Analysis of Various Technical Indexes of LED Display Screen

Related index analysis of LED display screen1) Physical indexPixel center to pixel pitch the distance between adjacent pixel centers. (unit: mm)

Density (number of points)Number of pixels per unit area (unit: points / m2). There is a certain calculation relationship between points and point spacing. The calculation formula is: density = (1000 pixel center distance). The higher the density of LED display, the clearer the image and the smaller the optimal viewing distance range.Optimum sight distance

Viewing distance diagramLevel up degreeConcave convex deviation of LED, pixel, display module and display module when forming the plane of LED display screen. The poor flatness of LED display screen is easy to cause uneven screen color when viewing.

2) Electrical performance indexGray scaleThe brightness level that can be distinguished from the darkest to the brightest in the same level of brightness of LED display screen. Gray scale is the so-called color scale or gray scale, which refers to the brightness. For digital display technology, gray level is the decisive factor of the number of display colors. Generally speaking, the higher the gray level, the richer the color displayed, the more delicate the picture, and it is easier to show rich details.

The gray level mainly depends on the A / D conversion bits of the system. Of course, the video processing chip, memory and transmission system of the system should provide the support of corresponding bits. Generally, there are no gray levels, 8 levels, 16 levels, 32 levels, 64 levels, 128 levels, 256 levels, etc. the higher the gray level of LED display screen, the richer the color and the more gorgeous the color; On the contrary, the display color is single and the change is simple.At present, the domestic LED display mainly adopts 8-bit processing system, that is, 256 (28) gray level. A simple understanding is that there are 256 brightness changes from black to white. RGB three primary colors can be used to form 256 two hundred and fifty-six 256 = 16777216 colors. That is commonly known as 16 trillion color. International brand display screen mainly adopts 10 bit processing system, i.e. 1024 gray level, and RGB three primary colors can form 1.07 billion colors.Refresh frame frequency

Frequency of LED display screen information update. It is generally 25Hz, 30Hz, 50Hz, 60Hz, etc. the higher the frame change frequency, the better the continuity of the changed image.Refresh frequencyThe LED display shows the number of times the data is repeatedly displayed per second. Usually 60Hz, 120Hz, 240Hz, etc. the higher the refresh frequency, the more stable the image display.

3) Optical indexBrightness of LED screenThe average brightness of the LED display in the normal direction. Unit: CD / m2.

Under the same point density, the brightness of LED display screen depends on the material, packaging form and size of LED chip. The larger the chip, the higher the brightness; Conversely, the lower the brightness. Illustration of brightness proportional to wafer size.When the brightness in the horizontal and vertical directions is half of the brightness in the normal direction of the LED display screen, the included angle between the observation direction and the normal of the LED display screen is called the horizontal angle of view and the vertical angle of view respectively. Generally, the left and right degrees and the up and down degrees are expressed in 1.

If the horizontal viewing angle of a display screen is 120 degrees and the vertical viewing angle is 45 degrees, all viewers can enjoy the best viewing effect within this viewing range. Beyond this range, the audience will be able to see a visual effect 50% lower than the normal brightness. The larger the perspective of LED display screen, the more its audience groups, the wider the coverage area, and vice versa. The packaging mode of LED chip determines the viewing angle of LED display screen. Among them, the viewing angle of surface pasted LED lamp is better, and the horizontal viewing angle of Oval LED single lamp is better. The viewing angle is inversely proportional to the brightness.

Perspective rangeLife of LED screenLED is a semiconductor device with a service life of 100000 hours. The service life of LED display depends on the service life of LED lamps and electronic components used in the display. Generally, the mean time between failures shall not be less than 10000 hours.

How to distinguish the gray and brightness of LED display?The gray scale of LED display screen is the so-called color scale or gray scale, which refers to the brightness. For digital display technology, gray level is the decisive factor of the number of display colors. Generally speaking, the higher the gray level, the richer the color displayed, the more delicate the picture, and it is easier to show rich details.

