What Makes a Projector? (Part 1)

What makes them what they are today? There are essentially two answers to that question.  One is LCD technology and the other is DLP technology.  They both use replaceable lamps, but how they use those lamps to create the image is what sets them apart.

This begins a short series on Projector display technology.  This post we will cover DLP Projectors.

DLP is an acronym for Digital Light Processing. Invented in 1987 by Larry Hornbeck of Texas Instruments the digital mirrored device chip(DMD).

The first video projector to use this technology was built by Digital Projection in 1997.  Both Digital Projection and Texas Instruments won an Emmy Award for the use of DLP technology.

A DMD chip(pictured above) is made up of microscopic mirrors arranged in an array on the chip surface.  The mirrors are controlled via small electrostatic pulses to adjust their angle.  The angle causes them to either reflect the light out the lens onto the screen or away from the lens to create black or lack of light.  Each mirror is made up a microscopic yoke, torsion spring, and the electrostatic pads that affect the memory cells that set the position.

The diagram to the right shows the construction of each pixel.  Depending on the resolutions the amount of pixels changes. For instance an 800 x 600 DMD has (800*600 pixels= 480,000) pixels or mirrors that are controlled.  However some chips use a method of oscillating the mirrors to have them act double duty, halving the amount of mirror elements needed in a DMD chip.  

DLP Chips are monochrome or single colored. They can only turn light on and off and adjust its brightness. It cannot color the light.  This is where the next most important device of a DLP projector comes into play.

The Color wheel is a glass wheel made up of multiple segments all with a different color glass light filter.  On average they use 4 segments: red, blue, green and clear. Some models have multiples of the same color.  Others use 6 segments colors red, purple, blue, light blue, green and yellow.  The projector syncs the rotation of the color wheel and the color needs of the DMD while the image is being projected. DLP uses optical persistence to “fool” our eyes into seeing a mix of colors when in reality the projector is only projecting one color at a time for a fraction of a second.  If you blink your eyes quickly while looking at the DLP projected image, you can see the colors by themselves.

These 2 devices more than any other set the DLP projector apart from LCD.  Other differences are in the electronics.  There are a whole set of electronics that only are there to support and control the DMD chip.  These electronics are not as physically obvious but are no less important. These chips are for driving the electrostatic control signals to the DMD, as well as the video processing chips that prepare the signal to the DMD controller.  There is a motor control chip that keeps the color wheel in sync so that why the DMD is projecting the red portion of the image, the red color wheel segment is in position.  These all work together to ensure a pleasant and vibrant image.  

The main benefit of DLP over LCD(at the time) was the contrast ratio. Contrast ratio is the difference between a full white and full black image. When using a lamp to create an image there is almost no chance of having true black as black is the absence of light.  Turning off the lamp for the black portions of the picture is not practical.  Rather the pixels that require black merely point their light away from the main lens creating black on the screen.  Since the mirrors are only pointing away, there is some minor light leakage so the “blacks” are not as black as they could be.  The higher the ratio, the darker the blacks and brighter the white bits of the image will be. This was more of an issue with DLP first came to market as LCD had abysmal contrast.  These days they are pretty much the same contrast ratio-wise.

The last and most obvious difference is the lamp used in DLP.  Both DLP and LCD use Short ARC Mercury vapor lamps but only DLP uses this particular arrangement.

DLP Lamps commonly use a non-optical lens. Meaning there is no focusing or change in direction of the light beam.  Rather they have lenses inside the optics that do any light adjustment needed.  The lamps do have a special coating on the lens(ND filter lens). That coating prevents UV(ultraviolet) and IR(infrared) light from being injected into the light path.

These can harm your eyes and the optics due to heat and radiation.  This coating is a big reason why we do not recommend replacing only the bare bulb. If that coating fails or fails just a little it can cause your color wheel problems and possibly even melt some of the lenses.

Here is a picture of the internals from a Polyvision PJ905 that uses a 2002031-00 Lamp that has the proper coating.

This about sums up the DLP video projector.  They are a reliable and well proven piece of technology.  Pureland Supply takes a lot of pride in our lamps and we make sure every lamp that is used in a DLP projector(as well as LCD) is configured correctly to perform as well as possible for you.  Check our selection of lamps here. 

Stay tuned for next week when we discuss LCD projectors. If you have any questions please do not hesitate to contact us at 1-800-664-6671 or Sales@purelandsupply.com