1 Brightness calculation method of display screen
Taking the full-color screen as an example, the balance ratio of red, green, blue and white is usually 3:6:1
Brightness of red LED: brightness (CD) / m2 points / m2 & times; 0.3 (white balance ratio accounts for 30%) 2
Brightness of green LED: brightness (CD) / m2 points / m2 & times; 0.6 (white balance ratio accounts for 60%)
Brightness of blue LED: brightness (CD) / m2 points / m2 & times; 0.1 (white balance ratio accounts for 10%)
(1) The brightness of a single tube is obtained by knowing the brightness of the whole screen.
For example: 2500 dots per square meter density, 2R1G1B, 5000 CD / m2 brightness requirement, then:
Brightness of red LED: 5000 2500 & times; 0.32=0.3cd=300mcd
The brightness of green LED is 5000 2500 0.6=1.2cd=1200mcd
The brightness of blue LED is 5000 2500 0.1=0.2cd=200mcd
(2) Calculate the brightness of the whole screen with known single tube brightness.
For example, take p31.25, Riya tube as an example.
Main tube core specifications of HSM display screen are red and green
Hsm-ph-a (Riya) 180-440mcd1020-2400mcd
Because white balance and brightness ratio Red: Green: blue = 3:6:1; The ratio of white balance is matched with the brightness of green tube to other tubes. Therefore, it is as follows:
From the Red: Green = 3:6, it can be seen that the brightness of the green tube is twice that of the red tube, that is, the brightness of the red tube is 2400 (blue) 2 = 1200mcd. Because there are 2 red tubes among the four red, green and blue tubes, the brightness of a single red tube is 1200 2 = 600mcd
From the green: blue = 6:1, the brightness of the green tube is 6 times that of the blue tube, that is, the brightness of the blue tube is 2400 (blue) 6 = 400mcd, because one luminous pixel = 2 red tubes 1 green tube 1 blue tube;
That is, the brightness of a pixel = 600 (red) 2 2400 (green) 400 (blue) = 3400mcd = 3.4cd
Brightness per square meter = brightness of 1 luminous pixel Pixel density per square meter (number) = 3.4cd 1024 (number of pixels) = 3482cd. Calculated by 20% light loss, the actual luminous brightness should be 2785.28cd
2 Method for controlling LED gray level of LED display screen
Next, we will introduce two brightness control methods of LED display screen. One is to change the current flowing through the LED. Generally, the continuous working current allowed by the LED core is about 20 mA. Except that the red LED is saturated, the brightness of other LEDs is basically proportional to the current flowing; Another method is to use the visual inertia of the human eye and use the pulse width modulation method to realize the gray level control, that is, periodically change the light pulse width. As long as the repeated lighting cycle is short enough, the human eye can not feel the shaking of the luminous pixels. Because PWM is more suitable for digital control, today, when microcomputer is widely used to provide LED display content, almost all LED screens use PWM to control gray level.
The control system of LED is usually composed of main control box, scanning board and display and control device. The main control box obtains the brightness data of each color of a screen pixel from the display card of the computer, and then redistributes it to several scanning boards. Each scanning board is responsible for controlling several lines on the LED screen, and the display and control signal of the LED on each line is transmitted in serial mode. At present, there are two ways of serial transmission of display control signals: one is to centrally control the gray level of each pixel point on the scanning board. The scanning board decomposes the brightness value of each row of pixels from the control box, and then transmits the turn-on signal of each row of led to the corresponding LED in the form of pulse (lit as 1, not lit as 0) in a row by row serial way to control whether it is lit or not. In this way, fewer devices are used, but the amount of data transmitted in serial is large, because in a repeated lighting cycle, each pixel needs 16 pulses under 16 gray levels and 256 pulses under 256 gray levels. Due to the limitation of device working frequency, generally, the LED screen can only achieve 16 gray levels.
Another method is that the content of serial transmission of the scanning board is not the switching signal of each LED, but an 8-bit binary brightness value. Each LED has its own pulse width modulator to control the lighting time. In this way, in a repeated lighting cycle, each pixel point only needs 4 pulses under 16 gray levels and 8 pulses under 256 gray levels, which greatly reduces the serial transmission frequency. With this method of decentralized control of LED gray level, 256 gray level control can be easily realized.