What is the grayscale, brightness and out-of-control rate of LED display

- Feb 14, 2021-

What is the gray scale of LED display


Gray scale is the so-called color scale or gray scale, which refers to the degree of brightness. For digital display technology, grayscale is the decisive factor for the number of colors displayed. Generally speaking, the higher the gray level, the richer the displayed color, the more delicate the picture, and the easier it is to express rich details.


The gray level mainly depends on the system's A/D conversion bits. Of course, the system's video processing chip, memory, and transmission system must provide support for the corresponding number of bits. At present, the domestic LED display screen mainly adopts an 8-bit processing system, that is, 256 (28) gray levels. The simple understanding is that there are 256 brightness changes from black to white. Using RGB three primary colors can form 256×256×256=16777216 colors. That is commonly referred to as 16 mega colors. International brand display screens mainly adopt a 10-bit processing system, that is, 1024 gray levels, and the three primary colors of RGB can constitute 1.07 billion colors.


Although grayscale is the decisive factor in determining the number of colors, it does not mean that the larger the limit, the better. Because firstly the resolution of the human eye is limited, and the increase in the number of system processing bits will involve changes in the system's video processing, storage, transmission, scanning and other links, resulting in a sharp increase in cost and a decline in cost performance. Generally speaking, civil or commercial grade products can use 8-bit systems, and broadcast grade products can use 10-bit systems.

Gray non-linear transformation


Gray-scale nonlinear transformation refers to the transformation of gray-scale data according to empirical data or a certain arithmetic nonlinear relationship and then provide it to the display screen. Since LEDs are linear devices, they have different non-linear display characteristics from traditional displays. In order to enable the LED display effect to 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 latter stage of the LED display system, and the number of data bits after the transformation will increase (to ensure that the gray level is not lost data). At present, the so-called 4096-level grayscale or 16384-level grayscale or higher by some domestic control system suppliers refer to the grayscale space size after nonlinear transformation. The 4096 level uses the non-linear transformation technology from 8-bit source to 12-bit space, and the 16384 level uses the 8-bit to 16-bit non-linear transformation technology. The non-linear transformation is performed by the 8-bit source, and the space after the conversion is definitely larger than that of the 8-bit source. Generally at least 10 people. Like the gray scale, this parameter is not as big as possible. Generally, 12 bits can do enough transformation.


What is the pixel out of control rate


The pixel out-of-control 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 blind spot, that is, blind spot, when it needs to be bright, it is called blind spot; the second is always bright spot, it keeps on when it needs to be off. Call it a bright spot. Generally, the composition of the pixel is 2R1G1B (2 red lights, 1 green light and 1 blue light, the same as below), 1R1G1B, 2R1G, 3R6G, etc., and out of control is generally not 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 the pixel is out of control. For simplicity, we perform statistics and calculations of out-of-control pixels according to the primary colors of the LED display (ie red, green, and blue), and take the maximum value as the out-of-control rate of the display.


The ratio of the number of out of control pixels to the total number of pixels in the full screen is called the "full screen pixel out of control rate". In addition, in order to avoid the concentration of out-of-control pixels in a certain area, we propose the "regional pixel out-of-control rate", that is, the ratio of the number of out-of-control pixels to the total number of regional pixels (ie 10000) in a 100×100 pixel area. This indicator quantifies the requirement that "out of control pixels are discretely distributed" in "General Specification for LED Displays" SJ/T11141-2003, which is convenient and intuitive.


At present, the domestic LED display screen will be aging (baking machine) before leaving the factory, and the LED lights of the out-of-control pixels will be repaired and replaced. The "full screen pixel out of control rate" is controlled within 1/104, and the "regional pixel out of control rate" is controlled. Within 3/104, there is no problem. Even some individual manufacturers’ corporate standards require no out-of-control pixels are allowed before leaving the factory, but this will inevitably increase the manufacturer’s manufacturing and maintenance costs and extend the delivery time. In different applications, the actual requirements for the pixel out-of-control rate can be quite different. Generally speaking, LED display screens are used for video playback, and the index requirements are acceptable and reachable within 1/104; If it is used for simple character information publishing, it is reasonable to control the indicator within 12/104.


What is the brightness discrimination level


The brightness discrimination level refers to the brightness level of an image that can be distinguished by the human eye from the darkest to the whitest. As mentioned earlier, the gray level of the display screen is very high, which can reach 256 or even 1024. However, due to the limited sensitivity of human eyes to brightness, these gray levels cannot be fully recognized. That is to say, it is possible that many adjacent levels of gray scale human eyes look the same. Moreover, the distinguishing ability of eyes varies from person to person. For the display screen, the higher the level of human eye recognition, the better, because the displayed image is for people to see after all. The more brightness levels the human eye can distinguish, the larger the color space of the display screen, and the greater the potential for displaying rich colors. The brightness discrimination level can be tested with special software. Generally, the display screen can reach 20 levels or more, even if it is a good level.


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