Choosing an LCD Projector
IntroductionOne of the main concerns in delivering multimedia presentations has always been unreliable or unsuitable projection equipment. Fortunately LCD technology has advanced significantly from the first dim VGA LCD panels and good quality, high resolution, projectors are now available. Where panels sat on top of a high powered OHP, projectors now contain their own light source. This review will look briefly at the technology behind the new projectors, consider the features you should look for, and finally give a personal view of seven LCD projectors.
LCDTraditional Liquid Crystal Displays consist of two plates of glass with a layer of liquid crystal material in between. When charge is applied to the cell, the crystals can rotate the plane of polarised light, effectively acting as an on/off switch for the light. Colour is created by using three cells for each pixel, one for each of the primary colours. Early models were passive, while most new projectors use an active matrix, which has a thin-film transistor (TFT) behind each element. Until recently, the TFT layer in most projectors and panels was amorphous silicon. These panels have a much higher refresh rate than passive LCDs and are suitable for displaying video as well as still images. There are a number of problems with this type of display, however:
Digital Light ProcessingProjectors that do not use LCD technology, but Digital Light Processing, based on Texas Instruments Digital Micromirror Device, are now becoming available. This is an SRAM (Static Random Access Memory) chip, covered with microscopic aluminium mirrors, each equating to a pixel. Charging one of the underlying memory bits causes the mirror to rotate due to the electrostatic charge, so acting as an on/off switch. DLP has a number of advantages:
LampsThere are two types of lamps used in projectors, tungsten-halogen and metal-halide. The metal-halide lamps are more efficient, providing greater brightness with lower power consumption. It is important to note that the lamp wattage does not reflect its brightness, this is measured in ANSI lumen, a value measured at the screen which takes into account the image size as well as brightness. For example, a 400W tungsten-halogen lamp may produce 200 lumen, while a 250W metal-halide lamp may produce 450 lumen.
Tungsten-halogen lamps are filament lamps with a low pressure halogen gas atmosphere. They run at a much higher temperature than normal lamps, so producing a whiter light. The colour temperature will shift during its lifetime, however, causing it to look dimmer and more yellow. The higher temperature and low pressure atmosphere both help to prolong the filament, and so lamp, life.
Metal-halide lamps use a high voltage discharge between two electrodes contained in a low pressure halogen and mercury vapour atmosphere. Initially a high current is drawn, which is reduced as the lamp warms up. These lamps are very efficient and produce a very white light, which is maintained throughout its life, though it will get dimmer.
Although the metal-halide lamps are initially much more expensive, often £300-400 compared with around £20, they last much longer, 2000 hours compared with 25-70, making the metal-halide cheaper in the long term.
What to look for
Image SizeImage size depends not only on the projector, but also on its positioning in the room, the further away, the bigger the image. It is very important when making a comparison to ensure all the models are the same distance from the screen, and that it is a similar distance to a real use situation. Although most of the model tested had zoom lenses, in all cases these had to be at maximum magnification to produce a reasonable image size at the test distance of 2.5m from the screen.
ResolutionAll the projectors reviewed below had a maximum physical resolution of 800x600. They differ, however, in how they cope with other resolutions. At lower resolutions (640x480), some are able to resize the image to fill the whole screen. To achieve a high quality image, however, resizing may not be desirable, so look for an optional, rather than auto-resize feature. At higher resolutions, some method of compression is employed. This means that data is lost, and the image quality will therefore decrease. Compression methods vary however, and it is worth checking the quality with typical screens if you plan to use this feature.
Colour and ContrastTo a certain extent, determining which projector provides better colour is a subjective exercise, and will depend on the exact contrast and brightness settings. However, it is worth displaying the same images through different projectors to compare. Test images should include some with a wide variation in shades, and some with large areas of solid colour.
Claimed maximum contrast ratios vary from about 100:1 to 300:1, and generally the higher the ratio, the better the image quality. Note that the type of screen used will also have some affect on the brightness of an image. Standard matte screens give the dimmest image, but it can be seen from anywhere in the room. Reflective screens are also available, which, while giving a brighter image, reduce the viewing angle.
MountingWhere to site a projector is a very important issue. It must be at a reasonable distance to give a good quality image with a reasonable size, but must not interfere with the view of any of the audience. Most of the reviewed projectors supported three projection directions:
KeystoningKeystoning occurs when the projected image is not square, e.g., the top of the image is large than the bottom. This is caused when the projected image is not perpendicular to the screen, and so can be corrected to some extent by tilting the screen. All the projectors had some degree of keystone correction built in, allowing the projector to be placed off centre. As the degree of keystoning is dependent on the position of the projector, it will be different if the projector is front or ceiling mounted. While a few projectors allow the degree of keystone correction to be altered, most have a fixed correction.
InputsMultiple inputs may be useful in a conference situation, where speakers are using different computers, allowing all the equipment to be setup before the session.
AccessoriesRemote controls are included with most projectors, and may act as a remote mouse. This is a useful feature, as the presenter is no longer tied to the computer. Most remote control units are infrared, with sensors mounted on the front of the projector capable of receiving commands bounced off the projection screen.
To prevent frustration later, it is important to check the projector comes with a full set of cables, including those for connecting to PC and Macs and where appropriate video and audio cables.
Scan Rate and AutosynchAutosynch is a common feature which allows the projector to automatically detect the input signal from the computer and adjust its scan or refresh rates etc, accordingly. In most cases some fine-tuning will also be required to get the best image quality.
Many graphics cards allow users to change the (vertical) refresh rate to eliminate any flicker. This may cause problems if the LCD projector is not capable of working at this rate, and should be checked if the projector cannot 'lock on to' a signal.
Flat Panel or Projector?Flat Panel LCDs do offer a number of advantages over projectors. They are cheaper, more portable, often higher resolution and tend to be more reliable. However, they require a high power OHP, produce a much dimmer image than can be achieved with a projector and usually have slower response times, resulting in poorer video quality.
A collection of papers on LCD technology from Sarif, an LCD manufacturer.
Smarter College Classrooms - information about designing and equipping multimedia classrooms
A university teacher's guide to overhead projection, by Tanya Slaughter, La Trobe University, Melbourne. http://ultibase.rmit.edu.au/Articles/slaug2.html
Master Classrooms - Classroom Design with Technology in Mind by Kathryn Conway, University of North Carolina. http://www.iat.unc.edu/publications/conway/conway1.html
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