How a SmartPhone LCD Works


Today’s smartphones allow you to install, configure and run a wealth of applications, from business tools to entertainment. The growing number of users who rely on their mobile devices to watch videos, play high-graphic games or browse rich web content, has sharpened the focus on smartphone screen technologies and heightened competition among the industry’s major players. That sharper focus on screen resolution and color saturation is also why screen technologies have become a major selling point, as each new generation of smartphones hits the market. Today’s smartphone touch screens exhibit a brightness, clarity and durability that is far advanced beyond those of only a few years ago. The two mainstream technologies in use for smartphone displays today are Liquid Crystal Display or LCD, and active-matrix organic light-emitting diode, known as AMOLED. While the latter is considered the newest smartphone technology, LCD has typically offered higher resolution and even greater availability for screen sizes appropriate for smartphone use.

Although today’s amazing smartphones may have you thinking otherwise, LCD technology is nothing new. It can actually be traced all the way back to the late 1800s when an Austrian botanist first discovered liquid crystals. But it wasn’t until the early 1970s that the first consumer products started utilizing LCD technologies’on items such as watches and calculators’ and those items appeared on store shelves.

From these early advancements, LCD technology quickly grew, addressing such sought-after features as more colors, greater brightness, multiple viewing angles, and more efficient power use. Today’s LCDs are considered a very mature technology.

The appeal of LCDs, which primarily lies in their advantages in size and power drain, over other display technologies like cathode ray tubes (CRT), is evident with the growing number of consumer products that make use of the technologyeverything from microwaves to digital clocks to laptop computers, as well as smartphones.

LCD technology is made possible by manipulating and blocking light and three major properties are involved, twisted nematics, polarization and the properties of liquid crystals. Light, whether from the sun or an artificial source, vibrates and radiates outward in all directions. That is to say that light waves oscillate, or vibrate, with more than one orientation, or on more than one plane. When the light waves oscillate on only one 2 dimensional plane, the light is considered linearly polarized. Polarizing filters, or polarizers, work by only allowing light to pass through that differs in orientation from the filter. A polarizing filter will block all light waves except for light waves oscillating on a specific plane. The resulting light is linearly polarized and only oscillates on one plane. This is critical to the functionality of an LCD screen, as an LCD works by blocking light.

An LCD is created with the use of two pieces of polarized glass. The second polarizer is placed perpendicular to the first. Orienting two polarizer 90-degrees from one another will block all light waves. The first layer creates polarized light on one plane and the second polarizer passes only lights waves oscillated perpendicular to the first plane, which blocks the polarized light passed by the first filter. Try this yourself by holding up two pairs or polarized sunglasses one after the other and twisted 90-degrees from one another. Looking through both lenses you will see only a black screen. Then, slowly rotate the second pair of sunglasses until the two pairs are parallel to one other. More and more light will be allowed through the lenses until you are able to see through both lenses normally. This is caused by polarized light.

So how does any light pass these two polarized layers and form the image we see on an LCD? The secret can be found in liquid crystals.

To allow light to pass through the two perpendicular polarizer, a layer of liquid crystals is sandwiched in-between.

To understand liquid crystals, it can be helpful to think back to science class, when you first learned about the three common states of matter: solid, liquid and gas. Solids maintain their physical traits because the molecules maintain their orientation and position with respect to one another. In liquids, however, molecules are constantly moving.

However, some substances like liquid crystals actually exist in a state that has features of both a liquid and a solid, though LCs lean closer toward a liquid. While these molecules tend to maintain their orientation, like a solid, they also move around to different positions, which makes them more like a liquid.

This ability to act as both these states of matter makes it difficult to classify such molecules as either, and so a new classification – liquid crystal – came into use. The sensitivity of liquid crystals to temperature also makes LCD a commonly used technology for devices like thermometers, and explains why cold or hot weather can have an impact on LCDs accessed outdoors.

How does this relate to polarized light?

Liquid crystals can exist in a number of forms and phases, depending on factors like temperature and the nature of the substance itself, but it is the nematic phase that makes widespread use of LCDs in electronics possible. One type of nematic liquid crystal in particular, known as twisted nematics (TN), are used in LCDs. Twisted nematic liquid crystals naturally exist in a helical or twisted structure. As light waves pass through the helical liquid crystal, they twist in a way that allows them to pass through the second polarizer. In other words, the light waves are rotated until they are oscillating on the one plane that the second polarizer allows to pass.

Liquid crystals are also affected by electric current, a principle that is critical to their use in LCD’s. As a current is applied to a twisted nematic liquid crystals, it untwist, which means the passing light waves are not rotated and are subsuquently blocked by the second polarizer. The reaction of a twisted nematic liquid crystal to an electric current can be predicted and varied, allowing light passage to be controlled by applying various amount of current. 

To apply the electrical current required to manipulate liquid crystals, LCD manufactures use a thin grid of transparent transistors.  Each transistor represents a single area in which an electric current can be applied to produce a unique shade, and is referred to as a “sub-pixel”. Each sub pixel is then filtered through one of the three primary colors, red, green, or blue. By manipulating and varying the electric voltage applied, each sub-pixel’s intensity can range over 256 shades. The combination of three sub-pixels, creates one pixel.

When sub-pixels are combined, it is possible to produce 16.8 million colors (256 x 256 x 256) since the eye only sees blended colors as a result of the three independent sub-pixels.

First, a backlight assembly creates an even light source that passes through the first glass polarizer, while at the same time, electric current allows the liquid crystal molecules to align in a way that allows differing levels of light to pass through the second piece of polarized glass. This manipulation of the light allowed to pass through the second piece of glass is what creates the images you then see on your smartphone or other electronic device.

The slimmer design of LCDs compared to the CRT technology they have replaced in many electronics has helped this technology to quickly grow in popularity. LCDs also consume less energy and offer much greater resolution – the number of individual dots of color or pixels found in a display – than screens utilizing the older CRT technology.

Consumers have also reported that LCDs cause less eyestrain. This is because the displays do not flicker the way older technology screens did. They also allow for greater ease of adjustment, since their thinness enables them to be held, tilted or swiveled to a comfortable viewing position.

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