Angles Of View
Virtually all projection screens utilize some kind of diffusion to disperse the light impinging on them. There are some screens, however, which have surfaces comprised of more than a diffuser. These screens have a tangible structure or profile which significantly alters the way in which they reflect or transmit light. What are the properties of these
Other Screen Surfaces?There are two principal distinctions between diffusion screens and surfaces with physical structure. One is obvious, the other subtle. The conspicuous difference is that profiled screens are not flat. Their surfaces are variously structured and periodic - the patterning on their surfaces repeats in some way. Usually the structure is coarse enough to be detected by running a fingertip across the surface. Diffusion screens have no such discernible profile; their only "structure" is molecular.
The second way that profiled screens are different is that their designs can permit them to disperse light asymmetrically. To see how this works, let's first consider a screen which Da-Lite makes called Super Wonder-Lite™. This is a soft, aluminized front projection screen which has a series of straight, parallel ribs embossed onto its surface.
The first thing the ribs do is render portions of the surface not flat. And if these raised ridges are oriented vertically, they will present to the projector a series of beveled slopes alternating with an intervening series of flat planes. Light rays which fall upon the plane portions of the surface will be reflected according to the law of specular reflection which states that the angle of incidence equals the angle of reflection. Thus light arriving at a 15o angle from the left will bounce off the screen at "an equal and opposite" angle of 15o to the right.
Rays falling onto the ridges will also obey the specular reflection law but, because their incidence angle to the screen is altered when the area they strike is sloped, their reflectance angle will be commensurately shifted. If the face of a rib rises from the surface of the screen at an angle of 20o , then the same 15o light ray we considered above will have an incidence angle of 35o (15+20) and thus will bounce off the screen at a 35o angle to its surface. Since the Super Wonder-Lite™ screen has a pitch of 42 ribs/inch, about half of its active surface area is sloped and the other half flat. In a sense, therefore, it is two screens in one: each has a high gain (due to its metallic coating which is much more mirror-like than the standard matte white diffuser) and a resultantly narrow viewing angle. But because one narrow viewing angle is aimed at the center of the audience field (this is all the flat parts of the screen) and the other narrow viewing angle is aimed at the edge of the audience field (all the sloped portions), the combination of the two produces a front projection screen with a gain of 2.5 and a horizontal half-angle of approximately 35o .
Super Wonder-Lite™ screens are frequently chosen for their ability to display 3D images. This facility is not produced by the existence of the ribs but results from the fact that the screen's coating is aluminized.
Another front projection surface with high gain is Da-Lite's new glass beaded High Power™ material. Although glass beaded screens have been around for years, the High Power™ surface represents a substantial advance in technical excellence. The surface of High Power™ is comprised of a huge number of tiny glass beads distributed evenly across a white vinyl field. In constructing this surface Da-Lite has found a way to get the diameter of the average bead reduced to about 9 microns. This is better than a conventional glass beaded screen by a factor of 7 since their typical bead diameter is about 65µ. The consequent improvement in resolution is of course equally great.
High Power™ has an additional advantage in that its special manufacturing process causes each of the beads to be firmly sunk about a third of the way down into the vinyl beneath it. This means that when the finished material is attached to a roller no beads will rub off when the screen is raised or lowered. This mechanical stability coupled with its exceptional resolution and 2.8 on-axis gain make the High Power™ material the best glass beaded screen on the market.
But what is it about glass beads that makes them useful to a projection screen in the first place? The answer is that the screen behaves as though it were partially retro-reflective.
When a screen (or any other reflecting device) is made to be retro-reflective the angle of reflection is not paired with an equal and opposite angle of incidence. The angle of incidence is the angle of reflection. In other words, when light rays strike a retro-reflective surface they only bounce back along the exact path they came in on and therefore end up returned to the projection source from which they originally came.
