Retroreflective material is commonly used on road signs and other objects when good conspicuity is required at night. As you approach, your car’s headlights illuminate the object and most of the light comes back pretty much toward your eyeballs, regardless of your direction of approach to the object.
There are two types of such material. One is made with tiny glass beads, each of which serves as a retroreflector. More recently, prismatic retroreflective material is being made. This has the plastic backed by a layer of foil, the surface of which features an array of numerous microscopic corner reflectors in its surface.
Question: How do they make those tiny corner reflectors on the surface of that foil?
I worked at Guide Div. of General Motors. We made reflectors as part of our rear, front, and side lamps. The section of each mold that made reflectors was a group of highly polished, angle-ground steel pins clamped together. The optics of the finished lens don’t need to be aluminized, they work fine without it.
I was talking about the section of the lamp that reflects incoming light without being turned on. We also did plenty of aluminizing, for the lighted part of the lamps. The back of every old-style “sealed beam” glass headlight was also aluminized.
There also is a plastic sleeve that fits over a road cone. It is made of transparent vinyl with the retroreflective surface stamped onto the back. These are sealed because if they get wet they lose their retroreflectivity.
That was interesting, thanks. Several of their patents speak of pressing the shapes into material using a roller that features protruding cube corners. This only leads to the question, how did they create the cube corners on that roller? To understand what I’m puzzling on, here’s some of the material I’m holding. Hover your mouse over one of those photos to see an enlarged version of it. Each of the triangles you see in that enlarged photo is about 1/8" on a side. With a magnifier, I counted a line of cube-corner reflectors along one side of a triangle. There were about 20 of them. So each cube-corner reflector is about 0.00625 inches across. These are extremely small features, and I’m wondering how they’re created in a way that produces mirror-smooth faces, and edges that aren’t so rounded as to make the flat faces completely disappear.
This is an area I work in, for one of the patent offices. There’s different methods for doing this, but probably the most common is “Diamond turning”, in which a very small cutting head is moved across a surface to produce a very small groove. The profile of the groove is usually a V-shape.
You make a lot of parallel grooves, spaced by the dimensions you want the cube corner elements to be. Then, you rotate the surface by a certain amount, and make another set of such grooves that cut across the first set and an angle. Repeat with a third set to produce a basic cube corner arrangement.
There are more advanced designs using different groove intersection angles, different groove V-angles, and different depths of grooves within each set. These are usually intended to tailor the reflection to match a desired output pattern.
Most systems use this to make a “master” array in some hard material like steel, which is then used in a press roller machine to either make sub-masters, or to directly produce the final product. Producing the original master can time some time, and be quite finicky, but once you have the master, you can use it in the press rollers for a long time, to produce the retroreflectors quite quickly.
Yeah, if you think about how small the pits and lands are on a CD - those are made by a mechanical embossing process - so making corner cube material by embossing shouldn’t be a problem.
“Embossing” implies that the desired features are forced into the substrate.
CD, DVD, BD are made by injection molding polycarbonate against the stamper. The desired features are not really pressed into the substrate - the substrate material is molten and flows into or around the features.
The prisms are manmade and are first designed depending on how tight the return light beam needs to be. Some reflective tapes disperse light in a wider arc than others for specific applications. Once the prism is designed an array is created and the design is embossed into the back of a polymer. Here is where the process takes one of two paths. There are two types of prismatic retro reflective tapes.
The first is an air backed prismatic and is made by glueing the top layer with the prisms onto a white backing that has raised octagons or honeycombs so that there is an air space between the prism and the white background. This type of reflective tape is bright and vivid but has the downside of delaminating. So it is mainly used in static applications where it is protected from the weather and any impacts.
The second type of tape is a metallized prismatic. The top layer is embossed with the prisms and then a layer of aluminum is applied to the back to create reflectivity. There is not an air space. Adhesive is applied to the back and the top is coated for protection. This type of prismatic reflective is much more durable and can be used in dynamic applications where the tape is beat up and it will still reflect. It is also much thinner and flexible than an air backed product.
Both tapes have their purpose. Here is a picture of an air backed film and here is an example of a metalized film. I hope this helps. Stan.
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.
The post gives no information on the manufacture of the reflective element, rather a recounting of the uses and the two links are not just pictures but to an online store.