Angles Of View

Despite the extraordinary profusion of very high quality equipment and hardware being continually produced for the improvement of audio/visual systems, a significant portion of a display's visual quality can often be greatly affected simply by the manipulation of the angular relationship between its parts. This article discusses two of those relationships in ways which we hope may encourage

Different Perspectives - From Other Angles

The most effective way to improve any single- image display which is being rear projected is to include in it a Fresnel lens. Why this is so barely requires reiteration. Only a Fresnel can minimize the angles through which light rays diverging from the projector must be bent in order to reach the eyes of the audience.



Figure 1

The two sketches above easily illustrate this advantage. In the left-hand drawing, light rays flowing from the projector through the right edge of the screen need to be bent through an angle of 50º if the seated figure is to see what they contain. In the right-hand drawing, that bend angle has been reduced to only 30º. This 40% improvement is accomplished by the Fresnel on the back of the right-hand screen which has collimated the incident light rays such that they all become parallel to the on-axis ray.

Although the only numbers which appear in Figure 1 are the two bend angles themselves, there are actually other numeric assumptions implicit in their deduction. One of these is that the screen has an aspect ratio of 3:4. The other is that the projector has a required throw of 1.2 times the screen's diagonal. These are absolutely standard numbers and apply exactly to an enormous number of existing screen installations throughout our markets.

Recently, however, there has appeared an inventive and interesting exception to these proportions and it deserves some attention. Because of the advent of numerous projection technologies which no longer require three, separate exit pupils (the R, G, and B guns of the classical CRT projector), it is now often possible to get a special optional lens for your new projector which permits a throw distance significantly shorter than the standard version's.

In rear projection setups particularly this development is often being viewed as welcome. Since everybody knows that "real estate" is the biggest single drawback to rear projection installs, it has to be acknowledged that the "wide angle" projection lens option is certainly attractive as a way to reduce minimum booth depth. And, no doubt about it, the availability of these short focal length lenses has helped projector manufacturers make sales which otherwise might have been lost to them.

From the point of view of a screen manufacturer, however, these new short lenses can cause as many problems as they solve. The issue, once again, is total bend angle. The graph in Figure 2 shows how the maximum incident angle onto a 3:4 aspect ratio screen varies inversely with the throw distance. Note that the slope of the resulting curve is not linear and instead becomes steeper as the throw distance is decreased. Thus, if at a throw of 1.2 times the diagonal the largest incident angle is something like 23º, by the time the throw has been reduced to .7, it has increased to more than 35º. For workstation screens with aspect ratios of 4:5, this angular progression is worse and for 9:16 aspect ratios, it's much worse.

What this means is that the use of wide angle lenses can't help but degrade the perceived uniformity of any image. Now, it is extremely important to state here that the observation just made has nothing whatsoever to do with either the quality or the design of the wide angle lens itself. We at Da-Lite have no doubt that optically these new devices are first rate technical achievements and, resultantly, are only to be admired.

The graph, you see, says absolutely nothing about the optics of the projection lens. It simply presents the geometric consequences of reduced throw distances which have the result of greatly increasing either angle labeled in Figure 1.



Figure 2

Thus, we at Da-Lite wonder whether the bend angle price is worth paying, especially if an acceptable alternative would be to use a mirror system to fold the light path and, thereby, enable the retention of a longer focal length lens. Yes, certainly the introduction of one or two mirrors into the light path will reduce by some measurable amount the overall brightness of the displayed image but that, in these days of ever more powerful projectors, may well be the less expensive of the two alternative optical tariffs.

Any screen, even one with a Fresnel on its entrance surface, cannot process very large bend angles with the same efficiency that it can manage moderate ones. Yet when a projection lens is designed such that it will completely fill a screen from a throw distance that may be as little as 2/3 of its diagonal, it is certain that the projected image will be seriously non-uniform. To be sure, a Fresnel backed screen like Da-Lite's Ultra will make this problem less bad, but even that device cannot render such wide angled geometry harmless.

There is a second matter having to do with Fresnel backed screens which also merits attention. This one has to do with the alignment of projectors and Fresnels on a display's vertical axis.

It is commonly supposed that the only proper position for a projector expected to fire through a Fresnel is exactly on-axis to the screen's center. The logic to this conclusion seems convincing and should be looked at. It generally goes something like this:

If the purpose of Fresnel is to collimate incident light rays such that all bend angles are reduced to a minimum, then, obviously, it is as important to have the projector centered vertically on the Fresnel as it is to have it centered horizontally.

On the face of it, that's perfectly true. The reason that this column disputes that conclusion stems from our long-standing conviction that the goal of any well designed display system is to deliver as many light rays from the projector as possible into the eyes of the audience.

Now, everybody knows that you don't want the bottom of a screen closer to the floor than, say, three to four feet. That being so, we also know that the center of our screen will (depending on its height), therefore, be five, six, or even seven feet above that floor. If we now recall that the average height of a seated audience's eye point is only 44 inches off that floor, then it's easy to see that the majority of the light emanating from a projector vertically centered on a screen will pass clean over the heads of everybody expected to look at it.

Alternatively, if we position the projector in such a way that its centermost light ray passes through the center of the Fresnel at whatever downward angle is permitted by the projector's optics and which as nearly as possible bisects the audience's vertical field, a significantly greater amount of the image light will be aimed directly at the audience.

Now, to be technical for a moment, let's look at whether pulling the projector off-axis to the Fresnel costs us anything optical which we might not wish to pay. We will find that it does not.

Since a Fresnel also is a lens, it too has a focal length which may be thought of as the distance back from its center that a projector needs to be placed in order to have all of its light rays collimated. If we put the projector at some distance which does not equal the Fresnel's focal length, one of two things will happen. If the source is outside its focal length, the Fresnel will converge the incident light rays to some extent. If inside the focal length, it will cause them to diverge to some extent.

Knowing this, we can now describe what happens when we position a projector off center vertically to a Fresnel. Because the incident angles of light rays passing through the Fresnel below its equator will now be steeper, they will converge. Because the incident angles of light rays reaching the Fresnel above its equator will now be smaller, they will diverge. Note, however, that both the divergence and the convergence of the rays is measured relative to the new, downwards (and, thus, no longer perpendicular) direction of the centermost ray. The overall effect, then, is precisely what we should desire and our audience is certain to be more enlightened.