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All About Custom RearScreen 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 briefquotations embodied in critical reviews.

CinemaSource is a registered federal trademark.

For information contact: The CinemaSource Press, 18 Denbow Rd. Durham, NH 03824

3All About Rear Screen Systems

Custom-built rear screen systems are the bigbrothers of the 45" to 80" rear screen televisionscommonly 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: Alarge rear projection surface, a high- brightness projectiondisplay and a mirror (or two) to fold the light beam.Although they are expensive to build, custom-built rearscreen systems appeal to many people because themechanics of the display are hidden from view. The resultis just a huge picture that appears to be “hanging on thewall”.

The imaging sources behind most custom-built rearscreen systems are three tube, front projection videomonitors. These are used because the manufacturers ofCRT projectors almost always build in circuitry that allowsthe picture scanning to be easily reversed for rear screenapplications.

In general, because the display applications are so similar,virtually all the features and requirements that apply tofront projection applications also apply to rear screenapplications. For example, if you want to display standardNTSC-rate 4:3 video images, you want a video-gradeprojector. If you have in mind line doubling, look for adata-grade projector, and if HDTV compatibility and 16:9images are important, you want a graphics-gradeprojector. One feature, however, you do want, regardlessof the grade of projector you chose is the capability ofremote-controllable convergence. For those who areunfamiliar with convergence alignment, this is theprocedure by which all three tubes in a display device areadjusted so the images they generate match each otheracross the screen surface. This procedure is oftenperformed on older video projectors by manually adjustinga bank of analog waveform controls inside the projectionunit itself. While this technique is still fine for frontprojectors, it is a real pain for rear projector applicationsbecause the units are often tucked into a small, closet-like

REAR SCREEN SYSTEMS

All About Rear Screen Systems4

enclosures with minimal accessibility. Projectors that allowremote convergence allow one to view the picture directly,usually by standing right in front of the display, and allowone to complete the alignment procedure with greatlyincreased precision.

Once the province of CRT-based projectors exclusively,custom-built rear screensare starting to beconstructed with LCD, DLPand other electronic imaging-based projectors. The mainreason is brightness, thesetechnologies offer totallumen outputs far in excessof CRT-based projectors. Incase you are consideringusing an LCD or DLPprojector in a rear screensystem, there is anadditional item to factor intothe design process. Theprojector pixel structure andthe surface structure ofoptical gain screens(lenticular/fresnel types) caninteract causing "moire"patterns. These are swirlingpatterns that often occurwhen fine detailed structures overlay each other. Thesolution for those who want to use solid state imagingprojectors in custom-built rear screens is to make surethat the screen manufacturer certifies the screen surfacefor electronic imaging use.Call the engineering staffat VUTEC for furtherinformation on this topic.

Using Mirrors To FoldThe Light Path

After the image isgenerated by the projector,the next step is to project iton the back of a rearscreen surface. Dependingon the amount of room youhave to build a rear screen“enclosure”, there areseveral design options.Some configurationssimply aim the projectordirectly at the back of therear projection screensurface. This is simple and works well enough, butbecause of the long throw distance of many video

projectors, can take up an awful lot of room. A far morecommon approach is to use one or two mirrors to "fold"the light path so the entire system can be fitted into amore narrow enclosure. As our diagrams on the followingpages illustrate, the narrowest enclosures typically require

two mirrors. Note: if you arebeginning to get nervousabout pulling out your oldgeometry text books just todesign this contraption, don'tsweat it. Vutec offers theirRSS-1 and RSS-2 Retro-Trac systems preconstructedfor custom rear screensystems. The way it works isthat you let them know thephysical parameters of thesystem (the size of thescreen, the size of theenclosure it will be built into,the type of projector used,etc.) and they will design oneof their Retro-Trac systemsspecifically for yourapplication. They will evenprovide you with the plottedCAD diagrams to assist youwith your design process.

