From the sleek monitor on your designer PC to the tiny display on your cellphone, display has grown smaller to keep up with the shrinking of computers into gadgets. This variety of display types is made possible because of different technologies. Here’s a walk-through of these.
CRT: The good ol’ one
CRT (Cathode Ray Tube) monitors work on a technology that debuted in TVs in the 1940s.
One of the more known innovations on this technology is Sony’s Trinitron which happened way back in the 1960s. This combined the three electron guns into a Pan Focus gun. While traditional tubes were made of sections of a sphere, Trinitron tubes were cylindrical. Trinitron resulted in a brighter display because the shadow mask occupied less of the screen and allowed more of the beam to strike phosphors.
LCD: Get sleek
Though the CRT is very cost-effective and can produce millions of colors at good resolutions, it’s not very power-conscious and lets out harmful electromagnetic radiation. Thus the world moved to flat-panel displays, LCD (Liquid Crystal Display) being the forerunner. LCD is less power-hungry and sleeker, making it the ideal choice for battery-operated mobile devices, like notebooks, cellphones and PDAs. Its disadvantages: lower brightness, contrast ratio, and viewing angle, and a slower response speed.
LCDs use liquid crystals. When light passes through them, they take the shape of the liquid-crystal molecule, but when an electric charge is applied to the liquid crystals, their molecular alignment (and thus the way light passes through them) changes.
LCDs are of two types: DSTN (Dual-scan Twisted Nematic) or passive matrix and TFT (Thin Film Transistor) or active matrix. Passive matrix consists of a glass sheet coated with a metal oxide that acts as electrodes that help to pass light to the liquid crystals. On top of this, a polymer is applied to fix the molecular alignment of the liquid crystals. Another glass plate with the same layer and some spacer beads to maintain space from the first glass sheet is then sealed with the first one, but with a small gap. Liquid crystals are then injected into this vacuum and the plates are sealed. The outsides of the glass sheets are coated with polarizing layers, and fluorescent tubes are added on the top and bottom at the back of the panel for backlighting.
Light from these is moved across the screen using a plastic light guide or prism. A color LCD display has red, blue, and green colored filters that create a single multi-colored pixel from three LCD elements. An LCD uses one cell for each pixel.
Light passes from the backlight, gets vertically polarized by the polarizing filters at the back, gets refracted by the molecular arrangement of the liquid crystals and appears from the horizontally polarized filter in the front. This creates the images and colors.
However, the response time of a passive-matrix screen is high, around 300 ms. TFT is an improvement on this, giving you a response time of 25 ms. Plus, TFT screens are lighter and have faster refresh rates. In TFT monitors, a matrix of transistors on a single silicon chip is connected to the LCD panel. Each transistor drives each color of each pixel, and determines how a pixel will behave when a voltage is applied and light passes through. This makes for faster response and takes care of problems like ghosting (where lit LCD pixels create shadows on the ones that are not lit by the passing of light).
Another major innovation in TFTs was replacing amorphous silicon (a-Si) in the transistors with polysilicons (p-Si). This helped in bringing down the cost, because the latter required less powerful circuitry.
Plasma: Clear picture on a large screen
Large wall-mounted displays in places like stock exchanges or cinema halls are PDPs (Plasma Displays Panels). PDPs display clear picture with good, detailed colors on the large screen—something that LCD is not able to do, but on the downside, their display life is shorter than LCDs’ or CRTs’.
Plasma is an emissive technology like CRT, but uses grids of electrodes like LCD. Since it’s emissive, color sharpness and viewing angle are no issues for this display. The matrix of pixels, or cells has three subcells in it—for red, green, and blue phosphors. Each of these also has three electrodes and a capacitor, and within each cell are sealed inert gases, like argon, neon, or xenon. When an electric charge is passed across the electrodes, the gases inside the cell get converted to plasma form (this is electrically neutral, and contains negative, positive and neutral particles). The plasma cells then release UV light, which strikes the phosphors and makes them glow. A technique called PCM (Pulse Code Modulation) is used to control the intensity of RGB color so as to generate the millions of colors that we see.
OLED: For minis and micros
A technology of some promise for small devices, OLED (Organic Light Emitting Diodes) displays have the advantages of low power consumption and paper-thin size, and also give sharp colors at wide viewing angles.
In OLED displays, each OLED cell makes up one pixel. Each cell consists of a thin layer of organic light-emitting materials between a transparent anode and a metallic cathode. When a charge of a few volts is applied to the OLED cell, the injected positive and negative charges combine in the organic layer to produce light (called electro-luminescence), giving a bright, sharp display. There are passive matrix and active matrix OLEDs. The former is good for low-cost applications that don’t have heavy information content, alphanumeric displays for example; while the latter is for high-cost applications where information can mean video and graphics.
In passive-matrix, an external controller circuit is used to generate the input power, and a charge is passed through selected pixels by applying a voltage to the corresponding rows and columns from drivers attached to each row and column. An active-matrix on the other hand, has a substrate that is an electronic backplane. This has two transistors for each pixel, which are connected to perpendicular anode and cathode lines. This gives better control of each pixel and results in brighter and sharper images. The downside is that active-matrix displays are more costly and complex to build.
This technology has been patented by Sanyo/Kodak, and will initially be used in devices like cellphones, PDAs or car navigation systems. It is still to cross some technological obstacles, the foremost being that some electro-luminescent materials have very short lives.
Touchscreen: How does it recognize my touch?
Another display system, seen at restaurants, airports or railway stations, is the touch-screen. You navigate through what you’re supposed to do by touching options on the screen.
A transparent panel with sensors built in is fit over the PC display. When you touch it, the voltage on the sensor changes, and this is passed to a touchscreen controller. The controller processes this information, and passes this data to the PC. The panel also comes with a software driver that lets it interact with the OS. What this does is make the OS read your touch events as mouse movements.
There are various types of touchscreen technologies, but those, perhaps, need a different story altogether.