DLP, Digital Light Processing, is a relative new technology that uses a digital mirror, composed of millions of small moveable mirrors, to project the image. Each mirror can either be switched on, and projects a ray of light through the lens, or off, which will not result in any light entering the lens. The actual image is formed on the mirror and reflected off of it through the projection lens onto the screen. Color is added in a DLP projector by using a rotating color wheel with the three primary colors, red, blue and green, in front of the mirror and then combining the images for these three colors faster than the human eye can see. Obviously with these three primary colors every possible color can be displayed, simply by switching the mirrors on and off at a higher or lower frequency in synchronization with the color wheel. A brighter red simply means that during the time that the red segment of the color wheel is in front of the mirror, the mirrors are more on than off and more of the red light is projected through the lens.
Fig 9. The Dell 2100/2200MP has no zoom lens, which limits the maximum distance from the screen.
The basic construction of a DLP projector isn’t that different from a LCD projector, with the biggest difference being the fact that we’re projecting onto a mirror instead of through a number of LCD displays. Again we see the use of a high pressure gas-discharge lamp as a light source. The light from the lamp shines through the rotating color wheel and the colored light is then projected onto the digital mirror and the image on the mirror is synchronized with the color wheel so the image is displayed properly. One of the advantages of using a rotating colorwheel is that often also a transparent segment is used which greatly enhances contrast, as white light is not created by combining the three primary colors, but by simply synchronizing the transparent segment. A downside of using the colorwheel obviously is the fact that not all colors can be displayed simultaneously; only one color can be displayed at any given time. That also is a disadvantage of a DLP projector as the rapid switching between colors cause for some people to see ‘rainbows’ across white text or white objects in movies. This is simply caused by the rotation of the colorwheel and may, or may not, be noticeable. During our testing we found that these effects wear off over time if we even noticed them.
Fig 5. A section taken from the images on page 3, notice the higher contrast of DLP on the right.
A well documented advantage of a DLP projector is a reduction of the screendoor effect, about 90% of the surface area of each mirror is used for projecting the image, and thus the effective pixel area is much larger. Another advantage is the high contrast ratio; most DLP projectors have a 2000:1 contrast ratio, which is much higher than most LCD projectors which, on average, have a 500:1 contrast ratio. The projected image therefore has blacker blacks and whiter whites in comparison to an LCD projector. Another optical advantage is the fact that the path the light has to travel in the projector is much shorter in a DLP projector. A DLP projector is pretty straightforward and the only object in between the image on the digital mirror and the projection screen is the projector’ lens. This obviously has a positive effect on color balance and reproduction as well as being a more elegant solution overall.
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