With a 100-year head start over more modern screen technologies, the CRT is still a formidable technology. It’s based on universally understood principles and employs commonly available materials. The result is cheap-to-make monitors capable of excellent performance, producing stable images in true colour at high display resolutions. But in the world of miniaturization, Cathode ray tubes (CRT) are giant dinosaurs waiting for extinction. A CRT uses a single-point hot electron source that is scanned across the screen to produce an image.
The CRT’s most obvious shortcomings are well known:
· It uses too much electricity.
· Its single electron beam design is prone to misfocus, misconvergence and colour variations across the screen.
· Its clunky high-voltage electric circuits and strong magnetic fields create harmful electromagnetic radiation.
· It’s physically too large.
Attempts to replace bulky Cathode ray tubes resulted in the introduction of the field emission display screens (FED) screens. It will be the biggest threat to CRT’s dominance in the panel display arena. Instead of using a single bulky tube, FEDs use tiny ‘mini tubes’ for each pixel, and the display can be built in the same size as a CRT screen.
The FED screens are lightweight, low power consuming and compact. The FEDs can be used instead of some other technologies are gaining market share in big screen and PC monitors, such as Projection TV, Plasma Displays, Liquid Crystal, and Organic Transistor Displays.
Various types of displays have become common in the every day life. The displays are used in televisions, computers etc. They also have wide use in laboratories and in medical applications. The displays are those devices by which we can view moving objects. The displays are manufactured depending upon their application.
One of the hottest markets driving physics research is the demand for a perfect visual display. People want, for example, large, thin, lightweight screens for high-definition TV and outside displays and very high resolution flat computer monitors that are robust and use little power. Several types of flat display are competing for these applications. Not surprisingly, the research departments of universities and the big electronics companies around the world are bustling with exciting ideas and developments. New university spinout companies are developing many new devices. The different types displays available are:
· Liquid crystal displays
· Plasma displays
· Electro luminescent displays
· Field emission displays
· Projection displays
Liquid crystal displays
Even the liquid crystal display (LCD), which has 85 per cent of the flat-screen market, is still a young technology and the subject of very active research. LCDs depend on arrays of cells (pixels) containing a thin layer of molecules which naturally line up (liquid crystals); their orientation can be altered by applying a voltage so as to control the amount of light passing through. Their main drawbacks have been poor viewing characteristics when seen from the side and in bright light, and a switching speed too slow for video. Electrically sensitive materials called ferroelectric and antiferroelectric liquid crystals show potential. These work slightly differently and are bistable so should use less power. They can respond 100 to 1000 times faster than current displays, and should give brighter images from all angles. One solution to the drawbacks of LCDs is to combine them with another technology. Indeed, the latest, high quality LCDs on the market incorporates a tiny electronic switch (a thin film transistor, TFT) in each pixel to drive the display.
Although LCDs up to a 42-inch diagonal have been demonstrated, for larger flat TV screens, companies have instead turned to plasma display panels. These employ gas discharges (as in a fluorescent tube) controlled by an electrical signal. The ionised gas, or plasma, emits ultraviolet light which stimulates red, green and blue phosphors inside each pixel making up the display panel to produce coloured light. The images on the latest displays are very clear and bright. Unfortunately they are still expensive.
Electro luminescent displays
One of the most promising emerging display technologies exploits ultra thin films of organic compounds, either small molecules or polymers, which emit light (luminescence) when subjected to a voltage. These organic light-emitting diodes (OLEDs) produce bright, lightweight displays.
Field emission displays
The other major technology competing for the flat screen, market is the field emission display. This works a bit like a cathode-ray tube, except that electrons are emitted from thousands of metal ‘micro-tips’, or even a diamond film, when an electric field is applied between the tips and a nearby phosphor coated screen. Printable Field Emitters, based at the Rutherford Appleton Laboratory near Oxford, has come up with a novel idea employing low-cost composite materials deposited and patterned using screen printing and simple photolithography. This technology could produce affordable large displays in the 20 to 40-inch diagonal range suitable for TVs.
Finally, a completely different approach showing potential is to direct light from an image source using wave-guides through a glass or plastic sheet onto a screen. A clever variation of this is ‘the Wedge’ developed by Cambridge 3D Display. Light rays pass up a thin wedge-shaped glass plate and emerge at right angles at various points depending on the angle of entry. The beauty of this device is that it could be used to project any kind of micro-display – LCD or OLED, for example – onto a large screen.
All of the technologies described here still have drawbacks and no one yet knows which will win the big prize of flat screen TVs. It is likely that all of them will find niche markets. The next five years will certainly see a revolution in flat screen development.
The FED screen mainly contains three parts:
1. Low-voltage phosphors.
2. A field emission cathode using a thin carbon sheet as an edge emitter.
3. FED packaging, including sealing and vacuum processing.