How to Calculate and Control LED Display Grayscale

There is an important parameter in the LED display industry: grayscale, that is, the brightness of the LED screen. The gray level is also called midtone, which is mainly used to transmit images, pictures, and videos. level to make the transmitted picture clearer. Whether it is a full-color screen or a two-color screen, to display images or animations, it is necessary to adjust the grayscale of each LED that constitutes a pixel, and the fineness of the adjustment is what we usually call grayscale.

1. Calculation method of brightness of display screen

Take the LED 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 whole screen to find the brightness of a single tube.

For example: density of 2500 dots per square meter, 2R1G1B, and brightness requirement of 5000cd/m2 per square meter, 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) Find the brightness of the whole screen from the known single-tube brightness.

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:

From 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 and because of the four tubes of red, green and blue, the red tube has 2 Therefore, the brightness of a single red tube is: 1200÷2=600mcd.

It can be seen from green:blue=6:1 that 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 light-emitting pixel = 2 red tube + 1 green tube + 1 blue tube;

That is, the brightness of one pixel=600(red)×2+2400(green)+400(blue)=3400mcd=3.4cd

Brightness per square meter = brightness of 1 light-emitting pixel × pixel density per square meter (number) = 3. 4cd × 1024 (number of pixels) = 3482cd. Calculated with a light loss of 20%, the actual luminous brightness should be: 2785. 28cd.

2. How to control grayscale of LED display

The following introduces two methods to control the grayscale of the LED display.

One is to change the current flowing through the LED. Generally, the continuous working current of the LED die is about 20 mA. Except for the saturation phenomenon of the red LED, the brightness of other LEDs is basically proportional to the current flowing through. In addition, the human eye can be used. It uses the pulse width modulation method to realize grayscale control, that is, the light pulse width is changed periodically. As long as the period of repeated lighting is short enough, the human eye cannot feel that the light-emitting pixels are shaking. Because pulse width modulation is more suitable for digital control, almost all LED screens use pulse width modulation to control gray levels today when microcomputers are generally used to provide LED display content.

The other is that the content of the 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 PWM to control the lighting time. In this way, in a cycle of repeated lighting, each pixel only needs 4 pulses under 16-level grayscale, and only 8 pulses under 256-level grayscale, which greatly reduces the serial transmission frequency. With this method of decentralized control of LED grayscale, 256-level grayscale control can be easily realized.

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