Tech 101: Modern LCD Displays
Displays have come a long way since the old CRT monitors and TVs. Advancements in display technology have made many modern devices possible such as mobile phones, mobile gaming platforms like the Sony PSP, thinner Televisions and Monitors. We use them everyday, for a variety of tasks but do we really know how they work? What is an LCD? What is a TFT Screen? How do they display so many millions of colours? This article will answer these questions and many more.
What is an LCD?
The term LCD stands for Liquid Crystal Display. But what exactly are Liquid Crystals? The term is quite confusing. A substance can either be a crystal, like quartz, which is hard as rock, or it can be a liquid which is obviously different. How can it be both? We all learned that matter can exist in 3 states, solid, liquid and gas. Solids have their molecules in a very rigid orientation while liquids and gasses are exactly the opposite. There are a few substances that have properties that are like solids and liquids at the same time. What this means is the the molecules of the substance tend to maintain their orientation, like a solid, but they also tend to move around, like a liquid. This is why they are called Liquid Crystals.
It takes a lot of energy to convert a solid into this state, and only a little more to convert it into a liquid. This is why LCD’s are so sensitive temperature changes. This property allows them to be used in the making of Mood Rings and Thermometers. There are many types of liquid crystals. The major type used in monitors and other displays are called Twisted Nematic Liquid Crystals. These type of crystals twist and untwist at varying degrees to allow light to pass through when a voltage is applied.
What is a TFT?
A Thin Film Transistor LCD or TFT-LCD is named after a very thin layer of transistors that are applied to the back of a Liquid Crystal Display. These transistors allow only one row of pixels to be updated at any given point in time. The speed with which this updation takes place fools your brain into thinking that its viewing a static image.
In small LCDs such as the ones used in calculators and other devices, a voltage can be applied across one segment without interfering with other segments of the display. This is impractical for a large display with a large number of pixels, since it would require millions of connections, two for each one of the three colors (red, green and blue) of every pixel. To avoid this issue, the pixels are arranged in rows and columns, reducing the connection count from millions to thousands. The column and row wires attach to transistor switches, one for each pixel. The one-way current passing characteristic of the transistor prevents the charge applied to the pixel from draining between refreshes, which creates the persistence of vision.
What is a Pixel?
A single pixel is the smallest addressable screen element in a display device, or it is the smallest unit of picture that can be represented or controlled. Each pixel has its own unique address which corresponds to its coordinates on the screen. Pixels are normally arranged in a grid pattern. Each pixel is made up of three Sub-Pixels which have a different color, Red, Green and Blue. The voltage applied to each subpixel decides its colour intensity. There are 256 possible gradations for each subpixel and together all three give a combination of 256 x 256 x 256 = 16,777,216 colours to each pixel on the screen.
How it Comes Together
A simple LCD monitor has many layers of technology that make it function. The basic principle behind these types of monitors is the Polarization of Light. What this means is that light is strategically allowed or not allowed to pass through any given point on the screen.
At the base of a monitor, you have a single line of white LED’s that provide a backlight. There are many layers of films placed on top of these LED’s to create even lighting across the back panel. The next layer consists of a plate of glass. At the back and front of this sheet of glass are two polarization films. Light passes through the back film and is twisted by the glass to be ‘in sync’ with the front film. If the light is ‘in sync’, it passes through, if not, it is blocked by the film.
The sheet of glass is an amazing piece of technology. It basically consists of a layer of TFT’s and a Liquid Crystal array that is arranged in a grid format. Each point or pixel of the grid consists of three subpixels. These subpixels are what give the pixel its colour. They have a maximum intensity of 255. What this means is that if we set the intensity of the red subpixel to 0, then no light passes through, and it appears black. If the intensity is set to 255, then all the light passes through and it appears bright red. If we wanted to create a single pixel of sky blue colour, the red subpixel would be set at 135/255 intensity, the green subpixel would be set at 206/255 intensity and the blue subpixel would be set at 250/255 intensity. This is how each pixel of your display is updated 60 times a second. Thereore, for a display of 1024 x 768 resolution, the monitor is making 141,557,760 updates per second. If it takes you half a second to flip a switch, making these many updates would take you 819.2 days or 2.24 years. Thats a LOT of updates (and finger cramps).
To conclude, we would like to point out how much all of us take technology for granted. We have really got no idea what goes in to making most things we use everyday. We bicker and complain about our Tech being too slow or too old. We cant wait for the next big thing. At iGyaan, we have decided to start this section to demystify the basics of technology, so that the end user/consumer understands the true nature of the products that he/she is buying. We hope that you enjoy this section and learn something new from it. We’ll continue to bring you updates on the basics of technology. If you have something specific that you’d like us to talk about, please leave your requests on the comment section below.
Stay tuned for the next update, continuing along the display line on OLEDs and AMOLEDs.