All About Custom Rear Screen Systems

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All About Custom Rear Screen Systems The CinemaSource Home Theater Design Handbook. Copyright 2002 All rights reserved. Printed in the United States of America. No part of this book may be used or reproduced in any manner whatsoever without written permission, except in brief quotations embodied in critical reviews. CinemaSource is a registered federal trademark. For information contact: The CinemaSource Press, 18 Denbow Rd. Durham, NH 03824 All About Rear Screen Systems REAR SCREEN SYSTEMS ustom-built rear screen systems are the big brothers of the 45" to 80" rear screen televisions commonly seen at A/V retailers. In fact, the internal construction is almost identical, just scaled up in size. Look inside one and you will find three basic elements: A large rear projection surface, a high- brightness projection display and a mirror (or two) to fold the light beam. Although they are expensive to build, custom-built rear screen systems appeal to many people because the mechanics of the display are hidden from view. The result is just a huge picture that appears to be “hanging on the wall”. C The imaging sources behind most custom-built rear screen systems are three tube, front projection video monitors. These are used because the manufacturers of CRT projectors almost always build in circuitry that allows the picture scanning to be easily reversed for rear screen applications. In general, because the display applications are so similar, virtually all the features and requirements that apply to front projection applications also apply to rear screen applications. For example, if you want to display standard NTSC-rate 4:3 video images, you want a video-grade projector. If you have in mind line doubling, look for a data-grade projector, and if HDTV compatibility and 16:9 images are important, you want a graphics-grade projector. One feature, however, you do want, regardless of the grade of projector you chose is the capability of remote-controllable convergence. For those who are unfamiliar with convergence alignment, this is the procedure by which all three tubes in a display device are adjusted so the images they generate match each other across the screen surface. This procedure is often performed on older video projectors by manually adjusting a bank of analog waveform controls inside the projection unit itself. While this technique is still fine for front projectors, it is a real pain for rear projector applications because the units are often tucked into a small, closet-like 3 All About Rear Screen Systems 4 enclosures with minimal accessibility. Projectors that allow remote convergence allow one to view the picture directly, usually by standing right in front of the display, and allow one to complete the alignment procedure with greatly increased precision. projectors, can take up an awful lot of room. A far more common approach is to use one or two mirrors to "fold" the light path so the entire system can be fitted into a more narrow enclosure. As our diagrams on the following pages illustrate, the narrowest enclosures typically require two mirrors. Note: if you are beginning to get nervous about pulling out your old “A Television Picture That Looks geometry text books just to Like It’s Hanging On The Wall” design this contraption, don't sweat it. Vutec offers their RSS-1 and RSS-2 RetroTrac systems preconstructed for custom rear screen systems. The way it works is that you let them know the physical parameters of the system (the size of the screen, the size of the enclosure it will be built into, the type of projector used, etc.) and they will design one of their Retro-Trac systems specifically for your application. They will even provide you with the plotted CAD diagrams to assist you with your design process. Once the province of CRTbased projectors exclusively, custom-built rear screens are starting to be constructed with LCD, DLP and other electronic imagingbased projectors. The main reason is brightness, these technologies offer total lumen outputs far in excess of CRT-based projectors. In case you are considering using an LCD or DLP projector in a rear screen system, there is an additional item to factor into the design process. The projector pixel structure and the surface structure of optical gain screens (lenticular/fresnel types) can interact causing "moire" patterns. These are swirling patterns that often occur when fine detailed structures overlay each other. The solution for those who want to use solid state imaging projectors in custom-built rear screens is to make sure that the screen manufacturer certifies the screen surface for electronic imaging use. Call the engineering staff at VUTEC for further information on this topic. Using Mirrors To Fold The Light Path After the image is generated by the projector, the next step is to project it on the back of a rear screen surface. Depending on the amount of room you have to build a rear screen “enclosure”, there are several design options. Some configurations simply aim the projector directly at the back of the rear projection screen surface. This is simple and works well enough, but because of the long throw distance of many video Did you think we skipped over the mirror part? Here too are several options to consider. First, standard rear surface "bathroom-type" mirrors are rarely used. This is because on common rear surface mirrors, light must pass through the front layer of glass first and then it is reflected off the rear surface. The problem is that during this