Basically there are two technologies commonly used in modern projectors and those are LCD and DLP. LCD, Liquid Crystal Display, is by far the oldest technology which has been developed decades ago and first debuted in wrist watches and more recently in flat panel monitors. The same technology is used in an LCD projector, although somewhat different than we’re grown accustomed to from, for example, flat panel monitors. In principle a LCD projector is not very different from a slide projector, with the slide being replaced by an LCD display. Modern LCD projectors use three LCDs, one for each primary color, red, green and blue. By making combinations of these colors a total of 16.7-million different colors can be generated.
If we look at the basic construction of a LCD projector we see a high-pressure gas-discharge lamp, a combination of mirrors and lenses and three LCD displays. The lamp obviously generates the light needed to project the image. Most lamps used in projectors have a 150-watt rating which means that 80 to 90% of the projector’s power consumption is due to the lamp. Besides a lot of light the lamp generates a substantial amount of heat which must be transported away from the lamp because of risk of melting and exploding (the inside of the lamp is filled with an inert gas that’s excited with a high voltage generator to produce the light). To get the excess heat away from the lamp and out of the projector one or more fans are used which generate the fan noise that projectors produce. Obviously small fans that turn at high rpms are more likely to be noisy than larger fans turning at moderate to slow rpms.
Fig 6. The Sanyo PLV-Z1 LCD projector uses wide screen, 16x9, LCD displays and has a very high contrast.
The LCD displays used in projectors are different than does we see in, for example, a flatpanel monitor, a notebook or a PDA. The LCD displays are manufactured using PolySi technology which gives them good resistance against high temperatures and excess doses of UV light from the lamp. Furthermore they feature a much higher contrast and resolution than normal LCD displays and are much smaller, with sizes up to about an inch measured diagonally. As mentioned three screens are used to form the image, each of which is only projecting one of the primary colors, green, red or blue. The colors are filtered out by mono- and dichromatic mirrors prior to entering the LCD displays. The images formed by the three LCD displays are then combined in a prism and projected to the screen through the projection lens.
Fig 4. Screendoor effect of a LCD and DLP projector, notice the reduced screendoor effect of DLP on the right.
The use of three LCD displays has a number of advantages; first off we’re now able to work with the three primary colors and can adjust the amount of red, green and blue in the final image as we please. If we’d have used a single color LCD, the color reproduction, brightness and the contrast would be fully accountable to that single LCD display, which will, in the end, result in a lesser quality image. A disadvantage of using three LCD displays is obviously that the images from all three LCDs must align perfectly at the screen to create a quality image. Another disadvantage of LCD technology is the rather low contrast which is limited by the amount of light still projecting through an LCD display when it is fully darkened. Another disadvantage is that every single pixel on the LCD display is surrounded by a small border, which will remain dark at all times. Upon looking closely at an image projected by an LCD projector we can see this effect, which is called the screendoor effect. It looks as if a fine grid is laid upon the image which can or cannot be clearly seen depending on the distance to the screen and the projected image size. Modern LCD projectors have about an 80% effective pixel area due to this effect.