Detailed explanation of relevant technical indicators of LED display screen

Analysis of relevant indicators of LED display screen

1) Physical indicators

Pixel center distance pixelEL pitch (dot pitch) The distance between the centers of adjacent pixels. (Unit: mm)

Density density (number of points)

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

Optimal viewing distance

Viewing distance diagram

Level up degree

The unevenness of the LED, pixel, display module, and display module when forming the plane of the LED display. The poor flatness of the LED display can easily lead to uneven color of the screen when viewing.

2) Electrical performance indicators

Gray scale

The same level of brightness of LED display screens is the number of brightness levels that can be distinguished from the darkest to the brightest. Grayscale is also known as color scale or grayscale, which refers to the degree of brightness. For digital display technology, grayscale is the determining factor of the number of displayed colors. Generally speaking, the higher the grayscale, the richer the displayed colors, the more delicate the picture, and the easier it is to show rich details.

Grayscale mainly depends on the number of A/D conversion bits of the system. Of course, the system's video processing chip, memory and transmission system must provide support for the corresponding number of bits. Generally, there are no grayscale, 8 levels, 16 levels, 32 levels, 64 levels, 128 levels, 256 levels, etc. The higher the grayscale level of the LED display, the richer the color and the more colorful it is; on the contrary, the display color is single and the change is simple.

At present, domestic LED display screens mainly use 8-bit processing system, that is, 256 (28) grayscale levels. Simply put, there are 256 brightness changes from black to white. Using the three primary colors of RGB can form 256×256×256=16777216 colors. This is commonly known as 16 trillion colors. International brand display screens mainly use 10-bit processing system, that is, 1024 grayscale levels, and the three primary colors of RGB can form 1.07 billion colors.

refresh frame frequency

LED display screen information update frequency. Generally, it is 25Hz, 30Hz, 50Hz, 60Hz, etc. The higher the frame change frequency, the better the continuity of the changing image.

refresh frequency

The number of times the LED display screen displays data repeatedly per second. Usually 60Hz, 120Hz, 240Hz, etc. The higher the refresh rate, the more stable the image display.

3) Optical Indicators

Luminance of LED screen ​

The average brightness of the LED display in the normal direction. Unit: cd/m2.
Under the same point density, the brightness of the LED display depends on the material, packaging form and size of the LED chip used. The larger the chip, the higher the brightness; conversely, the lower the brightness. Illustration showing that brightness is proportional to chip size.

When the brightness in the horizontal and vertical directions is half of the brightness in the normal direction of the LED display, the angles between the viewing direction and the normal of the LED display are called the horizontal viewing angle and the vertical viewing angle, respectively. Generally, ± is used to indicate the number of degrees left and right and up and down. If

the horizontal viewing angle of a display 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 that is 50% lower than the normal brightness. The larger the viewing angle of the LED display, the more audience groups it has and the wider the coverage area, and vice versa. The packaging method of the LED chip determines the size of the viewing angle of the LED display. Among them, the viewing angle of the surface-mounted LED lamp is better, and the horizontal viewing angle of the elliptical LED single lamp is better. The viewing angle is inversely proportional to the brightness.

Viewing angle range

Life of LED screen

LED is a semiconductor device with a lifespan of 100,000 hours. The lifespan of an LED display depends on the lifespan of the LED lamps used and the lifespan of the electronic components used in the display. Generally, the average failure-free time is not less than 10,000 hours.

How to distinguish the grayscale and brightness of LED display?

The grayscale of LED display is also called color scale or grayscale, which refers to the degree of brightness. For digital display technology, grayscale is the determining factor of the number of displayed colors. Generally speaking, the higher the grayscale, the richer the displayed colors, the more delicate the picture, and the easier it is to show rich details.

The grayscale level of LED display mainly depends on the number of bits of A/D conversion in the system. Of course, the system's video processing chip, memory and transmission system must provide support for the corresponding number of bits. At present, domestic LED display screens mainly use 8-bit processing systems, that is, 256 (28) grayscale levels. Simply put, there are 256 brightness changes from black to white. Using the three primary colors of RGB, 256×256×256=16777216 colors can be formed. This is commonly known as 16 trillion colors. International brand display screens mainly use 10-bit processing systems, that is, 1024 grayscale levels, and the three primary colors of RGB can form 1.07 billion colors.

