How Computer Display Technologies Work!

Learn About The Technology Behind Todays LED and LCD Display Devices

Working With Different Types Of Computer Displays and The Various Technologies used to implement them

There are currently alot of monitor technologies in the computer world today, LCD, LED, CRT, and Projection based technologies. LCD and LED are by far the most common in current use, and we will start our journey with them.

What’s the difference between LCD and LED? Which is better and why? Well, first we should start by clearing up a major misnomer here. They are actually both LCD (liquid Crystal Display) technologies. The difference lies in their source of backlighting. LCD’s generally have Fluorescent backlighting (Cold Cathode Fluorescent) to light up the screen, while LED monitors are also LCD screens, but instead use LED’s (Light Emitting Diodes) to provide their source of lighting. LCD monitors with fluorescent back lights have an “inverter card” which provides the source of power to monitor itself. The inverter card works by taking direct current and transforming it to the form of alternating current required by the LCD screen itself. Computers generally use direct current and filter this current through an internal power supply (desktops), or through an external power transformer (laptops, etc). A big problem with LCD fluorescent backlights is the inverter cards themselves. They tend to become very hot and have a higher than average failure rate for this very reason.

Some older LCD monitors didn’t have a backlight at all. They relied on power coming from the “Thin Film Transistor” (TFT) that interacted with the LCD pixels themselves in order to produce light. Most LCD monitors today use different variations of TFT along with a fluorescent backlight. The older form without a backlight was what we called “passive” LCD technology. These monitors had a polarizing agent that merely reflects the light coming into the screen itself as its source of light. They were hard to view in a room with low lighting for this very reason.

In general an LCD monitor works like this: an LCD monitor consists of a layer of “Liquid Crystal” and one or more polarizing layers of plastic or glass. The LCD displays a picture by using cells of liquid crystal that change their direction of light passing through them in response to an electrical charge. These liquid crystal cells are also known as “Pixels”, which literally means “picture elements.” When two polarizing materials are aligned with each other, light easily passes through them. However, when you align one polarizing agent with another at a 90 degree angle, light will be become blocked. By changing the voltage, LCD’s are able to change the direction of the liquid crystals in the screen itself, effectively changing the image.

What is TFT (This Film Transistor Technology)? Active TFT LCD displays work by using transistors to change the orientation of the liquid crystal cells, effectively changing the pattern of light being allowed to pass through. Remember, at 90 degrees the light can become blocked. Passive LCD monitors tend to blur images due to latency, while active TFT monitors are capable of displaying full motion video easily because they use fast transistor switching technology.

CRT: Cathode Ray Tube Monitors. This is an older technology which is quickly dying. CRT’s are analog only video components. At the rear of the monitor is an electron gun which sits inside of a vacuum tube that extends to the screen itself. The electron gun (also known as a cathode) fires electron beams (red, green, and blue) to the front of the wide end of the vacuum. As the electron beams travel through the vacuum tube they pass by an Anode which uses electro-magnetism to move the electron particles toward the proper areas of the screen. The electrons cause a coating of phosphors on the screen to light up displaying the colors corresponding to the images being produced. Before the electrons hit the phosphors on the screen they travel through a mesh “shadow Mask” or “aperture grill” that is located directly behind the phosphor coated screen which is used to filer out stray electrons and direct electrons to the proper pixels (picture elements). There are 2 main types of “masks” or “mesh” that CRT’s use:

Shadow Mask CRT’s consist of a metal sheet with millions of holes for the electrons to pass to their respective phosphors, causing them to light up, while “Aperture Grills” consist of a sheet metal with vertically aligned slots in which the electrons pass in order to reach the phosphors on the screen. The CRT “Dot Pitch” determines the distance of space between the holes or slots in either type of monitor. The lower the Dot Pitch, the better the image quality. Each pixel on a CRT screen is defined by lighting up combinations of red, green, and blue phosphors that make up the pixel. Different colors are produced by varying the strength of the electron gun on each phosphor. Each pixel is made up of a Phosphor Triad of red, green, and blue phosphors in a Shadow Mask CRT. The Aperture Grill CRT’s use vertical red, green, and blue phosphor strips. In an aperture CRT the dot pitch is actually called a “Striped Pitch.” Taking a close look at a CRT monitors screen you will be able to see the vertical phosphor strips, or phosphor triads.

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