Various forms of Monitors

Cathode Ray Tube

Definitions

• A cathode is a terminal or electrode at which electrons enter a system, such as an electrolytic cell or an electron tube.
• A cathode ray is a stream of electrons leaving the negative electrode, or cathode, in a discharge tube (an electron tube that contains gas or vapor at low pressure), or emitted by a heated filament in certain electron tubes.
• A vacuum tube is an electron tube consisting of a sealed glass or metal enclosure from which the air has been withdrawn.
• A cathode ray tube or CRT is a specialized vacuum tube in which images are produced when an electron beam strikes a phosphorescent surface.

Besides television sets, cathode ray tubes are used in computer monitors, automated teller machines, video game machines, video cameras, oscilloscopes and radar displays.

The first cathode ray tube scanning device was invented by the German scientist Karl Ferdinand Braun in 1897. Braun introduced a CRT with a fluorescent screen, known as the cathode ray oscilloscope. The screen would emit a visible light when struck by a beam of electrons.

In 1907, the Russian scientist Boris Rosing (who worked with Vladimir Zworykin) used a CRT in the receiver of a television system that at the camera end made use of mirror-drum scanning. Rosing transmitted crude geometrical patterns onto the television screen and was the first inventor to do so using a CRT.

Modern phosphor screens using multiple beams of electrons have allowed CRTs to display millions of colors.

LCD

Liquid crystal display (LCD) is a thin, flat panel used for electronically displaying information such as text, images, and moving pictures. Its uses include monitors for computers, televisions, instrument panels, and other devices ranging from aircraft cockpit displays, to every-day consumer devices such as video players, gaming devices, clocks, watches, calculators, and telephones. Among its major features are its lightweight construction, its portability, and its ability to be produced in much larger screen sizes than are practical for the construction of cathode ray tube (CRT) display technology. Its low electrical power consumption enables it to be used in battery-powered electronic equipment. It is an electronically-modulated optical device made up of any number of pixels filled with liquid crystals and arrayed in front of a light source (backlight) or reflector to produce images in color or monochrome. The earliest discovery leading to the development of LCD technology, the discovery of liquid crystals, dates from 1888.[1] By 2008, worldwide sales of televisions with LCD screens had surpassed the sale of CRT units.

Each pixel of an LCD typically consists of a layer of molecules aligned between two transparent electrodes, and two polarizing filters, the axes of transmission of which are (in most of the cases) perpendicular to each other. With no actual liquid crystal between the polarizing filters, light passing through the first filter would be blocked by the second (crossed) polarizer.

The surface of the electrodes that are in contact with the liquid crystal material are treated so as to align the liquid crystal molecules in a particular direction. This treatment typically consists of a thin polymer layer that is unidirectionally rubbed using, for example, a cloth. The direction of the liquid crystal alignment is then defined by the direction of rubbing. Electrodes are made of a transparent conductor called Indium Tin Oxide (ITO).

Before applying an electric field, the orientation of the liquid crystal molecules is determined by the alignment at the surfaces of electrodes. In a twisted nematic device (still the most common liquid crystal device), the surface alignment directions at the two electrodes are perpendicular to each other, and so the molecules arrange themselves in a helical structure, or twist. This reduces the rotation of the polarization of the incident light, and the device appears grey. If the applied voltage is large enough, the liquid crystal molecules in the center of the layer are almost completely untwisted and the polarization of the incident light is not rotated as it passes through the liquid crystal layer. This light will then be mainly polarized perpendicular to the second filter, and thus be blocked and the pixel will appear black. By controlling the voltage applied across the liquid crystal layer in each pixel, light can be allowed to pass through in varying amounts thus constituting different levels of gray.

The optical effect of a twisted nematic device in the voltage-on state is far less dependent on variations in the device thickness than that in the voltage-off state. Because of this, these devices are usually operated between crossed polarizers such that they appear bright with no voltage (the eye is much more sensitive to variations in the dark state than the bright state). These devices can also be operated between parallel polarizers, in which case the bright and dark states are reversed. The voltage-off dark state in this configuration appears blotchy, however, because of small variations of thickness across the device.

