Looking for DLP, LCD, and Plasma primer

Discussion in 'Beginners, General Questions' started by Jean Luc, Mar 23, 2004.

  1. Jean Luc

    Jean Luc Agent

    Dec 19, 2003
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    Just wondering if there is a primer on the above listed types of projections? Looking to find which is the best bang for the buck, as well as the technology behind it, pros and cons of each and most iomportantly, which one of these will be the most improved( in y'alls opinion) in the next 5 years. I looked at the primer here but couldn't find anything specific. Links would be sweet.
  2. Vince Maskeeper

    Vince Maskeeper Producer

    Jan 18, 1999
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    I recently wrote this for a book, but this should be some basic info for you. This was written specifically about front projection- but these are the same basic stuff found in rear projection TV (note, plasma was left off both because they are not used in front projection, and because i personally think they suck).

    Different projector technologies

    Cathode Ray Tube (CRT): One of the earliest types of front projection technology, CRT uses the same “scanning electron beam” technology used in old-fashioned picture tubes to create the video image. In the case of CRT projection, the projector increases the light output to project an image across a distance onto a screen. CRT display is an analog process, meaning it is not locked into a specific display resolution like a digital display, which are based on a fixed number of pixels. The only display limits on a CRT display is how fast the electron beam can draw (called the sync or refresh rate).

    You have probably seen a CRT projector in a conference room or your favorite sports bar. The majority of CRT projectors feature 3 lenses, one green, one red and one blue, that are specially configured to project the three overlaying colors to create an image on a reflective screen. Many of the big-screen rear projection sets also use CRT technology, but have the projection elements mounted inside the case, projection from behind the screen.

    The main advantages of CRT projection are the smoothness of picture, the solid black levels and the variety of sync rates it can display. The main disadvantages are the low level of light output, the overall size and the complexity to install and maintain. CRT will give you an excellent picture but the projectors tend to be large and unwieldy, require absolute darkness and will require hours to install and properly configure.

    Liquid Crystal Display (LCD): The first major technology in digital projection, LCD uses a light projected through three grids of colored Liquid Crystal pixels to create the image. The red, green and blue panels illuminate as needed, coloring the bright light passing through them accordingly, creating a projected video image. The LCD display process is digital, meaning that it has a native resolution tied to the number of pixels in its display panels.

    If a projector panel has a native resolution of 800 pixels by 600 pixels, it can only properly display 800x600 resolution signal, and will process incoming video (like DVD) to be compatible with this native resolution. This processing of incoming video signal is called “scaling.” You might have experienced this with a LCD computer monitor, using any resolution other than the suggested resolution results in a processed image.

    The main advantage of LCD technology is the size and high light output. LCD projectors tend to be smaller and brighter than their CRT projection counterparts. The main disadvantages include resolution issues outlined above, the dreaded “screen door effect” where pictures suffer from a visible grid structure due to the nature of the panel and occasionally LCD will suffer from “stuck pixels” where the grid will have a few single pixel spots as a result of a pixel getting stuck in an on or off position.

    Digital Light Processing (DLP): DLP has become the common catch name for what was previously more broadly referred to as Digital Micromirror Devices (DMD). A technology developed by Texas Instruments, these devices consist of a tiny chip with anywhere from hundreds to millions of tiny mirrors in a grid. These mirrors are attached to a “hinge” of sorts, with an electrical support post that allows it to be moved slightly when electricity is applied. By tilting each of the mirrors a few degrees in either direction using electricity and shining light onto the grid, you can manipulate the reflection of on/off pixels to create images.

    This on/off pixel grid is only half the equation. Now we have to add color. To add color to the reflected light, the white reflected light from the mirror grid is sent through a spinning wheel consisting of the primary colors: red, green and blue. The “brain” of the DLP engine divides the image up into these primary colors. The spin of the wheel is synchronized to the grid of mirrors, and as each color passes in front of the reflected beam of light, the grid triggers the needed pixels for that color. So, as the light is passing through the green section of the wheel, the pixels that need some green to them are turned on. This process repeats for each color, for each frame, thousands of times per second. Because it all happens so fast, your eye is able to integrate the overlaying colors into a single, full-color image.

    Texas instruments have heavily promoted this technology as the replacement to film projection in cinemas, and most major cities now have at least one theater fitted with a DLP projection system. This technology also appears in some of the more popular small format projectors and now is emerging in rear projection televisions.

    The main advantage of this technology is a further reduction in size, and a finer quality projected image. Because the mirrors on the DLP chip can be very close together, it does not have the “screen door” effect in LCD. The main disadvantage of this technology is the “rainbow effect.” Because of the nature of the spinning color wheel- some viewers see rainbow bands or halos around images projected using the DLP system. While increasing the speed of the color wheel spin has reduced the rainbow effect for many viewers, some with more sensitive eyes still complain and can even get headaches from viewing DLP images for extended periods.

    Liquid Crystal on Silicon (LCOS): A conceptual offshoot of traditional LCD technology, LCOS takes the liquid crystals off the glass plates and moves them to the surface of a silicon chip. The control circuits for the LCD are etched onto the chip and the chip surface is made reflective. While on the surface this is very similar to a traditional LCD chip, it opens a whole new world of options in LCD technology. LCOS chips can be smaller and higher resolution than traditional LCD chips, and are cheaper and easier to manufacture as well!

    LCOS display technology has been integrated into a few projector products, however for the time being the primary use has been in microdisplays (small LCD monitors that can be integrated into everything from wearable computers to greeting cards).

    The main advantage of LCOS would be the improved resolution, reduction of “screen door” and ability for even smaller size compared to traditional LCD. The main disadvantage of LCOS is that it is not very common so prices remain higher than they could be if manufactured on a large scale and selection of projectors is very limited.
  3. Allan Jayne

    Allan Jayne Cinematographer

    Nov 1, 1998
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    LCD tends to have a limited contrast ratio and blacks are not really black.

    LCOS is essentially LCD on a mirror. In a projector, light goes through the liquid crystals twice as it reflects off the mirror which greatly improves the darkness of blacks although not to as good as DLP. Due to the expense of the LCOS element, some LCOS projectors may, like DLP projectors, use a color wheel and just one LCOS element shared between the red, green, and blue colors and have the rainbow effect disadvantage.

    All three, DLP, LCD, and LCOS, can suffer from dead pixels.

    Plasma is a direct view technology. It, like CRT, uses phosphors. There are thin upright tubes about the thickness of the phosphor stripes on a direct view CRT, all the way across the screen to make the red, green, and blue pixels.

    Video hints:
  4. Cees Alons

    Cees Alons Moderator

    Jul 31, 1997
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    Real Name:
    Cees Alons
    Originally, DLP came as three aligned DMD's, each for one colour. A colour wheel is not needed then. Because the alignment is a pain staking operation, to be performed in the factory, they were much more expensive. They are coming back now, lately! Still a bit expensive, though.

    BTW, I'm one of those people who notices the rainbow effect on single-DMD projectors. They now use faster turning wheels, and with each colour more than just once on the wheel. Of course, the DMD has to switch much faster between colours too. I can tell you that the newest models no longer suffer from that particular rainbow/headache disadvantage!

  5. ChrisWiggles

    ChrisWiggles Producer

    Aug 19, 2002
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    3-chippers always seem to be a chip generation behind, and they suffer from lower CR than single-chip DLPs. Right now we're seeing the HD2 3-chippers, and some of these units are ungodly expensive.

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