Did you think we skippedover the mirror part? Here too are several options toconsider. First, standard rear surface "bathroom-type"mirrors are rarely used. This is because on common rearsurface mirrors, light must pass through the front layer of

glass first and then it isreflected off the rearsurface. The problem is thatduring this process somelight energy is scatteredand lost. Far preferable arefront-surface mirrors. Thefirst type uses glass as asubstrate with a polishedaluminum surface appliedto the front of the glass.The advantage is less lightloss; when light strikes thefront surface of the mirror itbounces directly off.Another second mirrortechnology uses metal-coated polyester film.Offered under the Mirror-Tec brand name by Vutec,these mirrors are highly

reflective (minimum reflectivity is 94%) and have theadded bonus of being extremely light. These “glassless

“A Television Picture That LooksLike It’s Hanging On The Wall”


Folding a projector light path withdual mirrors

Folding a projector light path with asingle mirror


A 16:9 CUSTOM-

BUILT REARSCREENSYSTEM

Speakers

mirrors” are rapidly becoming the preferred solution forrear projection system designers due to the ease ofinstallation and overall cost advantage when compared tofront surface glass types. If there is any downside toaluminized plastic mirrors, it is that they are slightly moredelicate than glass-based types so when it is time forcleaning and dusting, one just has to be a bit more careful.

The Rear Screen Surface

The key to building a high performance, custom-built rearscreen system is choosing the best final element in thedisplay system; the rear screen surface. There are twogeneral types available: diffusion screen surfaces andlenticular/fresnel screen surfaces. Let's look at standarddiffusion surfaces first. The original diffusion surfaces were"etched glass". These were sheets of optically clear glassthat are simply rough sandblasted on one side. The waythey work is that the sandblasted side, in effect, getsengraved with thousands of small irregular lenses thatfunction to disperse light rays uniformly in a wide 180degree viewing cone. Today, glass-based diffusion screensare still common but the diffusing surface is notsandblasted, instead it is created by spraying a specialchemical coating directly on the glass. Interestingly, theadvantage of this technique is that by applying differentamounts of the coating material, a diffusion screen canactually be given a slight amount of gain. Retro-Vu,Vutec’s diffusion rear screen material, for example, can beordered in gains of 1.0 to 2.5. This allows the systemdesigner a great deal of flexibility when designing thedisplay. When planning the construction, if muscling a 10foot piece of glass into a custom rear screen enclosuresounds scary, you may want to consider an acrylicdiffusion screen. These plastic-based screens are muchlighter and considered easier to install than their glass-

based counterparts, however, like anything constructed ofplastic, one has to be careful about scratches. People withchildren take note. Acrylic-based diffusion screens are alsooffered with different amounts of gain (also 1.0 to 2.5) viaspecial optical coatings.

Lenticular/fresnel screen surfaces operate in a morecontrolled way than diffuser surfaces. They produce videoimages using actual optical lens technology, albeit in arather unfamiliar way. Instead of the familiar smallerconvex and concave lenses, the ones you see on camerasand telescopes, screen manufacturers effectively fabricateentire sheets of plastic into giant lenses. Let’s start withthe first surface the light passes through: the fresnelsurface. Fresnel surfaces are scored with hundreds ofminiature circular slopes that function to "collimate"incident light rays so they exit the surface at right anglesto the surface of the screen. The result of this collimationprocess to the observer is that the picture is more uniformin brightness, without dimmer edges that can often beseen on diffusion-type screens. After passing through thefresnel surface, the light then passes onto the lenticularsurface. This surface is not one big lens, like the fresnelsurface, but is actually hundreds of vertical cylindricalones, arranged so that light is spread around horizontally.This is done to ensure a wide viewing cone so audiencemembers can view a bright picture regardless of seatingposition. Another feature you find on lenticular screens is aseries of vertical ribs that are painted black. These"unused" parts of the lenticular surface are designed toabsorb incident light in the room and thus improve thecontrast range of the picture. Lenticular screens with blackstripes typically are more expensive than the simplerlenticular surfaces, but offer greater performance.


Draper’s RPSRear ScreenAssemblySystem

Vutec’s RSS-1 andRSS-2 Retro-Trac

Universal RailSystems make rear

screen systemdesign easy


We can characterize reflected light in three generalways. As you may remember from high schoolphysics class, light itself is a form of electro-

magnetic radiation that travels in perfectly straight linesfrom the emitter to the source (at least in home theaterrooms, Astrophysicists please don’t write us). If we look ata highly reflective source like the surface of a still pond ora glass mirror, incident light rays bounce off at the sameangle they arrived at. Physicists refer to this as “the lawof specular reflection” and our diagram on the rightillustrates how this works. In short, the angle ofincidence, the angle the light strikes the surface, equalsthe angle of reflection. As you may surmise, mirrors makepretty poor projection screens because of the tendencyfor light rays to bounce directly off the surface withoutregard to where the audience is. A good projectionscreen requires a surface that diffuses the incident lightaround a large horizontal angle so that all audiencemembers can see the image. The best example of thisphenomena is a "Labertian diffuse surface" and is thebasis for the common matte-white front projection screen.However, diffuse labertian-type screens aren't perfect forall applications. They often are considered lackingbrightness because a lot of light is "wasted" by throwing itaround the room off-axis. A far better screen would beone that throws a majority of the light energy back to theviewing audience, and this is exactly the principle behindgain screens. (This topic is covered in more detail in thefirst chapter)

