The role of Liquid Crystal (LCD) in LED display

August 08, 2022


Gaurav Lokhande, B.Tech in Electronics & Telecommunication Engineering. Currently, he is working at Lesics Engineers Pvt.Ltd as a Product Developer and Visual Educator. His areas of interest are Embedded Systems, IoT, and Printed Circuit Board (PCB) Designing. To know more about the author check this link.

In the last article I explained the working of the display technology, construction of LEDs, control mechanisms and future scope of LEDs. In this article I will explain the fundamentals of light and LCD. I will also explain a pixel experiment through the Macro lens.

In a previous article, I have said that the vertical polarizer only allows a vertical component of lights and it blocks others. Lets see how the vertical and horizontal polarizer actually works.

Fundamental physics of light

The light has both electric and magnetic fields perpendicular to each other, and they are perpendicular to the direction that light travels. However, the electric field and magnetic fields are present in all directions. Here,I am showing only the electric field in fig 1 for simplicity. This kind of light is known as unpolarized light. When I pass this light through the vertically polarized sheet, the polarizer allows only the vertical components of the electrical fields and blocks all others (fig 1).

Fig 1: Unpolarized light passing through the vertical polarizer.

This vertical light passes through the LCD crystal. LCD crystal rotates this light according to our rotating signal. Let's see this in detail.

Role of Liquid Crystal in LED display.

What is a practical way to achieve different angles of rotations for the polarized light? This is done by liquid crystals. Normally, liquid crystals are found in a twisted state and rotate the incoming light by 90° as shown in fig 2. When we apply an electric field, the molecules untwist themselves and rotate the light accordingly. But what causes the light to rotate in the crystal?

Fig 2: Liquid Crystal and light controlling mechanism.

To understand this, let's consider linearly polarized light as equivalent to two circularly polarized lights. Let's call them clockwise and counterclockwise light according to their rotation as shown in fig 3. When they pass through twisted liquid crystals, they travel at different speeds relative to each other. One wave slows down as it passes through the twisted molecules. This differential speed causes a phase shift in the lights. Due to this phase shift, the resultant angle of output light is changed. This phase shift of light is determined by the degree to which crystal molecules are twisted. In an untwisted state, there is no phase shift because both light waves travel at the same speed inside the liquid crystals. This is the fundamental physics behind an LED display.

Fig 3: Clockwise and counterclockwise light

Experiment on Monitor screen.

Would you like to see the pixels of your monitor with the help of a cool experiment? Just purchase a simple macro lens. Attach the macro lens to the phone and keep it close to the monitor. The phone will strugle to focus but after some adjustment, you can see many colored boxes as shown in fig 4. They are the subpixels of the display. You can also see the transition between the two colors. The magic played by the sub-pixels is clear here. The sub-pixels color intensity needed for the yellow is different from the gray color.

Fig 4: Sub-pixels of LED

Drawbacks of LEDs.

We have been using LEDs for more than a decade, but there are some drawbacks. The main disadvantage is high power consumption. To display a small white dot, the whole backlight must be ON as shown in fig 5.

A second disadvantage is that color reproduction is not that accurate. A perfect black color is not possible due to the continuous backlight being on in the background.

Fig 5: Layers of LED Display

That's all in this article. I hope you might have learned how a LED display works.

Thanks for reading!

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