process some light energy is scattered and lost. Far preferable are front-surface mirrors. The first type uses glass as a substrate with a polished aluminum surface applied to the front of the glass. The advantage is less light loss; when light strikes the front surface of the mirror it bounces directly off. Another second mirror technology uses metalcoated polyester film. Offered under the MirrorTec brand name by Vutec, these mirrors are highly reflective (minimum reflectivity is 94%) and have the added bonus of being extremely light. These “glassless All About Rear Screen Systems Folding a projector light path with a single mirror Folding a projector light path with dual mirrors 5 All About Rear Screen Systems 6 A 16:9 CUSTOMBUILT REAR SCREEN SYSTEM Speakers mirrors” are rapidly becoming the preferred solution for rear projection system designers due to the ease of installation and overall cost advantage when compared to front surface glass types. If there is any downside to aluminized plastic mirrors, it is that they are slightly more delicate than glass-based types so when it is time for cleaning and dusting, one just has to be a bit more careful. The Rear Screen Surface The key to building a high performance, custom-built rear screen system is choosing the best final element in the display system; the rear screen surface. There are two general types available: diffusion screen surfaces and lenticular/fresnel screen surfaces. Let's look at standard diffusion surfaces first. The original diffusion surfaces were "etched glass". These were sheets of optically clear glass that are simply rough sandblasted on one side. The way they work is that the sandblasted side, in effect, gets engraved with thousands of small irregular lenses that function to disperse light rays uniformly in a wide 180 degree viewing cone. Today, glass-based diffusion screens are still common but the diffusing surface is not sandblasted, instead it is created by spraying a special chemical coating directly on the glass. Interestingly, the advantage of this technique is that by applying different amounts of the coating material, a diffusion screen can actually be given a slight amount of gain. Retro-Vu, Vutec’s diffusion rear screen material, for example, can be ordered in gains of 1.0 to 2.5. This allows the system designer a great deal of flexibility when designing the display. When planning the construction, if muscling a 10 foot piece of glass into a custom rear screen enclosure sounds scary, you may want to consider an acrylic diffusion screen. These plastic-based screens are much lighter and considered easier to install than their glass- based counterparts, however, like anything constructed of plastic, one has to be careful about scratches. People with children take note. Acrylic-based diffusion screens are also offered with different amounts of gain (also 1.0 to 2.5) via special optical coatings. Lenticular/fresnel screen surfaces operate in a more controlled way than diffuser surfaces. They produce video images using actual optical lens technology, albeit in a rather unfamiliar way. Instead of the familiar smaller convex and concave lenses, the ones you see on cameras and telescopes, screen manufacturers effectively fabricate entire sheets of plastic into giant lenses. Let’s start with the first surface the light passes through: the fresnel surface. Fresnel surfaces are scored with hundreds of miniature circular slopes that function to "collimate" incident light rays so they exit the surface at right angles to the surface of the screen. The result of this collimation process to the observer is that the picture is more uniform in brightness, without dimmer edges that can often be seen on diffusion-type screens. After passing through the fresnel surface, the light then passes onto the lenticular surface. This surface is not one big lens, like the fresnel surface, but is actually hundreds of vertical cylindrical ones, arranged so that light is spread around horizontally. This is done to ensure a wide viewing cone so audience members can view a bright picture regardless of seating position. Another feature you find on lenticular screens is a series of vertical ribs that are painted black. These "unused" parts of the lenticular surface are designed to absorb incident light in the room and thus improve the contrast range of the picture. Lenticular screens with black stripes typically are more expensive than the simpler lenticular surfaces, but offer greater performance. All About Rear Screen Systems 7 Draper’s RPS Rear Screen Assembly System Vutec’s RSS-1 and RSS-2 Retro-Trac Universal Rail Systems make rear screen system design easy All About Rear Screen Systems 8 The Physics Of Rear Screen Surfaces e can characterize reflected light in three general ways. As you may remember from high school physics class, light itself is a form of electromagnetic radiation that travels in perfectly straight lines from the emitter to the source (at least in home theater rooms, Astrophysicists please don’t write us). If we look at a highly reflective source like the surface of a still pond or a glass mirror, incident light rays bounce off at the same angle they arrived at. Physicists refer to this as “the law of specular reflection” and our diagram on the right illustrates how this works. In short, the angle of incidence, the angle the light strikes the surface, equals the angle of reflection. As you may surmise, mirrors make pretty poor projection screens because of the tendency for light rays to bounce directly off the surface without regard to where the audience