CRT (Cathode Ray Tube) Monitors
Still the most common type of monitor and capable of producing outstanding visual results. An electron beam is fired down the back-end of the monitor (or 'tube') and hits the screen. The inside of the screen is coated with a special type of phosphor that produces light when struck by these electrons the colour produced depends on the formulation of phosphor used.
Brightness is controlled by the voltage passed through the monitor that results in more or less electrons being fired at the screen. The electron beam scans from left-to-right and from top-to-bottom to form a grid. The picture is formed by controlling the brightness of each point or pixel (short for picture element) on this grid. The number of times per second that this grid is redrawn by the beam is called the refresh rate. Colour monitors use phosphor dots or strips of red, green and blue in pixels that are so closely grouped that the eye cannot make out the individual colours. These primary colours can thus be used to generate any colour required.
There are two major CRT technologies: Shadow Mask and Aperture Grill.
Shadow Mask CRTs
This type of CRT has a metal mesh placed just behind the screen that separates out the beams of the three guns (red, green and blue) so that they only hit the correct colour dot of phosphor. The width of each of these dots is called the dot pitch and, generally speaking, the smaller the better. Originally, Shadow Mask CRTs were noticeably spherical in shape but now the vast majority are of the FST (Flatter Squarer Tube) design that results in a much flatter screen and hence less distorted or curved images.
The Shadow Mask CRT design offers clear character formation, accurate colour rendition and offers good price/performance.
Aperture Grill CRTs
Instead of using a metal mesh like the Shadow Mask CRT, the Aperture Grill CRT uses vertical wires held under tension behind which the phosphor coating is painted on in strips (red, green, blue) instead of dots.
As there is less metal, more of the energy is turned into light and not heat. There is also a greater area of phosphor so brightness is generally improved. This additional brightness allows a darker tinted screen to be used, which in turn allows higher contrast. And finally, the screen shape is cylindrical (rather than spherical) which reduces reflections.
Flat CRT Monitors
The next step for the CRT is monitors that are not just flatter but completely flat. These technologies can be based on either the shadow mask or aperture grill CRT design. The advantages of a truly flat screen are two-fold: glare and reflections from sunlight or artificial light are significantly reduced and the warping of images by the curve of the screen is eliminated. To achieve a flat screen display, a special electron gun must be used that can vary it's focal length as there is no longer a curved screen to negate this issue.
Current flat CRT technologies include the DiamondtronÔ (shadow mask), FlatronÔ (shadow mask) and the FD TrinitronÒ (aperture grill). Each of these technologies will use different technological approaches. For example, the FD TrinitronÒ has a flat screen with a convex phosphor strip behind it to deliver visually (not just physically) flat picture. The important point is that each of these new CRT technologies aims to deliver totally flat pictures.
LCD monitors are becoming more and more popular due to their flat, space-saving design and picture quality that has improved massively over the past few years. They are also more economical to run than CRTs and have a longer service life.
LCD (Liquid Crystal Displays) generate images by using special chemicals called Liquid Crystals through which the amount of light transmitted can be controlled by electric fields that rotate the crystals. The screen is divided up into picture elements (pixels) and colour LCDs use pixels fitted with red, green or blue filters to produce all of the different colours.
The first form of LCD technology uses liquid crystal molecules called twisted nematic (TN). These twisted molecules are controlled by transistors that apply a voltage to a group of sub-pixels (red, green and blue). This electric current rotates them so that the appropriate level of light and colour is let through. They are usually described as TN + film LCDs as manufacturers usually add a film that helps to improve the viewable angle. This type of LCD offers excellent price/performance though, as the 'default' setting for each pixel is to let light through, dead pixels are a noticeable white.
Other LCD technologies include In-Plane Switching (IPS), Super In-Plane Switching (SIPS) and Multi-Domain Vertical Alignment (MVA).
IPS and SIPS LCDs use a second filter that is perpendicular to the first one so that if no voltage is applied the pixel remains black. This means dead pixels are much less noticeable as they are black rather than white. IPS LCDs may also provide better viewable angles than TN LCDs. The main limitations of IPS technology is that it requires more power and may have a slower response time (important for fast moving pictures) than a TN LCD.
MVA LCDs have the crystals of each subpixel aligned in a number of different directions and each crystal is able to rotate independently. This creates a wide range of display 'zones' so that users will only perceive one zone, no matter where they are in relation to the screen. The viewable angle is thus very good, the contrast ratio is very high and the response time may be better than TN or IPS/SIPS equivalents. However, whilst the image is viewable from a range of angles, colours will often be distorted unless viewed from head-on.
Projectors magnify smaller images to a size that is suitable for group viewing or public display. They will often be used for office meetings, presentations to clients or at lectures or conferences. The main criteria users should assess when selecting a projector are brightness (usually measured in ANSI lumens), contrast ratio (the difference in light intensity between the brightest white and deepest black), uniformity (how consistent the light and colour is throughout the image), resolution, weight and noise. You may also want to consider which extra features you will need such as digital zoom, built-in speakers, HDTV compatibility, picture in picture, etc.
There are four main front projection technologies: LCD, Digital Light Processors (DLP), Liquid Crystal on Silicon Devices (LCOS) and CRT.
LCD projectors are the most commonly used as they are efficient, robust and reliable. Using Liquid Crystal technology, an image made up of a pixel grid is projected onto a single mirror which reflects it through a lens for magnification and focusing. They provide excellent colour performance and are equally adept at displaying both data and video images. They are also relatively economical to buy, though you will need to purchase a model with a high brightness capability (1600+ lumens) if you wish to use this type of projector in an environment with high ambient light e.g. lit room.
DLP projectors are particularly effective at displaying video images. A single DLP chip has the entire pixel grid on it and every pixel on that chip has its own separate reflective mirror that helps to amplify the light being generated. Since each pixel has its own amplified light source, there is virtually no limit to the picture size because brightness and contrast will always be consistent. They are small, lightweight and very easy to carry around, though their small size means they are liable to become hotter than an LCD projector.
LCOS projectors use very small LCDs mounted on top of reflective silicon chips that generate an image with very high pixel counts. They therefore offer greater resolution than their peers, though contrast performance may suffer. The high image quality means they are especially useful when displaying video.
CRT projectors are still used where very high resolutions are required or outstanding video performance is essential. They are considerably larger than other types of projector as they require three separate colour tubes (red, green and blue). They are also much better at displaying images at a range of different resolutions than other projection technologies.