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Contrast ratio indicates the difference between the brightest part of a picture and the darkest part of a picture, measured in discrete steps, at any given moment. The implication is that a higher contrast ratio means more picture detail. Contrast ratios for plasma displays are often advertised as high as 5000:1. On the surface, this is a great thing. In reality, there are no standardized tests for contrast ratio, meaning each manufacturer can publish virtually any number that they like. To illustrate, some manufacturers will measure contrast with the front glass removed, which accounts for some of the wild claims regarding their advertised ratios. For reference, the page you're reading now (on a computer monitor) is actually about 50:1. A printed page is about 80:1. A really good print at a movie theater will be about 500:1 A French company, Nemoptic, has developed another zero-power, paper-like LCD technology which has been mass-produced in Taiwan since July 2003. This technology is intended for use in low-power mobile applications such as e-books and wearable computers. Zero-power LCDs are in competition with electronic paper. Vertical Alignment (VA)
1904: Otto Lehmann publishes his major work "Liquid Crystals" Transmissive and reflective displays When an electrical charge is applied to the electrodes, the molecules of the liquid crystal align themselves parallel to the electric field, thus limiting the rotation of entering light. If the liquid crystals are completely untwisted, light passing through them will be polarized perpendicular to the second filter, and thus be completely blocked. The pixel will appear unlit. By controlling the twist of the liquid crystals in each pixel, light can be allowed to pass though in varying amounts, correspondingly illuminating the pixel. During the 1970s and early 1980s, LCD technology was not yet mature. However, during the early 80's timeframe, a tabletop video game called Popeye was made with a color LCD, a device with technology ahead of it's time. Technologies used for portable devices made prior to the 1990s to use color graphics include tabletop video games that use Vacuum fluorescent displays and also, before modern laptop computers that used color graphics, the so-called luggable computer, Commodore SX-64 used color graphics on a mini-CRT. The Commodore SX-64 was however bulky hence the aforementioned term luggable. Vertical Alignment (VA) LCDs can be either transmissive or reflective, depending on the location of the light source. A transmissive LCD is illuminated from the back by a backlight and viewed from the opposite side (front). This type of LCD is used in applications requiring high luminance levels such as computer displays, televisions, personal digital assistants, and mobile phones. The illumination device used to illuminate the LCD in such a product usually consumes much more power than the LCD itself. Twisted Nematic displays contain liquid crystal elements which twist and untwist at varying degrees to allow light to pass through. When no voltage is applied to a TN liquid crystal cell, the light is polarized to pass through the cell. In proportion to the voltage applied, the LC cells twist up to 90 degrees changing the polarization and blocking the light's path. By properly adjusting the level of the voltage most any grey level or transmission can be achieved.
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In a monochrome plasma panel, control circuitry charges the electrodes that cross paths at a cell, causing the plasma to ionize and emit photons between the electrodes. The ionizing state can be maintained by applying a low-level voltage between all the horizontal and vertical electrodes - even after the ionizing voltage is removed. To erase a cell all voltage is removed from a pair of electrodes. This type of panel has inherent memory and does not use phosphors. A small amount of nitrogen is added to the neon to increase hysteresis. Reflective LCDs, often found in digital watches and calculators, are illuminated by external light reflected by a (sometimes) diffusing reflector behind the display. This type of LCD can produce darker 'blacks' than the transmissive type since light must pass through the liquid crystal layer twice and thus is attenuated twice, however because the reflected light is also attenuated twice in the translucent parts of the display image contrast is usually poorer than a transmissive display. The absence of a lamp significantly reduces power consumption, allowing for longer battery life in battery-powered devices; small reflective LCDs consume so little power that they can rely on a photovoltaic cell, as often found in pocket calculators. With prices starting around US$2,000 and going all the way up past US$20,000 (as of 2004), these sets did not sell as quickly as older technologies like CRT. But as prices fall and technology advances, they have started to seriously compete against the CRT sets. Some 42" sets fell below $1,500 at major retailers like Best Buy and Costco during the 2005 Christmas season, and many of the retailers reported that plasma TVs were among the hottest selling items for that season. Before applying an electrical charge, the liquid crystal molecules are in a relaxed state. Charges on the molecules cause these molecules to align themselves in a helical structure, or twist (the "crystal"). In some LCDs, the electrode may have a chemical surface that seeds the crystal, so it crystallizes at the needed angle. Light passing through one filter is rotated as it passes through the liquid crystal, allowing it to pass through the second polarized filter. A small amount of light is absorbed by the polarizing filters, but otherwise the entire assembly is transparent. Until quite recently (c. 2004) the superior brightness and viewing angle of color plasma panels, when compared to LCD, made them one of the most popular forms of display for HDTV. However since that time improvements in LCD technology have closed the gap dramatically. The much lower weight, price and power consumption of LCDs have seen them make large inroads into the former plasma market. Sony now only sell a very limited range of Plasma screens and appear set to quit this market altogether. PVA 1911: Charles Mauguin describes the structure and properties of Liquid Crystals.
* LCD screens occasionally suffer from image persistence, which is similar to screen burn on CRT displays. LCDs can be either transmissive or reflective, depending on the location of the light source. A transmissive LCD is illuminated from the back by a backlight and viewed from the opposite side (front). This type of LCD is used in applications requiring high luminance levels such as computer displays, televisions, personal digital assistants, and mobile phones. The illumination device used to illuminate the LCD in such a product usually consumes much more power than the LCD itself. Functional details The layout of the circuit is very similar to the one used in DRAM computer memory but rather than being built using silicon wafers, the whole structure needs to be created on glass. Many of the processing techniques used in creating circuits on silicon require temperatures in excess of the melting point of glass. The silicon substrate of normal semiconductors is grown from liquid silicon to produce a large single crystal with excellent properties for transistors. The silicon layer for TFT LCDs is deposited from Silane gas to produce an amorphic or polycrystalline silicon layer which is far less suitable for producing high grade transistors. Brief history