Light Emitting Diode (LED) display technology is in a fascinating stage now. The evolution of the display technology started with the cathode ray tube to cutting-edge OLED technology. Even though the OLED displays are superior, LED displays still hold a market share of around 40%. Scientists continue to make strides in improving the LED technology and soon it may once again become the king in the next few years. In this article, I will explain how this display technology works and what the future holds.
I have shown a pixel here in fig 1. Obviously a really big pixel, with three input LEDs; Red Green and Blue (R, G and B). The cool thing about pixel technology is that I can achieve any output color that I want, just by controlling the intensity of LED lights. So, If I take this pixel, reduce its size and replicate many such pixels, I can make any LED display.
The quality of the image will improve if I reduce the size of each of the LEDs. However, this causes an issue. I cannot reduce the size of LEDs after a limit due to manufacturing difficulty. To obtain a 4K quality image, the size required for the LED is approximately one-third of a millimeter(ref in fig 2), which is difficult to achieve.
So, how can scientists overcome this problem? Instead of making extremely small RGB LEDs, they used miniature color filters for the display and used normal-sized white LEDs as a backlight. However, this arrangement will just produce a white color when the backlight is turned on as shown in fig 3a and it produces a black color when the backlight is turned off. To produce different colors, we must be able to control the intensity of light for each color filter independently. This can be achieved by a clever arrangement of two polariser sheets and a material that can change the optical angle (fig 3b).
Now, let's understand the light intensity controlling mechanism. When the backlight incident on a vertical polarizer, it will allow only the vertical component of light to pass, blocking out all other components. The role of the optical sheet now comes into play. It is the size of a subpixel. When I give a rotate signal to this sheet, it will rotate the vertically polarized light to the angle I want. If this angle of rotation is 90 degrees, the next polarizer layer, the horizontal polarizer, will allow the light to pass through completely as shown in fig 4a. If I rotate the light less than 90 degrees, the horizontal polarizer will block out some of the light, and the intensity of the subpixel will be reduced. If the angle of rotation is 0 degrees, obviously the whole light will be blocked. This is the way I can control each subpixel's light intensity.
Using three such units, I can construct a pixel as shown in fig 4b. Now I can obtain any color pixel simply by varying the angle of light rotation. To get a cyan color, I simply need to block the red filter light. Similarly, to get a yellow color, I need to block the blue filter light. Using this clever arrangement, I can build an entire display system. This is the basic design behind an LED TV.
Let's get back to our LED display design which remains incomplete (fig 3b). Some pixels are brighter than others because the backlight LED arrays are providing uneven light. With the help of diffuser sheets, I can evenly spread the light. However, the diffuser sheet spreads light in all directions which reduces brightness in the desired direction. To solve this issue, let's add a prism sheet in front of the diffuser sheet. The prism sheet concentrates light in a forward direction. This is the actual construction of the LED display as shown in fig 5.
As I have explained earlier, the manufacturing of small LEDs is difficult, but not impossible. Scientists are trying to overcome those problems by developing new manufacturing methods. When the LED display is manufactured using minute LEDs, it is called a Micro-LED. The Micro-LED display technology will be superior to OLED technology. In OLED, the organic layer degrades with the time which causes pixel burn-in issues. Micro LED displays are obviously more durable due to the use of inorganic semiconductors. The manufacturers are using a modular design approach for better manufacturing capabilities. Since micro LEDs are inorganic in nature, they have a better brightness than organic technology. Currently, Samsung is the leading manufacturer of Micro-LED displays.
I hope you have learned how a LED display works. In the next article. I will explain the fundamentals of light and liquid crystal. Also I will guide you how to see actual pixels of the monitor.
Thanks for reading!