How LED tv displays work

WHAT IS LED

Before we look into how LED tv displays work, let us first define what LED means. LED stands for light-emitting diodes. Diodes are passive electrical components. They were primarily created to control the direction in which current flows; they allow current to flow in one direction while preventing it to flow in the other direction. So, light-emitting diodes are diodes that allow current to flow in one direction only while emitting visible.  They are a very effective source of light.

Many electronic devices that we use today use LED at some point. Devices where you can find an LED include laptops, phones, tablets, televisions among many others. LEDs are the most favored light source today because of their high level of efficiency and so they can save a lot of energy.

What is Polarized light?

In our previous article about how modern LCD tv display works, we discovered extensively about polarized light and its use in the display. If you remember, we said that Light is classified as an electromagnetic wave. This means as a wave, it contains both magnetic and electric fields.  The attributes of those fields are they are perpendicular to the direction of propagation. When the light is not polarized, the electrical fields are scattered in various planes randomly.

You should know that it is possible to filter out some of the electric fields and allow only others to pass through. When dealing with the display,  you can polarize the light vertically or horizontally. Vertical polarization allows electric fields that are vertical to pass through. When you polarize it horizontally, only horizontal electric fields are allowed to pass through. The tools you will need to achieve this are polarizing filters that are made up of long-chain molecules. How the molecules are aligned, determines whether the light is polarized vertically or horizontally. If you want vertically polarized light, the molecules are aligned horizontally and vice versa is also true.

What is LED tv

I want something that might shock you very much. LED tv or monitors (many manufacturers are creating devices that function as both) are LCD televisions.  Let me explain. What makes manufacturers call them LED televisions is because LEDs are used as the source of backlight. LCD TVs are classified into two categories depending on the type of backlight that is used. There is, of course, the LED that we are discussing now and there is the cold cathode fluorescent lamp.

CCFLs are very thin tubes that resemble the fluorescent tubes we use for light other than the size. CCFLs are being replaced with LEDs since the latter have proved to be cheaper and more power-efficient.

History

A solid-state diode emitting light was first recorded in the year 1907 by captain Henry Joseph.  It was a single discovery that altered the cause of history when it came to electronic technology. Unfortunately, the solid-state diodes remained an academic curiosity for many years until the 1960s.

Light-emitting diodes were introduced into the market in the early 1960s. while the first LED was created by General electric worker, Nick Holonyak, it was red. The first practical LED was developed by Hewlett-Packard  in the year 1968. HP introduced the first numeric LED indicators in their HP model 5082-7000.

primary colors

The last primary color to be created was the blue light in the 1980s. It was hardly power efficient. At the same time, the Aluminium indium Gallium and phosphide LED entered the market. They were widely favored in the information displays because of their appealing and efficient red and amber color. In those years, it was not possible to achieve full color, the available green color then was yellowish. The fully colored green and blue LED was made by an engineer at Nichia chemicals, Shuji Nakamura. It is only then that video display using LED became a possibility.

You should know that the idea of using LED for the display came into mind in 1997. Initially, the pixels were widely spaced to achieve a large image that was clearly visible at night. Such a display with a total of 150,000 pixels and a size of 17 m by 52 m was created. The company that made the first display of this kind was SACO technologies.  

A wide spectrum of colors

A wide spectrum of colors can be created today using high brightness diodes. There is a new form of LED that currently under development, the OLED. How OLED display works are already explained in an earlier article we did. The first OLED tv was created by the Sony-the Sony XEL-1 OLED screen. It was introduced into the market in the year 2009.

OLED possesses some advantages over LED types since they do not need any source of backlight like the screens LED tv does,( this is how LED tv works). In the OLED, the pixels do not work as color filters but they are the source of light themselves. They are highly efficient when it comes to power consumption. However, their drawback like many new technologies is they are very expensive. The first true LED tv was introduced in 2012 by Sony, a crystal LED tv.  Here the LED was not used as a backlight but was used to produce actual images.

micro LED

The micro-LED is a similar technology under development by many organizations. They are popular among the manufacturers since they are both easily scalable and the production process is streamlined. They are very expensive though.

After practical LEDs were created, they were just used as a seven-segment display for a very long. They replaced the incandescent indicators. Most expensive equipment was the only place where the LEDs were widely employed. However, several decades of technology have rendered them cheap and hence are widely used in many places such as calculators, phones, radios, and many other places.

How LED tv displays work

Now you won’t be confused when we say LED televisions are a special kind of LCD televisions which source of the backlight is the LED. This is the core principle behind the LED display technology.

How LED tv displays work

Full array LED backlight tv

The small size of the LED, as you see them in the figure above, is what makes LED screens thin, light, and very power efficient. This why CCFLs display types is being discarded in favor of the LED.