is. A good projection screen requires a surface that diffuses the incident light around a large horizontal angle so that all audience members can see the image. The best example of this phenomena is a "Labertian diffuse surface" and is the basis for the common matte-white front projection screen. However, diffuse labertian-type screens aren't perfect for all applications. They often are considered lacking brightness because a lot of light is "wasted" by throwing it around the room off-axis. A far better screen would be one that throws a majority of the light energy back to the viewing audience, and this is exactly the principle behind gain screens. (This topic is covered in more detail in the first chapter) W commonly used but the diffusing surface is generally a special coating whose application can be carefully controlled to produce different amounts of gain. Another method to produce gain screens uses optical lens technology, albeit in a rather unfamiliar way. Instead of the familiar smaller convex and concave lens, the ones you see on cameras and telescopes, screen manufacturers effectively fabricate entire sheets of plastic into giant lenses. The favorite configuration presently used is a sandwich of a lenticular and a fresnel surface. Let's start with the surface closest to the projector; the fresnel surface. This lens surface is actually scored with hundreds of miniature slopes that function to bend incident light rays so they exit the surface at right angles to the surface of the screen. Our diagram on the next page illustrates this effect. To the observer, the result of this "collimation" process is that the picture is distinctly more uniform in brightness. Absent are the perceptually dimmer How Rear Screens Work Rear screen surfaces behave in a similar fashion to front screens, the only difference is that the technology used to create rear screen surfaces is transmissive, not reflective. First, let's look at a standard diffusion rear screen surface. In general, there are three primary types of materials that are used: etched glass, coated glass, and coated acrylic. The first type, etched glass, consisted of a sheet of glass that was sandblasted (etched) on the viewing side. This side of the screen, in effect, gets stamped with thousands of small irregular lenses that disperse light rays isotropically, that is, around a 180 degree angle. Today glass and acrylic rear screens are Three Ways To Characterize Reflected Light All About Rear Screen Systems edges that you see on diffusion-type screens. After passing through the fresnel surface, the light then passes onto the lenticular surface. This surface is not a big lens, like the fresnel surface, but is actually hundreds of vertical ones, arranged so that light is spread around the horizontal axis. This is done to ensure a wide viewing cone. Note that this wide viewing angle is typically designed in the horizontal axis not in the vertical. As you can see in the diagram below, the angular gain plot of a typical rear screen surface’s vertical characteristics is very sharp. The rational here is that a wide viewing cone in the vertical dimension would waste light. Most everyone sits within a few degrees of each other vertically, so engineers design rear screen surfaces to project most of the image out in that direction. Another feature you find on lenticular screens is a series of small vertical ribs that are painted black. These "unused" parts of the lenticular surface are painted black so as to absorb any incident light in the room. In rooms filled with a modest amount of ambient light, from windows or other light sources, these screens display a much larger contrast range than ordinary rear screen surfaces. Lenticular screens with black stripes typically are more expensive than the simpler lenticular surfaces, but offer far greater performance in rooms with ambient light. How Rear Screen Surfaces Reflect Light How Lenticular and Fresnel Rear Screen Surfaces Work: 9 10 All About Rear Screen Systems Distribution Of Light F rom A Diffusion Rear Screen Distribution Of Light F rom A Coated Diffusion Rear Screen Distribution Of Light F rom A Lenticular/F resnel Rear Screen All About Rear Screen Systems 11 12 All About Rear Screen Systems REAR SCREEN INSTALLATION METHODS RSS-2 Dual Mirror Rear Screen System In order to assist you with your installation, Vutec’s engineering department will provide at no extra charge a custom design layout based on the Retro-Trac rear screen projection system. The criteria that you need to supply includes: 1) The screen size and aspect ratio 2) The projector throw distance (generally included in the projector’s installation manual) 3) The space available behind the screen 4) The sill height (height to the bottom of the screen). Once this information is given to Vutec, they will design a custom layout based on that criteria. It will show the mirror (s) sizes, dimensions of the Retro-Trac system and the position of the rear screen. For further details on Retro-Trac installation, call Vutec and request a copy of the RSS-1 or RSS-2 installation manuals (shown below). One the following three pages we illustrate the details involved with the physical installation of the HD and LD-series projection screen mounting systems. Both of these are designed for easy installation. All About Rear Screen Systems HD STYLE - REAR PROJECTION SCREEN FRAME LD STYLE - REAR PROJECTION SCREEN FRAME 13 14 All About Rear Screen Systems HD STYLE - REAR SCREEN FRAME INSTALLATION DETAILS All About Rear Screen Systems LD STYLE - REAR SCREEN FRAME INSTALLATION DETAILS 15