How LCD Screen Works?
How LCD Screen Works?
Figure: Different layer of LCD Monitor
[Note: TN means Twisted Nematic]
You might be curious about how an LCD screen produces different colours of light. Then this might be the right place for you.
Although there are many types of display, today we are going to discuss the most famous type of display, which is LCD. LCD stands for liquid crystal display. This article will explain how LCD screens work. What does “liquid crystal” mean? How does the screen produce different colours of light?
An LCD screen consists of a liquid crystal sandwiched between two polarized filters and background light. The background light is the only light produced by the LCD, and all other coloured lights are extracted from it. The background light is at the back of all these layers, and it’s white light. Why white light? Because white light is the mixture of all colours of light, we can extract any colour from white light.
An LCD screen is composed of millions of pixels. Inside, a single-pixel contains three sub-pixels. Those sub-pixels are nothing but red, green, and blue filters. The three primary colours are red, green, and blue (RGB). By mixing these three colours of light, we can make about 16.7 millions of light. Each sub-pixel is separately controlled to produce a certain colour of pixels.
How screen produced these different colours from these RGB sub-pixels?
The intensity of the backlight (white light) is changed independently for each sub-pixel. The intensity varies from 0 to 255. 0 means 0% intensity and 255 means 100% intensity. And these three sub-pixels make a pixel. A single pixel is so small that we don’t see three different red, green, and blue colours. We see a single different colour depending upon the intensities of the background falling on each sub-pixel.
For example,
If the backlight intensities are zero in all three sub-pixels, the pixel will appear black.
If the intensity of three sub-pixels is 255, then the pixel will appear white.
If the intensity of red and green is 255 and blue is 0, then the pixel appears yellow.
If the intensity of the red sub-pixel is 255, green around 100, and blue at 0, then the pixel will appear orange.
There are millions of combinations between 0 and 255, which means millions of colours. This is how different colours are made on the screen.
So, how is the intensity of the backlight adjusted?
Remember that intensity refers to the brightness of the light. You are probably thinking about why we just don’t alter the voltage supply in the backlight to alter the intensity. But remember, we have to alter the intensities of millions of sub-pixels independently. So there must be something in between the backlight and these pixels so that we can change the intensity of the backlight of each sub-pixel independently.
For that, we use liquid-crystal. As you can see in the first picture, it is just behind the RGB colour filter. A liquid crystal is an intermediate state between a liquid and a solid crystal. It means it has the properties of both a liquid and a solid crystal. When an electric field is placed across a liquid crystal, it can modify the polarization of the light. Changing polarization changes the quantity of light that passes through it, which eventually influences the intensity of the backlight.
Light is made up of several kinds of waves that vibrate in random directions. Light waves that vibrate along the horizontal axis are referred to as “horizontally polarized light,” whereas light waves that vibrate along the vertical axis are referred to as vertically polarized light. The backlight produces unpolarized light. To improve the image quality and contrast, we must transform the unpolarized backlight into polarized light. We’ll need a polarizing filter for this. Backlight light passes through the vertical polarizer, allowing only vertical polarized light waves to pass through. The light that is vertically polarized will now flow through the liquid crystal.
Twisted Nematic (TN) liquid crystal is used in the majority of LCDs. The molecules of TN are in twisted helical form. The first molecule’s orientation is perpendicular to the last molecule’s. And the molecule in between them adapts to it. As we can see from the figure, the red-coloured one is TN and the yellow one is polarized light.
Figure : Twisted Nematic(TN) Liquid crystal between two Polarized filter
In the absence of an electric field, the liquid crystal remains twisted (left image). This means that TN changes the polarization of light by 90 degrees. Vertically polarized light enters and horizontally polarized light exits. The horizontally polarized light coming out of TN now easily passes through the front horizontally polarized filter. Here, the intensity of the backlight remains the same (100%) throughout the liquid crystal. So the screen appears white.
In the presence of an extreme electric field, the liquid crystal untwists (right image). The molecular arrangements of TN are parallel to each other. This does not change the polarization of the passing light. The vertically polarized light from the back polarizing filter remains the same. Due to the presence of a horizontal polarizing filter on the front, vertically polarized light is blocked. Here, all light is blocked, so the intensity of the background light passing through the front polarizing filter is zero. Therefore, the screen is displayed in black.
If you start increasing the electric field slowly in TN, you can see the amount of light passing through the front polarized filter changing from completely passed to completely blocked. Therefore, from these, you can control the amount of light that enters the RGB colour filter to produce the light colour you want.
There are millions of pixels on the screen. To produce the required images, we need to change the electric field of every TN located behind the sub-pixels. How do we do this? The TFT (thin-film transistor) is a matrix of millions of transistors. It helps to control the amount of electricity applied to TN. So, because of this, we can control each pixel individually.
So, what is the point of having a polarized filter in the back? We know that the front polarized filter controls the amount of light passing through it. But what about a polarizing filter at the back? It is there because it converts unpolarized backlight into polarized light. The polarized light helps to create a clear and bright image.
This is how an LCD screen works.
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