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Choosing an LCD Projector

Introduction

One 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.

The Technology

LCD

Traditional 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:
  • Creating large active matrix LCD screens is difficult, the larger the screen, the more transistors involved and the more likely some of them are to fail.
  • A very strong light source is required, as the transistors absorb much of the light.
Newer active LCDs are made with polysilicon rather than amorphous silicon. These are much smaller than traditional LCDs, are more transmissive (i.e., brighter) and have higher switching speeds, but are more expensive to produce. These projectors usually use three LCD panels, rather than one. The light is split into red, green and blue components by dichroic mirrors, with each colour passing through a separate LCD panel driven by the corresponding component of the video signal. A prism is then used to recombine the light before it leaves the projector. Using three panels results in better colours than a single panel.

Digital Light Processing

Projectors 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:
  • As reflected light is used, the image can be much brighter than with standard LCD projectors
  • Because the mirrors are so small and closely packed the image produced is very high quality, with little pixellation.
  • The digital switch is very efficient, resulting in less noise and flicker.
In order to produce colour images, a spinning colour wheel is used to provide sequential red, green and blue light for the projector, synchronised with the mirror movements. The colour wheel absorbs two thirds of the light at any one time, i.e., only the red, green or blue component is transmitted. Therefore in projectors where the colour wheel can be removed from the path to give greyscale images, the brightness will increase up to threefold. Three chip DLP systems are also possible. These are equivalent to the three panel LCD systems, with dichroic mirrors and prisms used to split and recombine the light. Since all the light is reflected, much brighter images are possible.

Lamps

There 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 Size

Image 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.

Resolution

All 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 Contrast

To 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.

Mounting

Where 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:
  • Front - the standard viewing position. The main disadvantages are that the projector may obstruct the audience's view, or that the presenter may block the image path. All the projectors reviewed below were small enough not to cause a real obstruction, certainly smaller than an OHP and LCD panel.
  • Rear - this requires a special projection screen, and a similar amount of room behind the screen as would be required for front projection. It is usually only found in very large lecture theatres.
  • Ceiling - perhaps the best solution as the projector is not in line of sight, and is less likely to be accidentally damaged or stolen. As the projector will usually be mounted upside down, it must be possible to invert the image. In addition to the cost of the mounting, a cable booster may also be required if the projector is more than a few metres from the computer.

Keystoning

Keystoning 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.

Inputs

Multiple inputs may be useful in a conference situation, where speakers are using different computers, allowing all the equipment to be setup before the session.

Accessories

Remote 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 Autosynch

Autosynch 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.

Glossary

Active Matrix
Term used to describe LCDs which have micro-transistors that "open" and "close" each pixel.
AMLCD
Active Matrix Liquid Crystal Display
a-Si
Amorphous Silicon
Dichroic
A mirror or lens that reflects or refracts selective wavelengths of light.
DLP
Digital Light Processor, projector technology based on DMD.
DMD
Digital Micromirror Device, developed by Texas Instruments
Keystoning
Keystoning is caused when the projected image is not perpendicular to the screen, making the image appear wider at the top than bottom.
LCD
Liquid Crystal Display
Lumen
The unit of illumination on a screen or other surface.
p-Si
Polycrystalline silicon or Polysilicon
TFT
Thin Film Transistor

Online Resources

A collection of papers on LCD technology from Sarif, an LCD manufacturer.

http://www.sarif.com/sarif/library/menu1.html

Smarter College Classrooms - information about designing and equipping multimedia classrooms

http://classrooms.com/

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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