grayscale, the richer the displayed colors, and the finer the picture, the easier it is to express rich details. The grayscale mainly depends on the number of A/D conversion bits of the system. Of course, the video processing chip, memory, and transmission system of the system must provide the corresponding number of bits of support. At present, domestic LED display mainly uses an 8-bit processing system, that is, 256 (28) grayscale. A simple understanding is that there are 256 brightness changes from black to white. The three primary colors of RGB can form 256×256×256=16777216 colors. That is commonly called 16 trillion colors. The international brand display mainly uses a 10-bit processing system, that is, 1024-level grayscale, RGB three primary colors can constitute 1.07 billion colors.
Although grayscale is the deciding factor in determining the number of colors, it does not mean that the greater the limit, the better. Because the resolution of the human eye is limited first, and the increase in the number of processing bits of the system will involve changes in various aspects of the system's video processing, storage, transmission, scanning, etc., the cost will increase sharply, and the cost performance will decline. Generally speaking, civil or commercial-grade products can adopt an 8-bit system, and broadcast-grade products can adopt a 10-bit system.
3. Brightness discrimination level
The brightness discrimination level refers to the brightness level of the image that can be distinguished by the human eye from the darkest to the whitest. As mentioned earlier, the grayscale of the display is very high, which can reach 256 levels or even 1024 levels. However, due to the limited sensitivity of the human eye to brightness, these gray levels cannot be fully recognized. In other words, many gray levels of adjacent levels may look the same. And each person's ability to distinguish eyes is different. For the display screen, the level of human eye recognition is naturally the more, the better, because the displayed image is for humans after all. The more brightness levels the human eye can resolve, the greater the color space of the display, and the greater the potential for displaying rich colors. The brightness discrimination level can be tested with special software. Generally, the LED display screen can reach more than 20 levels, it is a relatively good level.
4. Gray-scale nonlinear transformation
Non-linear gray-scale transformation refers to transforming gray-scale data according to empirical data or some arithmetic nonlinear relationship and providing it to the display screen for display. Since LEDs are linear devices, they have different nonlinear display characteristics from traditional displays. In order to make the LED display effect conform to the traditional data source without losing the gray level, generally, the non-linear transformation of the gray data will be done in the rear stage of the LED display system, and the number of digits after the conversion will increase (guarantee that the gray will not be lost data). Now the so-called 4096-level grayscale or 16384-level grayscale or higher of some domestic control system suppliers refers to the size of grayscale space after nonlinear transformation. Level 4096 is a non-linear transformation technique using an 8-bit source to 12-bit space, and level 16384 is a non-linear transformation technique using 8-bit to 16-bit. Non-linear transformation is done by an 8-bit source, the space after conversion is definitely larger than an 8-bit source. Generally at least 10 people. As with grayscale, this parameter is not as large as possible. Generally, 12 bits can be used for sufficient conversion.
5. LED anti-static measures
LED attaches great importance to anti-static measures, the following are several instructions for static electricity and anti-static. The sources of static electricity that affect the circuit mainly include the human body, plastic products, and related equipment and instruments. Among them, the static power source from the use environment has the following items.
• Objects, materials
• Floor, work table and chair
• Work clothes, packaging containers
• Painted or waxed surfaces, organic and fiberglass materials.
• Cement floors, painted or waxed floors, plastic floor tiles, or floor leather.
• Chemical fiber work clothes, non-conductive work shoes, clean cotton work clothes.
• Plastic, packing boxes, boxes, bags, trays, foam plastic liners.
The failure mode of electrostatic discharge: sudden failure and potential failure.
The static failure of more than 90 in the use environment is a potential failure, which is manifested by the weakening of the circuit's ability to resist electrical overstress and shortened service life.
Anti-static measures.
• Conduct training on static electricity knowledge and related technologies for those who use static sensitive circuits.
• Establish an anti-static work area, use an anti-static floor, an anti-static workbench, an anti-static ground lead, and anti-static appliances in the area, and control the relative humidity to 40 or more.
• The harm caused by static electricity to electronic equipment may be anywhere from the manufacturer to the field equipment. The hazard is caused by insufficient, effective training and equipment manipulation failure. LED is a device sensitive to static electricity. InGaN chips are generally considered to be the "first" vulnerable to interference. And ALINGAP LEDS SHI "second place" or better.
• ESD-damaged devices can show dimness, blur, extinction, short or low VF or VR. Equipment damaged by ESD should not be confused with electronic overloads, such as: due to wrong current design or drive, chip hook-up, wire shield grounding or packaging, or general environmental induced pressure.
• SD safety and control procedures: ESD of most electronics and electro-optic companies are very similar, and have successfully implemented ESD control, manipulation, and main procedures of all equipment. These procedures have been used as instruments for detecting quality effects since ancient times. ISO-9000 certification also lists him as a normal control procedure.
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