Angles Of View

A cause for certain satisfaction in our industry is the increasing appreciation among our customers of the advantages and benefits of rear projection. This bright outlook has its dark side, however, because of the misconceptions and uncertainties regarding the techniques and equipment available for compressing rear projection systems into areas of the smallest possible depth. Perhaps no other issue risks so much misunderstanding and ill will between manufacturer and dealer. Let us, then, once more go into the projection booth and look into

Mirror, Mirror - Behind the Wall

Times are good; your business is booming. Your favorite projector manufacturer has just shipped you his latest and greatest machine and, sure enough, it actually is lighter and brighter than the other guy's. Better yet, you've got a client not only anxious to install it, but to install it in a Commercial Mirror System behind a rear projection screen both of which you'll also get to sell her. Does it get better than that?

Wind the clock forward a couple of weeks. The gear has all shown up right on time and your installer's over at the job site, finishing the set up. Then your phone rings and you're told the image through the screen doesn't look right. Maybe it's not square, or maybe a part of it doesn't seem to be in focus, or maybe it's not exactly filling the screen, or maybe a long list of other things; none of which is good news.

Then the client calls, reminding you of that meeting she has scheduled for first thing tomorrow morning and recalling for you just how much you're making her pay for all this high tech stuff and, "What do you mean, there's something wrong with my image?"

Not a pretty picture, is it? But the better question is, how do we fix it? Still better one is, how do we avoid it in the first place?

Let's take a typical application and go through it together, step by step. Let's assume that the screen size we need is six by eight feet and that the projector we're going to use is a single lens LCD device.

As we have stressed elsewhere, the very first thing we should do is determine where we want to place our screen in its wall. Second, we position our projector behind it as though there were no space constraints of any kind. These two decisions should in no way be influenced by our knowledge that we'll need eventually to use mirrors.

How high the screen should be off the floor depends only on its spatial relationship to its audience. The positioning of the projector relative to the screen on the vertical axis is dependent on two (but only two) factors.

The first and dominant of these is whatever off- axis limits are imposed by its manufacturer. How much flexibility does this particular projector permit us in moving it up or down from a direct, on-axis orientation to the screen while still maintaining a rectangular image that has four 90º corners?

Some projectors can't (or won't) make such a square image if their orientation is dead normal (0º) to the screen. They expect instead to be positioned off- axis such that they point either downwards (with their feet pointing toward the ceiling) at screen center or upwards at it (with their feet toward the floor). Whatever the case, it is essential to understand that whatever may be the positional constraints for each projector, they must categorically be observed and respected. Mirrors cannot ever be used to override these limits.

The second factor, however, determines where the projector should be placed within the limits imposed by its manufacturer. Even single position projectors can be mounted either with feet to the floor or, inverted, with their feet to the ceiling. Here, the decision should be made such that the center most light ray emanating from the device is aimed (as nearly as possible, anyway) through the center of the screen directly into the audience's eyes. Once that optimal position has been identified and established then, but only then, are we ready to look at mirrors.

Let's examine first Figures 1 and 2. They outline the orientation issue discussed above. The first figure shows a projector positioned normal to a screen. The second is placed such that it is shooting downwards with its feet toward the ceiling.



Figure 1

The triangle which has as its base the screen and as its apex the projector is different in each case. In Figure 1 it is an isosceles triangle with the lengths of the top most and bottom most light rays being equal and the center most light ray bisecting the screen at 0 (or, if you prefer, 90) degrees.



Figure 2

Note that the second (and more typical) triangle is a right triangle and it is the top most light ray which intersects the plane of the screen at 90 (or 0) degrees. The center most ray passes through the screen at one downwards inclination and the bottom most at yet another.

Both of these triangles have been drawn as though the rear wall of the projection booth didn't exist. This is as it should be. Always.

Note that either of these triangles can be transected inside the wall by an enormous variety of lines at a huge variety of angles. Theoretically, any one is as good as another, although some will be longer and some shorter than others. Whatever line we choose, of course, becomes the crease of our first fold and scales to the vertical dimension of the real life mirror.

Thus, Figure 2 becomes Figure 3 and if this first fold finishes with all or even part of the projector outside the box, we'll either have to start over with an alternative fold or we'll have to make a second fold in order to fit everything neatly in.



Figure 3

The difficulty with many of the first fold lines that we could draw is that in real life these lines are mirrors which have to be supported by racks. And racks are not nearly so variable. Therefore, we need to understand that an underlying but major constraint of any mirror system is the limited flexibility of its racking configuration.

The uprights of self-supporting rack systems, for instance, need generally to be parallel to and equidistant from the screen. This structural fact severely limits the number of possible mirror sizes and angles. Yes, technically, a rack could be built to accommodate any and every mirror orientation, but such a product line would be wholly impractical and prohibitively expensive.

In the case we have chosen, the final fold looks like this:



Figure 4

Please look carefully at this drawing. While it's obvious to note that the three lines extending to the screen from the large mirror are exactly the same as the comparable three line segments in Figure 2, it may not be so clear that the mirrors have to be set in their racks at angles exactly equal to what are shown in the drawing. A moment's further reflection will reveal that the projector must be inverted in its rack and set exactly at its specified angle or, inescapably, the image will somehow be skewed.

Anyone who believes that she can correct for a departure from one of these parameters by fiddling in the field with the others is in for serious trouble. No, if the cad drawing you're provided decrees that the angle for the large mirror must be 27°, you can't make it 25° and then "fix" the resultant distortion by fiddling the other mirror and/or the projector. It won't work. Please don't do it. If you do, you'll only blame us, your screen and mirror company. And we're sensitive.

If, on the other hand, you make sure that the entire mirror and rack assemblage is aligned symmetrically with the screen then, yes, your mirror system should indeed be the squarest of them all.