• Uniformity: Even Brightness in Projection Systems

    MyCockpit ģ Presents "Mikes Tips" by Mike Powell<div>
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    Uniformity describes how evenly a projection system can distribute light across an image. Ideally, each point on the screen will be capable of producing the same peak brightness. In reality, the center of the screen is almost always brighter than the corners, sometimes annoyingly so.
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    The human eye is quite tolerant of smoothly changing brightness levels across an image. Brightness has to drop by almost half before it becomes an issue for most of us. This may be why uniformity isnít often considered when planning a new projection system, only bemoaned when the paid for and installed system has dingy corners and edges. Not too long ago getting any flight sim scenery up on a screen was a vast improvement in immersion, but now we have better scenery, better equipment and higher expectations. A little forethought can brighten up those edges and corners.

    Uniformity is expressed as a percentage. A system with an 80% uniformity rating, for example, will have some area of the screen that will only have 80% the brightness as the most brilliant area.

    Uniformity starts with the projector. Contemporary DLP and LCD projector achieve roughly 90% uniformity. CRT projectors, on the other hand, come in somewhere around 70%. Iím not aware of any DIY projector plans that offer even that.

    The projection screen can affect uniformity too. When we look at a screen each area is viewed at a different angle. The left screen edge is several degrees to the left, the right edge is several degrees to the right, and so on. As long as we have even illumination an ideal unity-gain screen will disperse light such that all areas appear equally bright regardless of the different viewing angles, so this type of screen has minimal effect on uniformity. But thatís not the case with other gains.

    As screen gain increases uniformity drops because gain is the result of directing more of the projected light in an increasingly narrower direction. This chart shows the gain curves of three commercially produced screens.
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    Imagine that we have a screen directly in front of us that fills a 60 degree horizontal field of view. If the screen has unity gain and is evenly illuminated, the image has perfect uniformity because each area of the screen receives the same amount of light and disperses it evenly regardless of viewing angle.

    Now letís consider a screen with a gain of 1.3. The edge of the screen will be viewed at a 30 degree angle, so the effective gain (or reflectivity) has dropped to about 1.1. This means the screen edge is reflecting only 85% as much light to the viewpoint as is the center.

    We still have to take projector uniformity into account, because the reality is that the screen is NOT evenly illuminated. Assuming weíve got a projector capable of 90% uniformity, the screen edge will receive about 90% the illumination that the center does. (Actually, it may be a bit more, but letís stick with easy numbers.) So the light reflected toward the viewpoint is (0.9)*(85%) or 77% as bright as that being reflected from the screen center.

    This is a decrease in brightness of 23% which is probably not too noticeable, but look what happens if we decide to use a screen with a gain of 2.0.

    The effective gain at the screen edge is about .85 which is 43% of the 2.0 gain at the center. Once we factor in the 90% projector uniformity we have (0.9)*(43%) or 39% as much light coming from the screen edge as we do from the center. This is a 61% decrease in brightness which would be both noticeable and distracting.

    These are the sorts of results expected from a projection system having a long enough projection throw so that the projected light is nearly perpendicular to the screen. In home simulator set ups, we are often constrained for space, and will use shorter throw distances. With shorter and shorter throw distances, the angle of the light hitting the screen edge is increasingly far from perpendicular. This changes things.

    An ideal unity-gain screen does a good job of widely dispersing light. The angle of the light illuminating it isnít irrelevant, but itís generally not an issue. Thatís not the case for screens with higher gains. As the gain increases, a screen acts more like a mirror. The light that is reflected off the edge of the screen not only is less bright, most of it is going away from the viewpoint.

    A shorter throw distance makes this worse. The closer the projector is placed to the screen, the more skewed the projection angle becomes for light reaching the screen edge, and the light reflected from the edge will have an even greater angle AWAY from the viewpoint. The net result is an even more dimly lit screen edge.
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    Falling prices and better performance make projection systems increasingly appealing. Itís easy to put together a system that works, but may not perform as well as it might. Here are a few thoughts about getting the best performance.

    Consider uniformity when you design your projection system. (You ARE designing your system, right? Not just throwing pieces togetherÖ?) Aim for a delivered image uniformity of 70~80%. Most people will not notice the variation in brightness as long as itís not an abrupt jump. When uniformity drops to 50% most people WILL notice.

    Be aware of projector characteristics. Most contemporary LCD and DLP projectors provide about 90% uniformity, but check the spec sheet to be sure. CRT projectors and the DIY projectors offer substantially less. This doesnít mean you shouldnít use them, just that you should be prepared to accept or compensate for their shortcomings.

    Lean toward unity gain screens. If youíve got a brightness problem and think you need a high gain screen, try resolving it first by controlling extraneous room light. If you must add more gain, add sparingly.

    If youíre using screen paint, donít assume unity-gain paint means ideal (very wide) light dispersion. Find spec sheets on the product, talk to a technical person with the manufacturer, or run some small scale tests before committing to a large purchase.

    Ditto with regard to contrast enhancing (gray) screens, DIY or purchased: donít assume. Find the specs. You need to know how light is reflected from the screen to predict system uniformity.

    A curved screen can cure a lot of problems. With the screen wrapping around the viewpoint, more of the light is directed to the viewer, and uniformity improves.

    Mike Powell, author of

    Building Recreational Flight Simulators and

    Building Simulated Aircraft Instrumentation.