There are actually a lot of different types of retroreflectors out there. I devoted an entire chapter of my book on optics to then, because people don’t give them the attention they deserve.there are a lot of patents for them.
The “beaded” kind is the easiest to make, and the least demanding of design – you just place glass or plastic beads on a painted surface. The beads don’t really need to be perfectly round and the refractive index isn’t critical. The beads act like lenses that roughly focus the light onto the surface behind them, although usually the rays are intercepted before they reach a full focus. The surface then reflects the light in all directions, but many of the rays are collimated or nearly collimated by the bead back along the direction tghey came. It’s the same principle that causes the natural effect of retroreflecting sun from morning dew on grass (called Heiligenschein) or from tree leaves (which some have called sylvanschein.
A more sophisticated means is to make a glass 9usually) or plastic bead so that the light passing through the front comes to focus on the rear surface, which acts as a perfect reflector to redirect the light back through the bead as a collimated beam. Because this requires a specific (and uncommon) refractive index to work for perfect spheres people generally cheat and make the radius of the back part of the bead different from the front so that they can use whatever material they want. Sometimes the back surface is silvered, but it needn’t be. These are called “Cat’s Eyes” They were invented – twice – in Britain. Once you have a glass or plastic mold made you can simply crank these out.
The kind you’re asking about is essentially a sheet of what are called “corner cube” retroreflectors (although one of my professors took umbrage at this, insisting that they be called “Cube corners”) If you lopped off the corner of a cube of glass you’d have it. Genberally there’s no need to coat it – Fresnel reflection sees to it that almost all the light comes back. You can also make this from three mirrors forming the hollow corner of a cube. It was probably done this way in Germany, where the thing seems to have been invented in the 19th century, and was called a tripelspiegel (“Three-Mirror”). You can buy precision glass corner cubes from optics suppliers. But the sheets of reflectors you’re talking about are made, as has been pointed out above, by injection molding. A symmetrical sliced-off corner of a cube has the cross-section of an equiolateral triangle, which you can use to tessellate a plane, but they generally lop off the three corners to tuirn the shape into a hexagon, and tile the plane of the retroreflective sheet with those. If you look at the surface of the sheet you;ll see it has a kind of “chicken wire” texture to it.
There are lots of other retroreflectors, as well. There’s the Lunenberg Lens, ideally made from gradient index materials, but practical ones use layers of materials. I myself have come up with three more retroreflectors I’ve never seen anywhere else. One of these days I’ll patent them (if I can find some advantage they have), or write a column about them.
