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How CRT Works

LCD displays are slim and their small footprint makes them ideal where unobtrusive technology is required – but what are the drawbacks?

Flat panel technology is completely different from traditional Cathode Ray Tube (CRT) technology. There are two main types of flat panel technology: conventional Thin Film Transistor (TFT) and Enhanced TFT.

Conventional TFT has several limitations, such as 90-120( viewing angles, as well as colour, clarity and contrast problems. Monitors with standard TFT technology present a good image only if the viewer is directly facing the screen. Even slight angles degrade the picture quality and more severe angles make it disappear all together.

Enhanced TFT is available from a few vendors and each one offers a slightly different version. Hitachi uses SuperView technology to provide full 160( viewing, improve clarity and images. Hitachi SuperView is the brand name that refers to Hitachi's LCDs with full 160( horizontal and vertical viewing angles. Hitachi accomplishes its SuperView by using STFT or MDVA technology, which both provide a wider viewing area.

Viewable image size vs screen size

Many customers look at flat panel displays and feel that the images look bigger than they do on the same size CRT monitors. In fact, they look bigger because they are in fact bigger. CRT monitors are measured by their bezel sizes, such as 17in, 19in, or 21in. But a CRT monitor's viewable image size is smaller than its bezel size. This is why CRTs list their viewable image area in addition to their bezel size. For example, a 17in CRT monitor usually has a 15.6-16in viewable image area. By contrast, the new flat panel displays are rated by their viewable image size only, because a 15in flat panel display has a 15in viewable image area.

How Hitachi overcomes viewing angle limitations

Conventional TFT displays begin by using twisted liquid crystal pixels. When electricity is applied to these pixels, they straighten. Most of them provide images perpendicular to the screen, and some shoot off at irregular angles to provide the limited viewing angle of 90-120(.

These displays have colour, clarity and contrast problems. If you think of the pixels as short cylinders, then in conventional TFT the light you see is being emitted from the ends of the cylinders. The long sides of the cylinders, which emit most of the light, are facing the sidewalls - making the image lose much of its viewing angle.

Some enhanced TFT vendors fix this problem by turning the pixels and having their long sides parallel to the glass, and this is certainly an improvement. However, images still suffer significant loss of colour, brightness and clarity as the viewer moves to side viewing angles.

Other vendors combat this problem in both of their flat panel technologies. Hitachi's patented STFT technology not only places the pixels parallel to the glass, it also spins the pixels (think of the disco balls of the 70s) so that equal light, image, colour and clarity are delivered to all areas of the screen.

On their 15in flat panel, Hitachi uses Multi-Domain Viewable Angle (MDVA) technology. With MDVA, the long sides of the pixels are still aligned parallel to the glass for wider viewing angles. In addition, MDVA uses multiple substrates to orient the pixels at multiple angles, so light reaches all areas of the screen with the same bright, crisp and colourful image they have at the centre. This technique combats the loss of colour, clarity and brightness that other companies face when using the parallel design alone. With MDVA, Hitachi has succeeded in creating non-degraded, full 160( viewing angles.

Image resolution and refresh rate

Flat panel displays are all set up like a grid. Unlike CRTs, which have 1800 or more physical pixels, virtually all 14in and 15in panels are 1024 physical pixels wide and 768 pixels high. This is why all vendors of flat panel displays suggest using the 1024x768 resolution exclusively for 14in and 15in panels. By the same token, the best resolution for 18in panels is 1280x1024, because of their physical size.

Brightness and contrast

Flat panel displays are significantly brighter, and feature higher contrast, than traditional CRTs. For example, the average 17in monitor features a brightness of 90-130 candelas per square metre, or nits, while flat panel displays feature brightness ratings of 150-250 nits. There are also significant differences in contrast ratios. CRTs feature contrast ratios around 190:1 to 245:1, while flat panels have contrast ratios from 192:1 to 300:1 significantly higher brightness and contrast even when compared to other 15in flat panel displays.

Reduced glare and eyestrain

One of the benefits of flat panel displays is reduced glare and eyestrain. Because of the liquid crystal panel screen, these displays can be placed facing a window and still provide comfortable viewing, with reduced glare that significantly reduces eyestrain. The difference between flat panels and CRTs in this area is amazing ( especially if your work area is by a sunny window.

Enhanced text display

LCD technology provides some of the sharpest, clearest text available. Images are stable and easy on the eyes, without any of the fading or blurring found in many panels. Some LCD manufacturers use a performance-enhance function to improve text quality at all resolutions by outlining the pixels used to create letters with an additional thin-line, grey scale-smoothing algorithm. The panel uses data and intelligent chips on the glass to calculate the grey scale level for each new pixel and provide a smoothing effect.

Space-saving and environmental benefits

Flat panel displays offer a whole new world of convenience and energy savings. Incredibly thin, LCD 14in and 15in panels each weigh less than 15lbs ( making them light to move, swivel and carry. In fact, they are so light that some users even use these panels rather than a projector for small client meetings.

Most panels are approximately 8in deep. Compare that to the average depth of a 17in monitor that is usually around 17-17.5in deep. Not only does this save space on your desktop, but some panels can even be mounted on a flexible wall mount or desktop-mounting arm, so they can be moved to any angle.

Because of their liquid crystal medium, flat panel displays emit very low magnetic and radiation fields ( significantly less than CRT monitors. In addition, most LCD display manufacturers displays are TCO certified ( they are built and tested to TCO 1995 or 1999 specifications, which are the strictest European specifications for ergonomics, emissions, energy savings and ecology.

LCD panel displays on average use less than one third of the energy to operate than the average CRT. For companies that run hundreds of monitors every business day, cutting monitor power requirements by about two-thirds represents a significant savings in electricity costs.

Why a CRT may still suit your needs

While flat panel displays are amazing and offer numerous benefits, CRTs are still the best choice for some consumers. People who need to work with multiple resolutions, especially 1600x1200 and higher, are better off with CRTs because flat panels cannot support very high resolutions, and will show degraded images at non-prime resolutions.

In addition, CRTs are significantly less expensive than flat panel displays, so if budget is an issue, you can get larger screens on CRTs for the same money as a smaller flat panel.

There is no doubt that flat panels are the wave of the future and have a host of great benefits, from space savings to improved brightness and reduced glare, but there are still some cases where CRTs are a better fit.

Compiled by Paul Phillips

( Hitachi-NSA 1999


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This was first published in June 1999

 

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