Angles Of View
Deciding on a screen for your Home Theater involves a variety of choices not all of which may be obvious. This article will try to present some of the most important criteria for screen selection so that you can enjoy the very best
Home Theater - Behind the ScenesWhen it comes to looking at front projection screens, the biggest single difference between their use in Home Theater and industrial environments is whether or not you leave the lights on while you do it. In the Pro A/V world of presentation systems, significant levels of ambient light are de rigeur and, thus, inevitable. And, for reasons explained in detail earlier in this series, well they should be. In the residential environment of Home Theater, however, exactly the opposite is the caseor at least it sure should be.
Justification for viewing your Home Theater screen in ambient light that is as low as possible is not obscure. As viewers, we are psychologically very much more likely to become immersed in (or by) the program material we are watching if, indeed, it is the only thing our eyes easily can see. Dimming the light levels of all the objects occupying our field-of-view other than the screen greatly brightens our physiological perception of it.
Before moving to consider surface and image qualities of a Home Theater projection screen, let's leave the lights on for a minute and decide how big to make it. As of this writing, this is a tricky question, obliged as it is to include in its answer a bevy of aspect ratios which are inconveniently dissimilar.
If you believe, as we at Da-Lite certainly do, that the 9:16 format is sure eventually to prevail, then you will want to procure a screen of that overall aspect ratio. Here's the way most efficiently to size it. Measure the distance from the wall in front of which you plan to hang it to the chair or couch from which you plan to watch it. Divide that distance by 3.3 and you will know how high your screen ideally should be. (Its width, of course, is 1.777 times that deduced height.)
The reasoning behind that geometry is complex but reliable. (A detailed discussion can be found at http://www.da-lite.com/educational_materials/angles.php?action=details&issueid=11 ) Suffice to say here that if you sit back 3.3 screen heights from an image with HDTV resolution (1920 x 1080) you will have a viewing experience that has been very carefully designed to yield high degrees of immersion and illusion from your viewing experience.
If, on the other hand, you size your screen so that it may be filled by 3:4 material, your optimal screen height (for the same viewing distance) historically needed to become a good deal smaller. That is less true today because of the tremendous advances in video processing (scalers and the like). Nevertheless, because of all the varying aspect ratios which our various sources and software produce for us to look at, multiple aspect ratio screens like Da-Lite's Horizon are a particularly nifty addition to your Home Theater.
Ignoring resolution (which none of the screen surfaces discussed in this article affects at all), we can now look at the two attributes to which a screen's surface may meaningfully contribute: contrast and uniformity. We already know that the magnitude of external contrast (between screen and ambient light) should be as high as is practicable and so, in turn, should everything else possible be done to maximize the contrast internal to the screen. This, however, involves some projection issues which, while hardly more important, are nevertheless more subtle than just turning off the lights.
When we speak of contrast within an image and projected on a screen, what we're actually describing is the relative intensities of its lightest and darkest elements. The greater the disparity between those bright parts and those dim parts, the greater will be our satisfaction in looking at both of them. Since, these days, projectors with enough energy to make those bright parts really bright are commonplace, what we really need to worry about is keeping those dim parts dark.
To see why managing this "Black Level" is such a challenge, consider the following "image."
If we pretend that what we're looking at here is a screen onto which has been projected this random assortment of black and white "blocks," there are some useful things we can learn. The first is, that (unless you're reading this on the Internet) you'll never, never find a way to get blacks up on your screen as black as these printed blacks are black. Although this has already been said a thousand
times, the fact is that the only thing your projector can project is light and, since black is the absence of light, you can never, never project it.
That being said, if we take the whole rectangle above to be some arbitrary area of our overall projection screen, what is the closest we can ever come to reproducing the color of Block 1 on our
screen? You're absolutely right, the answer is Block 2 (or 3, or 5) and, therefore, the only way to fool our eyes into believing that Block 1 is "black" is to make Blocks 2 and 3 so much brighter that the ratio between them becomes convincingly large.
Now let's look at this same subsection of a screen with an eye to uniformity. What is meant by Uniformity in this context is that, if the software intends, for example, for Blocks 2 and 7 to be equally bright, that the screen does nothing to encourage our eyes to decide otherwise. A more careful way of stating that might be to say that the same percentage of the light falling on them reaches our eyes from Block 2 as from Block 7 even though the latter may proportionately be much closer to the center of the image. This concept of uniformity, leads us, then, by a slightly alternative route to the question of screen gain.
In the beginning, we screen manufacturers developed screen surfaces which, one way or another, were significantly non-uniform in the way
that they reflected projected light back toward the audience looking at them. Thus, if Block 2 in our illustration were to be the upper right hand corner of such a screen and Block 7 nearly its center, a light meter positioned perpendicular to that center would read a much larger value than it would when pointed at a corner. The more we make a screen surface reflective and, thereby, raise its gain, the greater the disparity between its sections will become.
So, if you care about uniformity in you image, the one thing you don't want your screen to have is a high gain. If you select a screen with no gain, which is to say, a matte white surface whose gain is "unity", pointing a light meter at any section of it will give you exactly the same reading as may be taken from any other.
But what about contrast? In addition to perfect uniformity, does a non-gain screen also display the maximum potential contrast ratio? The answer, regrettably, is no. Here's why. If we point our light
meter at Block 3 of a screen that is Matte White beneath and get, say, a reading of 100 and then point it at Block 5 and get a reading of 4, we know that our setup has a contrast ratio of 25:1. Now, let's
switch out that screen for a new surface like Cinema Vision which has gain of 1.3 (and is, resultantly, 30% more reflective).
Repeating the same measurements, we will discover that a maximum number will vary slightly, up or down, depending on which white block we point the meter at. The black blocks, however, are the important ones because the meter will read from them much lower numbers, perhaps down in the 3 range (fully a 25% reduction from 4, you see). This lowering of the system's Black Level will conspicuously improve the contrast of our display.
It's easy to understand why this is so once we recall that in addition to the "absence" of light entering a meter when we point one at Block 5, there is also light from Blocks 2, 3, 6, and 7 leaking into it. Since the screen is Matte White, the exact same amount of light is disbursed with perfect uniformity from each and every one of those white blocks throughout the entire viewing area. This includes
wherever we choose to position the light meter.
But, if all the "mini-screens" comprising the white blocks have positive gain, then, because they are all more directional, less of the light re-radiated from them will be spread into the lens of our light meter. And so, as the screen gain goes up, the measurable contrast ratio just gets better and better.
But, following this prescription for increasing contrast almost immediately causes side effects which so badly debilitate image uniformity that corners grow weak and the center inflamed.
A little gain, then, may be good for you; a lot, however, is uniformly toxic.