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What is the Relationship between the Gray Level and Brightness of LED Display?

What is the Relationship between the Gray Level and Brightness of LED Display?

LED Gray Level of LED Display

Led gray level can also be called LED brightness. Gray level, also known as half tone, is mainly used to transfer pictures. There are three ways to transfer pictures: level 16, level 32 and level 64. It uses matrix processing to process the pixels of files into level 16, level 32 and level 64, so that the transferred pictures are clearer. Whether it is monochrome, bicolor or tricolor screen, in order to display image or animation, it is necessary to adjust the gray level of each LED light-emitting pixel, and the fine level of adjustment is what we usually call gray level. 

Two Ways to Control the Gray Level of LED

One is to change the current flowing, the other is pulse width modulation. 1. Change the current flowing through the high resolution LED screen. Generally, the continuous working current of LED tube is about 20mA. Except for the saturation of red LED, the gray level of other LEDs is basically proportional to the current flowing through them. Another method is to use the visual inertia of human eyes to realize the gray level control by means of pulse width modulation, i.e. to periodically change the light pulse width (i.e. duty cycle). As long as the cycle of repeated lighting is short enough (i.e. refresh frequency rate is high enough), the human eye can't feel the flicker of the luminous pixels. Because pulse width modulation is more suitable for digital control, it is widely used in computer to provide LED display content today, and almost all LED screens use pulse width modulation to control the gray level. LED control system is usually composed of three parts: main control box, scanning board and display control device.

The main control box obtains the luminance data of one 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 rows (columns) on the LED screen, and the display and control signals of each row (column) are transmitted in serial mode.

Two Ways of Serial Transmission of Display Control Signals

1. One is to control the gray level of each pixel point on the scanning board. The scanning board decomposes the gray value of each row of pixels from the control box (i.e. pulse width modulation), and then transmits the opening signal of each row of LED in the form of pulse (lit to 1, not lit to 0) to the corresponding LED in line serial mode to control whether it is lit. In this way, fewer devices are used, but the amount of serial transmission data is large, because each pixel needs 16 pulses under 16 levels of gray, and 256 pulses are needed under 256 levels of gray in a cycle of repeated lighting. Because of the limitation of device working frequency, generally, only 16 levels of gray can be achieved for LED screen.

2. One is pulse width modulation. The scanning board serial transmission content is not each LED switch signal but an 8-bit binary gray value. Each LED has its own pulse width modulator to control the lighting time. In this way, in a period of repeated illumination, only four pulses are needed for each pixel at 16 levels of gray, and only eight pulses are needed at 256 levels of gray, which greatly reduces the serial transmission frequency. The method of distributed control of LED gray level can easily realize 256 level gray level control.

  1. Yuan, S., Liu, Y., & Zhao, X. (2013). High grey level and high refresh rate to the LED screen based on PWM. Guangdian Gongcheng/Opto-Electronic Engineering.

  2. Mishra, S., Singh, N. K., & Rousseau, V. (2016). Display Interfaces. System on Chip Interfaces for Low Power Design, 53-126. doi:10.1016/b978-0-12-801630-5.00004-9

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