Angles Of View

Now that most of the universal screen attributes for Home Theater have been established, it is time to turn things around for a while and look at Home Theater screens from an entirely different perspective. We refer, obviously, to rear projection screens whose virtues, while considerable, are not always as obvious as those belonging to their front projected counterparts. Here, then, are a few remarks concerning

Rear Projection in Home Theater - Front to Back

Although it might at first seem a misnomer, even a contradiction in terms, a Home Theater that is rear projected is not only a viable idea, it's a really good one. Yes, it must be acknowledged, all movie theaters we attend are front projected but, then again, all of them are showing film and we in our homes are not.

This article will contend that, if you can, making your Home Theater screen a rear projection device is superior, even much superior to the more familiar front screen choices.

For starters, a rear screen is aesthetically attractive because you can take the projector and all of its ancillary equipment and hide them away behind it. Space and sight lines are thereby liberated in your living room (or wherever you look at the screen) which, therefore, can remain looking more like a living room-a benefit, we are told, particularly popular with spouses.

In addition to that physical advantage, rear screens provide a large enough variety of optical benefits that it may sometimes seem hard to choose between them. The selection may be simplified, however, if we reduce it to a few, basic principles. The first of these is that all rear screens utilize refraction to control and manipulate the light passing through them while all front screens depend for the same purpose largely on reflection. Reflection does play a role in rear projected displays as well, but never a welcome one.

Refraction is essentially the process by which light rays may be caused to change their direction when they cease traveling through a medium of one density and enter a medium of another. Any such ray when projected at a screen through air (a low density medium) will swerve by some amount away from its initial direction when it enters the back surface of a plastic rear projection screen (a higher density medium).

For completeness sake, we need to add that when that same light ray exits out of the front surface of the screen (and thus switches back to traveling through low density air), it will swerve back toward the direction in which it was originally projected. The magnitude of each of those swerves made by the ray is variable according to the angle at which it consecutively encounters each surface. By causing the front and/or back surfaces of a rear screen to become other than flat and/or other than parallel, quite a lot of clever things can be done in its manufacturing processes which usefully manipulate this angular behavior.

Some historically prominent examples of this are lenticulated rear screens which have a series of fine vertical grooves molded into their front surface and which, according to the radius of those grooves, cause the projected light rays to fan out from the screen in a pattern that is very much wider than it is high. In the days of what we can now correctly term "brightness-challenged" projectors, lenticulated screens were highly prized because their asymmetric distribution pattern ensured that a higher percentage of a projector's total (if meager) lumen output into the eyes of an audience conventionally arrayed before it. Home Theater enthusiasts are surely aware that virtually every off the shelf projection television set has been designed to include a lenticulated screen.

Unfortunately, there is a problem with lenticulations which can become pronounced and worrisome with displays whose images are projected in very high resolution. Space does not permit a full description of the mathematics which underpin this relationship (a full discussion can be found in Volume III, Issue 2), but it can be said here that the regular configuration of the vertical grooves extending horizontally across the screen can cause significant interference patterns with the equally regular horizontal rows of pixels being projected through it. The higher the pixel density becomes, the more likely it is that distracting optical artifacts will appear.

Given the bright future of projectors in Home Theater, we @Da-Lite Screen Company see no reason why lenticulated screens particularly should share in it. Instead, the entire resolution issue (which certainly is an important one) can be obviated by choosing a diffusion screen whose front surface at least is always flat and without periodic structure. Since the resolution "limit" of such a screen is way, way beyond any conceivable pixel density, one may as well characterize it as being infinite.

Our recommendations continue, however, and now that we've counseled you to pick a diffused rear projection screen we're going to go on and offer it to you in two entirely different shades. One "shade" is a pure, milky white and its virtues have to do with color. The other "shade" is a quite chromatically complicated gray and its merits have to do with contrast.

First, Video Vision, the white one. This is a unity gain surface which has exceptionally high transmission and hence is extremely uniform throughout large viewing angles. The real purpose of Video Vision, though, is to be white, so white that it will reproduce any and all colors projected through it with exact chromatic fidelity. Because, technically, Video Vision causes no shift in any wavelength, the purist may be assured when he chooses it that the blush he will see on Scarlet O'Hara's cheek can be the very same pink which was visible in the original Selznick print.

Now the type with the gray shade, the DA-100 and the DA-130. As its number indicates, the second screen has a gain of 1.3 which, therefore, is 30% higher than the DA-100. They both, however, are the same gray and it is this grayness which makes rear projection screens so superior to front projection.

Everybody knows that the color of this box �  is the color of the blackest black you can ever exhibit on a white front projection screen. Since black, after all, is not a color created by the presence of light, but rather by its absence, it cannot, having no wavelength, ever be projected. Thus, if you want some area of your image to look like ■ and not like � , it will very helpful if the underlying "color" of the screen can be something like which is a quite reasonable approximation of what Da-Lite's Neutral Gray screens in fact look like. Not only will the resultant "black level" of your imagery be much better, but its actual contrast ratio will also be much enlarged.

To understand how this could be so, we need to recall that all screens are obliged to interact with two light sources: projected and ambient. In the case of front projection, light from both of those sources falls onto the screen in the same way and from the same direction I│. In rear projection the two sources originate on opposite sides of the screen and, thus, travel in different directions →│←.

A front projection can't do anything else than reflect both sources with equal efficiency. But since a rear projection is designed to **transmit** rather than reflect light it can be made to be a very poor reflector (which is why its coated side should **always** face its audience).

Now a word about ambient light. Some Home Theater people suppose that they can eliminate it. They can not. Even if the only light source in or behind the room is the projector, the light radiated out from the screen (front or rear) will inescapably end up illuminating a much, much bigger area than its audience's eyes. (That's how come midway through a movie in a purportedly darkened theater you can still easily see not only whether there's a person sitting next to you but whether he's wearing a blue sweater.)

Some amount of that image light will splash back up onto the screen and inevitably degrade the contrast ratio of the picture. In front projection, there's nothing the screen can do with **that** light except re-radiate it. In rear projection, however, the screen being, if you will, a two way street will re- transmit it backwards into the projection booth where it may safely be absorbed. Thus, only a very small percentage of the ambient light incident to a rear screen's gray surface actually gets reflected in such a way as to noticeably degrade contrast.

The precise color (wavelength) of the Da-Lite gray, incidentally, is quite carefully chosen so that it is "neutral" and has an exact set of coordinates in what is called color space. (Imagine a sort of oddly drawn triangle with pure Red, Green, and Blue at each of its corners and all the colors which are combinations of those three established by their spacial position within it.) The color shift, therefore, imparted by our gray is minimal.

The distinction, then, between front and rear projection may not be as sharp as Black and White. But Gray and White? We think so.