Liquid Crystal Displays

LCDs have fairly recently become a popular for the home computer market. But the LCD technology is not new, indeed it was discovered in the early 1960s and used for a range of devices like pocket calculators and digital watches from the early 70s. There is however important differences between the early devices and the current range of computer monitors.

The early devices where monotone (mostly black and white) the display area was fairly small and they used reflected light. This last point means that they are not visible in the dark. . This type of LCD has higher contrast than the transmissive type. 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.

In the current computer monitors (and TVs) the Liquid crystal is back-lit. 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.

Although the way LCDs actually do thier job is part of a computer science degree. This syllabus does not concenr itself with the detail of how a liquid Crystal display works. Other than to say that the monitor has a RGB cluster of pixels in the same way as a CRT monitor. For those of you ineterested in this level of detail. See How stuff works or Wikipedia - both of which have good sections on this

Comparision of Monitor Types

Brightness
The typical ViewSonic LCD monitor has a brightness of 250 to 300 nits, compared with a typical CRT brightness of 100 nits. Since an LCD acts like a shutter, it can be made brighter by increasing the brightness of the backlight.
No flicker
There is no flicker on an LCD display because, while a CRT must be refreshed, the LCD has a constant source of light over the whole screen. Once a pixel is on, it stays on until turned off.
Focus
In a CRT, the electron beam is circular when aimed directly forward, but becomes distorted when aimed up, down, left or right as it sweeps across the screen and may cause image clarity or focus issues at the screen edge.
In contrast, an LCD has millions of pixels, each one effectively independent from its neighbor, with no scanning electron beam, so distortion problems are negligible. The image is always perfectly "focused" over the entire screen.
Perfect geometry
LCD monitors provide geometrically perfect, distortion-free images-a huge advantage for advanced users such as artists and designers.
Longevity
The only item that ages on an LCD monitor is the backlight, which is composed of one or more tiny fluorescent tubes. The typical life of a backlight is 50,000 hours to the half brightness point-the point at which brightness is one-half of the original brightness, and the industry standard measure for product life.
A CRT ages in two ways: An oxide layer forms on the cathode of the electron gun, decreasing beam current; and the phosphor ages and becomes less efficient. The typical CRT half-brightness point occurs between 10,000 and 20,000 hours.
Power consumption
The power required to run an LCD is about one-third of that required for a CRT with the same screen area. In addition, the amount of heat generated by an LCD monitor is considerably less than a CRT monitor, resulting in a lower load on air conditioning. Building cooling needs may be decreased by up to 20%.
And, if an LCD monitor is used with an uninterruptible power supply, the lower power required provides precious extra minutes to store critical data and shut down gracefully in the event of a power failure.
Low emissions
An LCD is essentially emission-free, while a CRT monitor can generate electric, magnetic and even X-ray emissions due to the high-voltage power supply necessary to drive the CRT. An LCD causes no electromagnetic interference.
Ergonomics
The size, dimensions and weight of an LCD allow it to fit into locations that a CRT can't—even mounted on a wall. An average 17" CRT monitor may measure almost 17 inches deep and weigh up to 40 pounds, while an average 17" LCD takes up half the space, with a depth of just around 8 inches and a weight of 15 pounds.
In addition, some LCDs can also pivot from landscape to portrait mode to enhance certain applications, making it easier to work on spreadsheets or two-page layouts, for example.
Robustness Both monitors can be completly broken by agressive action, in a school enviroment experience shows that LCDs are less durable.
Cost
A CRT is Cheeper to buy but more expensive to run.

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