There is an important parameter in the LED display industry: grayscale, that is, the brightness of the LED screen. Gray level is also called midtone. It is mainly used to transmit images, pictures, and videos. There are three methods of 16, 32, and 64 respectively. It uses matrix processing to process the pixels of the file into 16, 32, and 64. Hierarchical levels to make the transmitted pictures clearer. Whether it is a full-color screen or a two-color screen, to display images or animations, it is necessary to adjust the luminous gray level of each LED that constitutes the pixel. The fineness of the adjustment is what we usually call the gray level.
1. The brightness calculation method of the display
Take a full-color screen as an example, usually the red, green, blue and white balance ratio is 3:6:1
Red LED light brightness: brightness (CD)/M2÷points/M2×0.3 (white balance ratio accounts for 30%)÷2
Green LED light brightness: brightness (CD)/M2÷points/M2×0.6 (white balance ratio accounts for 60%)
Blue LED light brightness: brightness (CD)/M2÷points/M2×0.1 (white balance ratio accounts for 10%)
(1) Know the brightness of the entire screen to find the brightness of a single tube.
For example: 2500 dot density per square meter, 2R1G1B, the brightness requirement per square meter is 5000cd/m2, then:
The brightness of the red LED light is: 5000÷2500×0. 3÷2=0. 3cd=300mcd
The brightness of the green LED light is: 5000÷2500×0. 6=1. 2cd=1200mcd
The brightness of the blue LED light is: 5000÷2500×0. 1=0. 2cd=200mcd
The brightness of each pixel is: 0.3×2+1. 2+0.2=2. 0cd=2000mcd
(2) Know the brightness of a single tube to find the brightness of the whole screen.
For example: Take P31.25, Nichia tube as an example.
HSM display main die specifications red and green
HSM-PH-A+ (Nichia) 180-440mcd1020-2400mcd
Because the white balance matches the brightness ratio red:green:blue=3:6:1; and the white balance ratio uses the green tube brightness to match other tubes. So as follows:
It can be seen from red: green=3:6 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 and because of the four tubes of red, green and blue, the red tube has 2 So, the brightness of a single red tube is: 1200÷2=600mcd.
From 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, 1 luminous pixel = 2 red tube + 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 = 1 luminous pixel brightness x pixel density per square meter (number) = 3. 4cd x 1024 (number of pixels) = 3482cd. Calculated with a light loss of 20%, the actual luminous brightness should be: 2785.28cd.
Second, the method of controlling the gray scale of the LED display
The following describes two methods of controlling the grayscale of the LED display.
One is to change the current flowing through the LED. Generally, the LED die allows a continuous working current of about 20 mA. Except for the saturation of the red LED, the brightness of other LEDs is basically proportional to the current flowing; in addition, the human eye can be used For visual inertia, the pulse width modulation method is used to achieve grayscale control, that is, the light pulse width is periodically changed. As long as the repeated lighting period is short enough, the human eye will not feel the luminous pixels jittering. Because pulse width modulation is more suitable for digital control, today when microcomputers are commonly used to provide LED display content, almost all LED screens use pulse width modulation to control gray levels.
The other is that the content of the serial transmission of the scan board is not the switch 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 period of repeated lighting, each pixel needs only 4 pulses in 16-level grayscale and only 8 pulses in 256-level grayscale, which greatly reduces the serial transmission frequency. With this method of distributed control of LED grayscale, 256-level grayscale control can be easily achieved.
