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. 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 tm screen surface. 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. Instead of air they strike a matte white diffuser into which the sphere's underside has been tightly embedded.
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. 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. It does not matter whether the light passes through the curved surface before the flat surface or after it.