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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. 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. 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. TFT LCDs are a variant of liquid crystal display which use thin-film transistor technology to improve their image quality. TFT LCDs are one type of active matrix LCD, though this is usually synonymous with them. They are used in both flat panel displays and projectors. In computing, TFT monitors are rapidly displacing competing CRT technology, and are commonly available in sizes from 30 to 77 cm (~12 to 30 inches). As of 2005, they have also made inroads on the television market.
General characteristics 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 Normal Liquid Crystal Displays like those found in calculators have direct driven image elements – 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 - top and bottom connections for each of red, green and blue of every pixel. To avoid this issue, the pixels are addressed in rows and columns which reduce the connection count from millions to thousands. If all the pixels in one row are driven with a positive voltage and all the pixels in one column are driven with a negative voltage, then the pixel at the intersection has the largest applied voltage and is switched. The problem with this solution is that all the pixels in the same column see a fraction of the applied voltage as do all the pixels in the same row, so although they are not switched completely, they do tend to darken. The solution to the problem is to supply each pixel with its own transistor switch which allows each pixel to be individually controlled. The low leakage current of the transistor also means that the voltage applied to the pixel does not leak away between refreshes to the display image. Each pixel is a small capacitor with a transparent ITO layer at the front, a transparent layer at the back and a layer of insulating liquid crystal between. (Patterned Vertical Alignment) is a more advanced version of MVA technology offered by Samsung. Developed independently, it suffers from the same problems as MVA, but boasts the best contrast ratios of any TFT technology. * While CRTs are capable of displaying multiple video resolutions without introducing artifacts, LCD displays usually produce only crisp images in their "native resolution" or even fractions of it.
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* LCD displays generally have a lower contrast ratio than that on a plasma display or CRT. This is due to their "light valve" nature: some light always leaks out making black grey. A Color LCD is a descendant of the primative monochrome LCD. This kind of LCD can emulate almost any real-life image compared to a monochrome LCD. A general purpose alphanumeric LCD, with two lines of 16 characters. 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. Value TFT screens and most 38 cm (~15 in.) sized LCDs usually fail to include a digital signal compatible DVI interface, thus their future proofing may be limited. The upper end of 43 cm (~17 in.) or 48 cm (~19 in.) gamer and office TFT screens may have dual analog-VGA and DVI sockets; almost all professional screens have DVI and pivot mode for letter-mode display. However, the use of a DVI video signal does not automatically guarantee better image quality: a good video card RAMDAC and properly shielded analogue VGA cable may produce a better display than a bad video card and DVI. Each pixel consists of a column of liquid crystal molecules suspended between two transparent electrodes, and two polarizing filters, the axes of polarity of which are perpendicular to each other. Without the liquid crystals between them, light passing through one would be blocked by the other. The liquid crystal twists the polarization of light entering one filter to allow it to pass through the other.
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. Small monochrome displays such as those found in personal organizers, or older laptop screens have a passive-matrix structure employing supertwist nematic (STN) or double-layer STN (DSTN) technology (DSTN corrects a color-shifting problem with STN). Each row or column of the display has a single electrical circuit. The pixels are addressed one at a time by row and column addresses. This type of display is called a passive matrix because the pixel must retain its state between refreshes without the benefit of a steady electrical charge. As the number of pixels (and, correspondingly, columns and rows) increases, this type of display becomes increasingly less feasible. Very slow response times and poor contrast are typical of passive-matrix LCDs. 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.