Although grayscale is the determining factor for the number of colors, it does not mean that the larger the better. First of all, the resolution of the human eye is limited. Secondly, the increase in the number of system processing bits will involve changes in various links such as system video processing, storage, transmission, scanning, etc., which will increase costs dramatically and reduce cost performance. Generally speaking, civilian or commercial products can use 8-bit systems, and broadcast products can use 10-bit systems.

Grayscale Nonlinear Transformation

Grayscale nonlinear transformation refers to transforming grayscale data according to empirical data or some arithmetic nonlinear relationship and then providing it to the display screen for display. Since LED is a linear device, it has different nonlinear display characteristics from traditional displays. In order to make the LED display effect conform to the traditional data source without losing the grayscale level, the nonlinear transformation of grayscale data is generally performed in the back stage of the LED display system, and the number of bits of the transformed data will increase (to ensure that the grayscale data is not lost).

At present, the so-called 4096-level grayscale or 16384-level grayscale or higher by some domestic control system suppliers refers to the size of the grayscale space after nonlinear transformation. 4096-level adopts the nonlinear transformation technology from 8-bit source to 12-bit space, and 16384-level adopts the nonlinear transformation technology from 8-bit to 16-bit. The space after nonlinear transformation from 8-bit source is definitely larger than that of 8-bit source. Generally, it is at least 10 bits. Just like grayscale, this parameter is not the larger the better, and generally 12 bits can do enough transformation.

Pixel loss rate

The pixel out-of-control rate refers to the proportion of the smallest imaging unit (pixel) of the display screen 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, which is not lit when it needs to be lit, which is called blind spot; the other is constant bright spot, which is always on when it needs to be off, which is called constant bright spot. Generally, the composition of pixels is 2R1G1B (2 red lights, 1 green light and 1 blue light, the same as below), 1R1G1B, 2R1G, 3R6G, etc., and out-of-control generally does not mean that the red, green and blue lights in the same pixel are all out of control at the same time, but as long as one of the lights is out of control, we consider this pixel to be out of control. For simplicity, we count and calculate the out-of-control pixels according to each primary color (i.e. red, green and blue) of the LED display screen, and take the maximum value as the pixel out-of-control rate of the display screen.

The ratio of out-of-control pixels to the total number of pixels on the whole screen is called the "whole screen pixel out-of-control rate". In addition, to avoid the concentration of out-of-control pixels in a certain area, we propose the "regional pixel out-of-control rate", which is the ratio of the number of out-of-control pixels to the total number of regional pixels (i.e. 10,000) in a 100×100 pixel area. This indicator quantifies the requirement of "out-of-control pixels are discretely distributed" in the "General Specification for LED Displays" SJ/T11141-2003, which is convenient and intuitive.

At present, domestic LED display screens are aged (baked) 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 "whole screen pixel out-of-control rate" within 1/104 and the "regional pixel out-of-control rate" within 3/104. Some individual manufacturers' corporate standards even require that out-of-control pixels are not allowed before leaving the factory, but this will inevitably increase the manufacturer's manufacturing and maintenance costs and extend the delivery time. In different application scenarios, the actual requirements for pixel out-of-control rate can vary greatly. Generally speaking, for LED display screens used for video playback, 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.

Brightness identification level

Brightness discrimination level refers to the brightness level of the image that the human eye can distinguish from the blackest to the whitest. As mentioned earlier, the grayscale level of the display screen is very high, which can reach 256 or even 1024 levels. However, due to the limited sensitivity of the human eye to brightness, these grayscale levels cannot be fully recognized. In other words, many adjacent grayscale levels may look the same to the human eye. And the eye's discrimination ability is different for each person. For the display screen, the more levels the human eye can recognize, the better, because the displayed image is for people to see after all. The more brightness levels that 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 dedicated software . Generally, a display screen that can reach more than 20 levels is considered a relatively good level.