 

You won’t be hyped when someone tells you that LED is a major kind of invention when it comes to televisions since they are simply LCD with high efficient backlight source. So, you will never tell the difference between the qualities of images from the LED tv and CCFL tv. BUT!!! Do not get disappointed yet. Some screens LED instead of crystal displays though it is very expensive. This is the same principle that the OLED tv displays manufacturers are using. You can check here how OLED tv displays work, it is very interesting.

How are LED colors produced?

The topic on how LED tv displays work will not be complete if we do know how the colors are produced. The LED produces a bright white light at the back of your tv. There are ways that the light-emitting diodes are arranged behind your screen.  The first method is referred to as fully array LED and the other method is the edge-lit LED. In the former method, full array, the LED are placed uniformly behind your tv. The other method, edge-lit, the LED is placed at the edge of your tv panel. Most of the LED screens in the market use the full array method so a technique called local deeming can be applied. This method also offers images with better quality.

local dimming

Local dimming is an LED feature where the other than the pixels themselves, the backlight is also dimmed to offer the best image quality. Diming is done on places where the image appears dark. It is not possible for the LCD to completely block all the light from the LED backlights. So dimming is an easy and affordable way of making the dark regions darker to achieve a better contrast ratio. So through well development components, it is possible to selectively dim the LED of the regions where the image is supposed to be dark.

These characteristics allow the LED tv to closely behave like the OLED which are known for their high level of contrast that not possible for LED and CCFLs.

How LED tv displays work

How do you get to see an image on LED tv type.

So now we will explain how light from the LED becomes a clear image that you are seeing now on your screens. The LCD panel has so many components.  They include the diffusers, LCD panel itself. Once the light is emitted by the LED whether, in the full array or edge lighting, they come into contact with the diffusers.  The diffuser on your tv will distribute the white light uniformly on the entire screen. They are are several sheets.

The LCD panel.

LCD panels are created such that they have many cells. See an image of an LCD panel. An LCD cell has two polarizers, color filters, and liquid crystal molecules. The polarizer has the light electrical field directed towards a single direction. It is placed at the point where the light is meant to enter the cell. The second polarizer is placed at the point where the light is leaving the cell. Each cell has liquid crystals in its cell between the polarizers.

The liquid crystal molecules change the direction of polarization of the light after it leaves the first polarizer. How much the direction becomes changed is a function of the liquid crystal molecule alignment on the cell.

physical states of liquid crystal

There are several physical states of liquid crystals. The states are the nematic phase, discotic phase, cholesteric phase ad smectic phase. While the nematic phase has the molecules in a state similar to the liquid, molecules point in the same direction. As shown in the figure below, a smectic phase has the molecules arranged in layers.

How LED tv displays work
several physical states of liquid crystals
cholesteric phase

The cholesteric phase is also referred to as the chiral nematic phase. The arrangement of molecules is in layers but each layer is oriented such that they form a spiral.  The last shape is only possible for liquid crystals that are disc in shape. The molecules used for the LCD are elongated or calamitous.

How color is produce

So, say you want a dark pixel.  Once the light hits the cell, the liquid crystals receive no electricity so there is no change in the electric field direction. Now the light moves unaltered into the second polarizer.  If the first polarizer was meant to perform vertical polarization, the second one will be used for horizontal polarization.

polarization

Now the light did pass without any changes since the liquid crystal molecules were not charged. When that unchanged light tries to exit through the second polarizer it would pass and hence a dark pixel result. The light is not able to pass through the second polarizer since the vertical polarizer will block rays of light that have other characteristics other than the vertical electric field.

How LED tv displays work
vertical and horizontal polarization

An image showing how the application of current affects the alignment of molecules.

If now some current is allowed into the cell, the liquid crystal will change the polarization proportional to the amount of current applied. So, the more current allowed into the liquid crystals in the cell, the more the electric field becomes twisted, and hence more light will be allowed to pass through the second polarizer. Below is a simplified schematic showing how the whole process works.

How LED tv displays work
how the polarization process takes place

The electrodes that pass current to the cells are aligned in so many ways in the LCD panel.  The commonly used types are vertical alignment (VA), twisted nematic (TN), and in-plane switching (IPS).

The amount of light that is allowed to pass through the second polarizer is then passed through color filters, red, blue, and green. So, each color is intensified differently so that various colors can be born. And when all the filters give the required colors, an image is displayed on your screen. This is how LED tv displays work.

Advantages of LED screens

now that we have looked into how LED tv displays work, we can look into  some of their advantages. There are many advantages of LED screens and this has made them be widely used in televisions and as a source of light in homes and other places. We only hope that soon, actual LED screens will be affordable.

 

 

 

 

 

 

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  1. Pingback: How CCF Lamps work - Tech1Resource

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