Both the liquid crystal material and the alignment layer material contain ionic compounds. If an electric field of one particular polarity is applied for a long period of time, this ionic material is attracted to the surfaces and degrades the device performance. This is avoided either by applying an alternating current or by reversing the polarity of the electric field as the device is addressed (the response of the liquid crystal layer is identical, regardless of the polarity of the applied field).

When a large number of pixels are needed in a display, it is not technically possible to drive each directly since then each pixel would require independent electrodes. Instead, the display is multiplexed. In a multiplexed display, electrodes on one side of the display are grouped and wired together (typically in columns), and each group gets its own voltage source. On the other side, the electrodes are also grouped (typically in rows), with each group getting a voltage sink. The groups are designed so each pixel has a unique, unshared combination of source and sink. The electronics, or the software driving the electronics then turns on sinks in sequence, and drives sources for the pixels of each sink.
 

Gas Plasma Displays 

An overview of plasma displays

Gas plasma technology is a new way to build video and computer monitors. Essentially plasma units have the brightness and look of a CRT monitor, but they offer a much larger image and are thin enough and light enough to hang on any wall. This combination makes them ideal where lighting conditions would favor a monitor, but audience size indicates a projector. Like LCD displays, plasma monitors do not exhibit the distortion and loss of clarity in the corners inherent to CRTs.

How do plasma monitors work?

Plasma monitors work much like CRT monitors, but instead of using a single CRT surface coated with phosphors, they use a flat, lightweight surface covered with a matrix of millions of tiny glass bubbles, each having a phosphor coating. These phospors are caused to glow in the correct pattern to create an image.

What are the advantages of plasma?

Plasma monitors have several advantages over CRT-based monitors:

• Thin and lightweight: at only 4" - 6" thick and about 60-100 lbs., they’re easy to hang on any wall.
  
• Very bright: less sensitive to ambient light than most LCD projectors, plasma monitors have the brightness and contrast of CRT-based sets.
  
• 160° viewing cone: ideal when your room is wide and people may view the monitor from farther off-axis than normal.
  
• Stable and distortion-free: unaffected by magnetic fields; useful in many applications where CRT monitors or LCD and CRT projectors are problematic. Entire image always in perfect focus, not just in the center, but all the way to the corners.
  
• Look and feel: plasma somehow looks different--better--than monitors and projectors alike. It's hard to quantify that difference, but most people would say they have more depth and warmth than other types media. They look very, very good.
  

What are the disadvantages of plasma?

This new technology has several disadvantages worth mentioning.

• Cost: plasma is expensive. For that reason alone, plasma is not for everyone. But prices are coming down, as they do for most new technologies.
  
• More susceptible to burn-in than CRT monitors. It's not a good medium on which to display a company logo for two or three hours at a time. But with the appropriate precautions, and in some situations a screen saver, you should not expect problems.
  
• Resolution restrictions: plasma is subject to the same type of resolution problems as LCD or DLP projectors. You'll get the best images when the resolution of your source matches the "true" resolution of the monitor. But, as with LCD, the monitors will incorporate compression or expansion circuitry to automatically resize other resolution sources to match their native resolution, and most people will be very happy with the result. Still, if sharpness is critical for your application and you'll be using a variety of computer sources, you may be better off with a CRT-based unit.
  
• Doesn't travel well: plasma is not portable. These monitors weigh 60 - 100 pounds and they don't do well if you drop them. If you want to travel with a plasma monitor, plan to invest in a good shipping case.
  

There's one other rumored "problem" with plasma that turns out not to be true. It has been said by some that plasma units do not have a long lifespan. Actually, the estimated life span for plasma monitors (according to Sony) is about 30,000 hours-- which translates to approximately 15 years at 8 hours a day, 5 days a week (comparable, or maybe a bit better, than a CRT-based monitor).