• Rear Projection and Image Contrast

    MyCockpit ® Presents "Mikes Tips" by Mike Powell<div>
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    A Better Option?
    Rear projection is often seen as a work around, something to use when no clear front-projection path exists. The reality is that a rear projection system can provide better image quality under adverse conditions because it’s much better at preserving contrast.

    A front projection system positions the viewpoint on the same side of the screen as the projector. The screen scatters projected light back toward the projector and viewer’s eyes. If no light other than that directly from the projector hits the screen, the image contrast matches the projector contrast ratio. Of course, there is always more light hitting the screen. Light leaks in around doors and windows. Simulator equipment has lighted gauges and indicators. Light from the screen will be reflected by the room furnishings back onto the screen. The results can be devastating.

    Image contrast is often measured using a four by four check-board array of black and white areas. Contrast is the ratio of the brightness of the light areas to that of the dark areas. Since we don’t have a physical screen or projector for these measurements, we’ll have to calculate the brightness. Brightness in foot-Lamberts of an area equals the screen gain multiplied by the number of lumens hitting that area divided by the square footage of the area.

    Imagine a six foot by eight foot unity-gain screen illuminated by a 1000 lumen projector having a 1000:1 static contrast ratio. If the checker-board test pattern is sized to fill the screen, 24 square feet will be illuminated by 500 lumens while the remaining 24 square feet will receive 1/1000th of that, 0.5 lumen. Of course, this is just the light from the projector. Let’s also throw in a bit of room light. Let’s say there’s dim lighting equivalent to half a lumen per square foot. Look what happens.

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    Extraneous room light yanks the contrast of a front-projected image down to 41:1.

    Positioning the projector on the far side of the screen relative to the viewer for a rear-projection system means that the projected light must pass through the screen to reach the viewer’s eyes. Ideally all of the light passes through. In reality, most will pass through a commercial quality rear projection screen, a fraction, perhaps 10 to 20 percent will be reflected back toward the projector, and a few percent will be lost entirely.

    Now imagine a six foot by eight foot unity-gain rear-projection screen with the same 1000 lumen projector and same contrast test pattern. We’ll have the same 500 lumens lighting up the bright areas and same 0.5 lumen hitting the dark ones, but the half lumen per square foot room light has a far different effect.

    Most of the room light passes through the screen. Only a small percentage, let’s say 20%, reflects back toward the viewer. If the screen has a matte surface, the 20% reflectance is like saying the front screen surface has a gain of 0.2. So the amount of extraneous room light reflected back to the viewer is reduced by a factor of five compared to a front-projected screen and image contrast is much better.

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    While extraneous room light also affects the contrast of rear-projected images, most of that light is not reflected to the viewer, so we’re left with a respectable 173:1 contrast ratio.

    There’re a few caveats of course. If you expect to get contrast results like this you must use a rear projection screen with similar characteristics. A white bed sheet and a translucent shower curtain will both form an image when used as a rear-projection screen, but neither is likely to perform as well as a screen designed for rear projection. You’ll also have to control extraneous light on the projector side of the screen, perhaps by enclosing the projection light path.

    Mike Powell, author of

    Building Recreational Flight Simulators and

    Building Simulated Aircraft Instrumentation.