The gray level of LED display screen mainly depends on the A / D conversion bits of the system. Of course, the video processing chip, memory and transmission system of the system should provide the support of corresponding bits. At present, the domestic LED display mainly adopts 8-bit processing system, that is, 256 (28) gray level. A simple understanding is that there are 256 brightness changes from black to white. RGB three primary colors can be used to form 256 two hundred and fifty-six 256 = 16777216 colors. That is commonly known as 16 trillion color. International brand display screen mainly adopts 10 bit processing system, i.e. 1024 gray level, and RGB three primary colors can form 1.07 billion colors.

Although gray is the decisive factor to determine the number of colors, it does not mean that the larger the unlimited, the better. Firstly, the resolution of human eyes is limited. Moreover, the improvement of system processing bits will involve the changes of system video processing, storage, transmission, scanning and other links, resulting in a sharp increase in cost and a decrease in cost performance. Generally speaking, 8-bit system can be used for civil or commercial products, and 10 bit system can be used for broadcast products.Gray nonlinear transformation

Gray nonlinear transformation refers to the transformation of gray data according to empirical data or some arithmetic nonlinear relationship, and then provided to the display screen for display. Because LED is a linear device, its nonlinear display characteristics are different from those of traditional display. 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 gray data will be done at the rear level of the LED display system, and the number of transformed data bits will increase (to ensure that the gray level data will not be lost).

At present, the so-called 4096 gray level or 16384 gray level or higher by some domestic control system suppliers refers to the size of gray space after nonlinear transformation. Level 4096 adopts the nonlinear transformation technology from 8-bit source to 12 bit space, and level 16384 adopts the nonlinear transformation technology from 8-bit to 16 bit. The space after nonlinear transformation by 8-bit source must be larger than that of 8-bit source. Usually at least 10. Like the gray level, the larger the parameter, the better. Generally, 12 bits can make enough transformation.Pixel out of control rate

Pixel out of control rate refers to the proportion of the smallest imaging unit (pixel) of the display screen not working normally (out of control). The pixel out of control has two modes: one is the blind spot, that is, the blind spot. It is not bright when it needs to be bright, which is called the blind spot; The second is the constant bright spot. When it needs not to be bright, it is always bright, which is called the constant bright spot. Generally, the composition of pixels includes 2R1G1B (two red lights, one green light and one blue light, the same below), 1R1G1B, 2R1G, 3r6g, etc. generally, out of control will not mean that all the red, green and blue lights in the same pixel are out of control at the same time, but as long as one of the lights is out of control, we consider it as out of control. For simplicity, we count and calculate the out of control pixels according to the primary colors (i.e. red, green and blue) of the LED display screen, and take the maximum value as the out of control rate of the pixels of the display screen.

The ratio of the number of out of control pixels to the total number of pixels on the full screen is called "out of control rate of pixels on the whole screen". In addition, in order to avoid out of control pixels concentrated in a certain region, we propose "regional pixel out of control rate", that is, at 100 The ratio of the number of pixels out of control to the total number of pixels in the 100 pixel area (i.e. 10000). This index quantifies the requirement of "pixels out of control are distributed discretely" in general specification for LED display screen SJ / t11141-2003, which is convenient and intuitive.

At present, the domestic LED display screen will be aged (toaster) before leaving the factory, and the LED lights of out of control pixels will be repaired and replaced. It is no problem to control the "out of control rate of whole screen pixels" within 1 / 104 and the "out of control rate of regional pixels" within 3 / 104. Even some enterprise standards require that out of control pixels are not allowed before leaving the factory, However, this is bound to increase the manufacturing and maintenance costs of manufacturers and prolong the delivery time. In different applications, the actual requirements of pixel out of control rate can vary greatly. Generally speaking, LED display screen is used for video playback, and the index requirement is acceptable and achievable within 1 / 104; If it is used for simple character information release, it is reasonable to control the index within 12 / 104.

Brightness discrimination level