Angles Of View

Common to all visual displays are three principal elements: projector, screen, and audience. Ideally the configuration and arrangement of the first two should function so that the maximum number of light rays emitted from the first will be relayed by the second directly into the eyes of the third. All other distributions of the light are superfluous. With this basic precept in mind, let's take a closer look at

Some Reflections on Front Projection Screens

Although there are many different types of projection devices, there are not so many types of screens. In fact there are only three. There are screens that reflect light, screens that scatter light, and screens that refract light. Understanding a little about how each of them works can help improve our ability constructively to choose between them.

Let us consider first screens which are reflective. These are the surfaces which are governed principally by the Law of Reflection which states that the Angle of Incidence = the Angle of Reflectance. When light falls on such a surface, it bounces off in a direction which is largely determined by the direction it originally came from. This is comparable to the behavior of a ball on a billiard table. If we propel it toward a cushion at an angle of, say, 30^o, it will rebound at an angle of -30^o.

The second kind of screen is the type that scatters. Light bounces off this surface in a remarkably uniform way that is completely independent of its incident angle. To extend the billiard ball analogy, whatever direction we chose to aim at the cushion will have no effect whatsoever on the way the ball will bounce off it. In fact, the "ball" doesn't bounce at all. Instead it spontaneously breaks up into millions and millions of tiny little balls which flow outward from the cushion, spreading evenly across the entire table. This, of course, is the Matte White screen surface and for a detailed discussion of its properties see Vol I, No. 6 of this series.

The third kind of screen is refractive. Since refraction is a phenomenon which occurs when light ceases traveling through one medium and enters another, we don't normally think of it in connection with front projection screens. Nevertheless, the optics which underlies all glass beaded screens is definitely refractive. Light projected from some angle at this type of screen neither bounces off at an opposite angle nor scatters uniformly. Light directed at a glass beaded screen is returned along the very same path it arrived on. It is because of this peculiar attribute that glass beaded screens have come to be called retro-reflective. If a billiard table were retro-reflective, a ball aimed at one of its cushions from whatever angle would rebound only along a path leading directly back to the tip of the cue stick.

Among these three basic types of front projection screens, glass beaded surfaces have been the most maligned and the least understood. That is unfortunate because, as we shall see, they can be exceptionally useful devices which no longer deserve the reputation they once had.

Historically, there were two drawbacks to glass beaded screens as they were initially manufactured. One was mechanical, the other optical. To create a surface that's glass beaded, a screen manufacturer has to find a way to apply a very large number of very small glass balls onto a substrate (a backing) which is thereafter going to get rolled up and down possibly thousands of times. If it was hard enough getting the beads glued to the substrate in an acceptably uniform way, getting all of them to stay stuck to that substrate as it went through all those ups and downs was pretty well impossible. What often occurs with typical glass beaded screens is that some of the beads do get rubbed off as the surface supporting them gets repetitively furled and unfurled. Worse than the litter they create when they fall onto the floor are the bare spots they leave behind on the screen.

The optical problem has had to do with the size of the beads themselves. Most conventional glass beads used on projection screens have diameters which, by optical standards at least, are quite large. This deficiency is most apparent under data or graphics projection when the otherwise highly resolved images can exhibit a kind of granular, sparkly appearance, which is often distracting.

Recent breakthroughs in screen manufacturing techniques have overcome both of these defects in extremely successful ways. Da-Lite Screen Company has introduced a fabric called High Power™. To ensure complete mechanical stability, all of the beads in the High Power™ fabric are covered by a thin, protective top layer which stretches over their upper contours like a tight elastic skin, holding them permanently in place. Not only is the resultant surface washable, it is completely smooth to the touch and the adhesion is so perfect that it requires a powerful magnifier to confirm that the surface really is beaded at all.

Da-Lite has surmounted the resolution problem by finding a way to use beads that are only 9 microns in diameter. This is a sevenfold improvement over the best of the traditional beaded surfaces and it completely eliminates all traces of granularity and scintillation. (To put that 9µ diameter in perspective, 10µ is the typical width of a single human hair.)

Because the development of the High Power™ fabric has eradicated so convincingly the defects associated with earlier glass beaded surfaces, designers need no longer hesitate in specifying retro-reflective screens. Now all three screen types can be considered equally with the choice between them dependant only on their optical merits. To select efficiently among those merits, it's best to begin by examining the spatial relationship between the projector and the audience.

Front projection systems are rarely set up with the projector dead normal to the screen. When a projector is placed exactly perpendicular to screen center, it tends to be right in the middle of the audience, too close to the tops of the heads of the people seated beneath it and too obstructive to those seated behind it. To escape these difficulties, projectors are often hiked up above the normal and configured so that they shoot downwards at the screen. Since we always want a screen surface to redirect the light from the projector into the eyes of the audience, we want in this case to choose a screen which obeys the Law of Reflection. In fact the rule could be:



Figure 1

Whenever the projector and the audience are on opposite sides of the normal, use a screen that is reflective. [See Figure 1]

Next we want to ask what happens when the projector isn't top mounted, when its most convenient placement is on a tabletop, below screen center? Using a reflective screen in this configuration would be a mistake because the upwardly directed projection beam will only get bounced yet higher still. Unless our audience happens to be seated "up there" (as might be the case in an amphitheater, for example) the majority of the projector's brightness will never reach their eyes. This, then, is a good opportunity to take advantage of a glass beaded screen. Thus the second rule could be:

Whenever the projector and the audience are on the same side of the normal, use a screen that is retro-reflective. [See Figure 2]



Figure 2

Notice that both of these configurations differentiate only between the vertical projection geometries. Although projectors rarely, if ever, are positioned off-axis horizontally, viewers often are. Whenever, therefore, the horizontal half-angles get larger than 20-30 degrees, the screen specification should probably revert to a Matte White surface. When, however, the requisite horizontal viewing angles are more modest, reflective or retro-reflective surfaces can significantly improve image brightness. Da-Lite's Pearlescent or Video Spectra™ 1.5 are both reflective surfaces which offer on-axis gains of +1.5. The new Cinema Vision surface delivers a slightly reduced on-axis gain of +1.3. If a retro-reflective surface is appropriate, the High Power fabric has an extremely bright on-axis gain of +2.8 which is produced by the optical action of the glass beads. Interestingly, outside of their viewing cone, the High Power™ material behaves nearly identically to a Matte White scatterer.

When selecting a front projection screen with gain, this article wishes to propose that choosing the magnitude of the gain is less important than choosing the type of reflectivity producing it. The goal of any display system must be to deliver the maximum number of information bearing light rays from the projector into the eyes of the audience. Ensuring by our surface choice that the fewest number of those rays end up anywhere else is sure to reflect well on all of us.