How Rear Screens Work

Rear screen surfaces behave in a similar fashion to frontscreens, the only difference is that the technology usedto create rear screen surfaces is transmissive, notreflective. First, let's look at a standard diffusion rearscreen surface. In general, there are three primary typesof materials that are used: etched glass, coated glass,and coated acrylic. The first type, etched glass, consistedof a sheet of glass that was sandblasted (etched) on theviewing side. This side of the screen, in effect, getsstamped with thousands of small irregular lenses thatdisperse light rays isotropically, that is, around a 180degree angle. Today glass and acrylic rear screens are

commonly used but the diffusing surface is generally aspecial coating whose application can be carefullycontrolled to produce different amounts of gain.

Another method to produce gain screens uses opticallens technology, albeit in a rather unfamiliar way.Instead of the familiar smaller convex and concavelens, the ones you see on cameras and telescopes,screen manufacturers effectively fabricate entire sheetsof plastic into giant lenses. The favorite configurationpresently used is a sandwich of a lenticular and afresnel surface.

Let's start with the surface closest to the projector; thefresnel surface. This lens surface is actually scored with

hundreds of miniature slopes that function to bend incidentlight rays so they exit the surface at right angles to thesurface of the screen. Our diagram on the next pageillustrates this effect. To the observer, the result of this"collimation" process is that the picture is distinctly moreuniform in brightness. Absent are the perceptually dimmer

The Physics OfRear Screen

Surfaces

Three Ways To CharacterizeReflected Light


How Lenticular andFresnel Rear Screen

Surfaces Work:

edges that you see on diffusion-type screens. Afterpassing through the fresnel surface, the light thenpasses onto the lenticular surface. This surface is not abig lens, like the fresnel surface, but is actuallyhundreds of vertical ones, arranged so that light isspread around the horizontal axis. This is done toensure a wide viewing cone. Note that this wide viewingangle is typically designed in the horizontal axis not inthe vertical. As you can see in the diagram below, theangular gain plot of a typical rear screen surface’svertical characteristics is very sharp. The rational here isthat a wide viewing cone in the vertical dimension wouldwaste light. Most everyone sits within a few degrees ofeach other vertically, so engineers design rear screensurfaces to project most of the image out in thatdirection.

Another feature you find on lenticular screens is a seriesof small vertical ribs that are painted black. These"unused" parts of the lenticular surface are paintedblack so as to absorb any incident light in the room. Inrooms filled with a modest amount of ambient light, fromwindows or other light sources, these screens display amuch larger contrast range than ordinary rear screensurfaces. Lenticular screens with black stripes typicallyare more expensive than the simpler lenticular surfaces,but offer far greater performance in rooms with ambientlight.

How Rear Screen Surfaces Reflect Light


Distribution Of Light F rom ADiffusion Rear Screen

Distribution Of Light F romA Lenticular/Fresnel Rear

Screen

Distribution Of Light F rom ACoated Diffusion Rear Screen


REAR SCREENINSTALLATION

METHODS

In order to assist you with your installation, Vutec’s engineeringdepartment will provide at no extra charge a custom designlayout based on the Retro-Trac rear screen projection system.The criteria that you need to supply includes:

1) The screen size and aspect ratio2) The projector throw distance (generally included in theprojector’s installation manual)3) The space available behind the screen4) 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-Tracinstallation, 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-seriesprojection screen mounting systems. Both of these are designed for easy installation.

RSS-2 DualMirror Rear

ScreenSystem


HD STYLE - REAR PROJECTION SCREEN FRAME

LD STYLE - REAR PROJECTION SCREEN FRAME


HD STYLE - REAR SCREEN FRAME INSTALLATION DETAILS


LD STYLE - REAR SCREEN FRAME INSTALLATION DETAILS

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