Figure 1 shows a series of brightness measurements (the Y-axis) made from just behind a projector (0o on the X-axis) that was first positioned normally to a High Power™ screen (the solid line) compared with another series taken when the projector was moved 20o off of the normal (the dashed line). Noticing the degree to which the two plots are unshifted and identical in slope, it is easy to see why glass beaded screens are assumed to be retro-reflective.
To understand how glass beads really work, however, we first need to recall a little bit about an optical phenomenon called refraction. This is the process which governs the change in direction which light rays undergo when they cease travelling through one medium (air, for example) and start to travel through another with a different density (glass or plastic, for example).
If the medium the light is leaving is less dense than the medium, the light is entering the refraction process will bend the light towards what is called the "normal" of the denser medium. When light exits a denser medium it is bent away from that same "normal." How much bending (in either direction) is proportional to the difference in densities between the two media.
Figure 2 illustrates a bundle of projected light rays striking a single glass bead located somewhere out near the left-hand edge of a High Power™ screen surface. Because glass is denser than air, each ray is refracted through a specific number of degrees towards the "normal" - which in the case of a sphere is a radius, a line connecting the point on the surface where the incoming light ray strikes and the center of the sphere. Notice that the spherical shape of the refracting surface causes all of the light rays in the bundle to converge such that they will reach the bottom of the sphere very much more tightly clustered than when they entered it.
If, after passing through the back surface of the sphere, the light rays encountered air (a less dense medium) they would of course be refracted away from the normal (all those radii); but they don't. Instead of air they strike a matte white diffuser into which the sphere's underside has been tightly embedded. (For contrast purposes only this diffuser has been shaded black in Figures 2 and 3.)
Because it's a perfect diffuser, the matte white reflects all of the light back up through the sphere which now can be thought of as a microscopic rear projection screen which images just that little area of illuminated diffusion beneath it [Figure 3]. Note that when the reflected light rays reach the top of the sphere and reemerge into the air, they are refracted away from the normal (those radii again) which conveniently happens to mean that they're bent back towards the projector.
Refraction is also the operative process controlling profiled rear projection surfaces. When one of these is ribbed it is said to be lenticulated which signifies that its shape is going to act as a refracting lens for light rays passing through it. When both surfaces of a rear screen are parallel to one another (as in the case of the flat sheets used as substrates for diffusion screens) all refractive effects can be ignored. Since the normal to both surfaces is the same, the incoming bend toward the normal is exactly cancelled by the outgoing bend away from it.
When the two sides of a sheet are intentionally rendered out of parallel interesting things happen. Suppose for example that a projected light beam first strikes the plano side of a lenticulated sheet of plastic. The angles of incidence within the beam will of course vary greatly between its central ray (0o ) and its outermost ray (which might be 25o ). The normal toward which all of these rays will proportionately be bent, however, does not vary: it is the perpendicular to the sheet's flat surface.
When these refracted rays get ready to come out through a back surface that is shaped like an undulating series of ridges and valleys they discover an infinite range of new normals only two of which (the point at the exact bottom of each valley and the one at the exact top of each ridge) are parallel with the entrance normal.
By varying the radii of these curves (the ridges and valleys), screen manufactures can control the degree to which exiting light rays are dispersed by the two surface system. It does not matter whether the light passes through the curved surface before the flat surface or after it. Da-Lite sells the Da-View™ screen whose lenticulations face the audience.
The reason that all of these screens provide such wide viewing angles perpendicular to the axis of their lenticulations can now be seen to be the result of the series of unequal normals between the flat surface and the curved. If the lenticulations were curved along both axes (if they were craters instead of ditches) then viewing angles parallel to their original axis would also be refractively controlled. As it is, the dispersion about that parallel axis (generally the vertical) is largely unaffected by the lensing and thus remains small by default.
We saw in earlier issues that pure diffusion screens scatter the light projected at them. We now see that alternative, structured surfaces can be used to reflect that light (Super Wonder-Lite™) or to refract that light (the glass beaded and lenticulated surfaces). These are the only things a projection screen can do: Reflect, Scatter, or Refract.