LCDs Explained: How Liquid Crystal Displays Work

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We all know what screens are, we practically stare at them 25 hours a day, but have you ever given a bit of thought as to how they actually work?

Many types of screens exist. The ancient displays, used in the 90s they were usually heavy and very power-hungry/ inefficient. Now they are lightweight, ultra-slim, foldable, and crisp.

LCD Displays

An LCD display which means Liquid Crystal display are those displays most common in computer screens and monitors. These displays work in a completely different way than the Cathode ray tube technology did, in the early 80s.

An LCD is divided up into tiny segments called pixels, (picture elements). Millions of them are in an everyday LCD screen. Each pixel is further divided into 3 subpixels, each emitting red, green and blue light. These pixels can be turned on and off individually.

From basic physics, we know that white light is a combination of red, green, and blue light, or that white light can be split into red, green, and blue light. So when your display shows, white, it means that all subpixels re shinnying at the same brightness.

The difference between plasma screens and LEDs is the way they control the turning off and on of the pixels. For Plasma screens, each pixel is basically a fluorescent lamp that can be switch on and off automatically.

While in LCDS the pixels are switched on or off using Liquid Crystals to rotate Polarized light. This is where LCDs get their name from.

What are Liquid Crystals Anyways?

We all know that 3 basic states exist, Solid Liquid And Gas. OR so we thought. By the late 19th century, an Austrian chemist named Friedrich Reinitzer (1857–1927) discovered liquid crystals. Which are a different state on its own.

Liquid crystals are like both in the solid and liquid state at the same time. Solids usually have tightly packed stationary atom arrange In distinct regular arrangements, called crystals, or crystalline. The atoms of liquids are free-flowing, even in the liquid is stationary, the atoms keep moving in-between them.

Liquids crystals have both characteristics of solids and liquids. The crystals are distinctly range but can still move around. Have both solid and liquid characteristics.

Liquid crystals can be in one of several possible ‘substrates’ or phases if you will…. The 2 most important phases are called the Nematic and Smectic phase.

In the Nematic phase, the liquid crystals are more like a liquid, the molecules can move around each other but all the molecules broadly point in 1 direction.

In the Smectic phase, the molecules arrange themselves into layers that can slide past each other, They molecules can move within the layers but cannot move from 1 layer to another. They all still maintain a certain orientation.

When the crystals are in the Nematic phase, they can adopt a twisted up structure and when electricity is applied to them, they straighten up again. This is essential for allowing the LCD to turn pixels on and off.

Polarized Light

Before moving further, we need to know what polarized light is. UnPolarized light e.g. sunlight is light that vibrates in all possible directions. Light has what we call a wave-particle duality. It means that light acts as both a wave and a particle at the same time. We notice the effect of polarization when we look at light as wave.

Polarized light is basically light that vibrates on 1 plane or directions. Unpolarized light can be polarized by passing it through a polaroid. A polaroid allows the part of the light that vibrates in a particular plane to passé through and the light that does not vibrate in that place is not allowed to pass through. The most common example of a polaroid is your everyday sunglasses.

Your sunglasses have tiny bands of opaque lines running through them either horizontally or say vertically, this allows for light vibrating only in the vertical plane to pass through.  Since this block out some of the light, the amount of light passing through is dimmer.

Adding another polaroid say horizontally after the first i.e. 90 degrees, means that the vertical light that passed through the first polaroid can no longer pass through the 2nd polaroid so no light can pass through. So light cannot pass through 2 polaroids 90 degrees apart.

How LCDs Switch Their Pixels On/Off

An LCD screen uses this idea of polarization to turn on/off pixels. The first layer of an LCD screen is a large Light emitter, as a diffuse softbox, usually called a backlight. In front of this are the pixels that have their subpixels.

Each pixel has 2 polaroids, 1 in front and 1 behind at 90 degrees apart. Which means that the pixels are normally dark. An in-between those polaroids are liquid crystals in the Nematic phase. That can be switched on or off (twisted or untwisted) by passing electricity through it. The switch is a transistor, in which each pixel has its own.

The backlight is made of what is called CCFL (Cold-Cathode Fluorescent lamp) as a backlight source. They are basically ultra-thin long florescent light bulbs covered with a diffuser.

Light from the backlight gets plane-polarized at the polaroid behind the filter. When the liquid crystal is switched on, it straightens and does not rotate the light so the light passes through it unchanged and gets blocked completely by the filter in front of the pixel which is 90 degrees apart. So no light goes through so the pixel is effectively off.

For the pixel to be on, the transistors send to electricity to the liquid crystal so it remains coiled up or twisted. Light passing through a twisted liquid crystal will be rotated by 90 degrees and will be able to pass through the polaroid in front, and the pixel will effectively become on.

A red, green, or blue filter then gives the light the pixel emits its color.

One noticeable problem arises from LCD technology. It is not very electricity efficient. Since the backlight shines all the time, Whether the pixel is on or off, the light will always shine which leads to a wastage of electricity.

Resolution

Resolution is basically a count of how many pixels a screen has. We hear of numbers like 1080p or 4k(2160p).

This number basically means that there are 1080 vertical pixels in the display for a 1080 p display or that there are 2160 vertical pixels in a display for 4k displays.

They always measure resolution with the least number of pixels is has. Typically, in most landscape displays, the vertical side has the least number of pixies.

Depending on the aspect ratio of the screen, this number usually goes with another. For a 1080p display, we typically hear “The screen has a resolution of 1920*1080p” ‘1920’ refers to the number of horizontal pixels in the display. So multiplying 1920 by 1080 gives the total number of pixels in the display.

We also hear phrases like ’this is a 408 PPI display’. ’PPI’ means ‘pixels per inch’ and it usually refers to the number of pixels in a given area usually 1 inch.

This is also used to determine the resolution of a screen. A 6-inch 1080p display has a higher pixel density or more pixels per inch than a 10-inch 1080p display. If they could fit 1080 vertical pixels into a 6-inch display, it means the pixel sizes are smaller than the pixels used in a 1080p 10-inch display.

 

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