A retroreflector is an optical device that returns an oncoming beam of light back to its source. It can be as simple as a tiny glass sphere or a type of prism formed from glass or plastic. (For example, you can build a Glass Bead Projection Screen.)
While ordinary flat mirrors also reflect light, the light isn’t reflected back to the source but off to the side at the same angle the beam arrived. Only if the light beam is perfectly perpendicular to the surface of a flat mirror does it act like a retroreflector.
Retroreflectors are so much a part of everyday life that typically they don’t attract much attention. But they attract plenty of attention while driving at night, when they seem to be almost everywhere. They’re incorporated into the taillights of vehicles, safety barriers, traffic signs, and the painted stripes that separate lanes of traffic.
Recently while waiting at a traffic light on a dark night, I noticed seven brightly glowing traffic signs coated with retroreflective paint. These signs were illuminated by the headlights of my pickup, and they each reflected the oncoming light back toward me.
A nearly full moon was overhead, and it has retroreflective properties, too. That’s because the Apollo 11, 14, and 15 astronauts left arrays of precision retroreflectors on the moon. For 40 years, various observatories have pointed powerful laser beams at the moon and detected the light returned by those retroreflectors to accurately measure the distance between Earth and the moon.
Retroreflection was observed long before artificial retroreflectors were invented. That’s because the eyes of many animals double as retroreflectors that glow at night when viewed from the same direction as a fire or lamp. When I was a Boy Scout I observed the eyes of alligators in a Florida lake, glowing bright orange in the light of a campfire. Years later I used a flashlight to find caimans in Brazil’s remote Cristalino River. Drivers notice the eyes of animals at night glowing brightly in the headlights of their cars.
The retroreflection exhibited by animal eyes is called eyeshine. A bright, head-mounted light provides the best way to observe eyeshine. Retroreflection from an eye occurs when some of the light focused onto the retina by the lens is reflected back through the lens, where it is refocused into a narrow beam that travels back to the source of light. While this occurs whenever the eye is opened, we notice it only at night when a bright light source held close to our eyes is pointed at the eyes of an animal. The retina alone is not a particularly good reflector, but in many vertebrate animals it is backed by a highly reflective layer of tissue called the tapetum lucidum.
The red glowing eyes of people in flash photographs is known as red eye. Human eyes lack a reflective tapetum, so human eyeshine is not nearly as bright as that of animals. The best way to eliminate red eye is to move the flash away from the camera’s lens. The light will be reflected back toward the flash, and most of it will miss the lens.
For decades highway signs and painted stripes on roadways have been coated with tiny retroreflectors. The earliest and still the most common are clear glass beads, which can be sprayed or poured over freshly painted road stripes and highway signs. Various kinds of reflective sheets are also used to coat warning barriers and signs. Some employ a layer of glass beads, while others use sheets embedded with tiny plastic corner prisms called microprisms.
Microprisms are tiny versions of much larger retroreflectors cut from a corner of a solid cube of glass or silica. These reflectors are called corner cubes or cube corners, and they provide the best performance. The retroreflector arrays on the moon are silica corner cubes. Retroreflectors can also be made by mounting 3 mirrors to form an open corner of a cube.
Retroreflective glass beads, like those used on highway stripes, are available from various sources. I bought an 8oz bag of standard glass beads for $6 (plus shipping) from colesafety.com.
Any surface that can be painted can be made retroreflective by sprinkling glass beads onto freshly applied paint. Here’s how I transformed a plain wood letter “M” from a hobby shop into an attention-attracting object.
1. Place the wood letter on a sheet of paper and apply a thick coat of white enamel.
A simple retroreflector can be made from 3 mirrors arranged to form the corner of a cube. You’ll need three 2″×2″ glass mirrors, double-sided tape, and a standard 2-1/8″×2-1/8″×4-1/8″ plastic box, all available from a hobby shop. Use care working with small mirrors, as they have sharp edges and are easily broken; supervise children. Follow these steps:
1. Clean your hands and work surface. Hold the mirrors by their edges and use glass cleaner to clean their surfaces.
2. Place a 1″ strip of double-sided adhesive tape on the back of a mirror and insert the mirror, shiny side up, inside the box lid.
3. Place double-sided tape along the lower half of the back of a second mirror and stick it against one of the inner sides of the lid.
4. Stick the third mirror adjacent to the second one so that all 3 mirrors merge to form a corner inside the lid.
5. Look inside the reflective corner of the cube. With one eye closed, your open eye will be directly centered at the apex of the corner. It will stay there even when you move the cube at various angles.
If the sides of the plastic container lid are slightly angled, the reflection from the cube corner will form separate beams that can be seen as 3 spots of light on a white wall behind the light source. When the sides of the box are perfectly square, these separate spots will merge into a single beam.
CAUTION: Use suitable laser goggles and avoid eye damage by never looking directly at a laser beam or its reflection.
Retroreflectors are ideal for aligning laser intrusion alarms and communication systems.
If you want to make a really big glass bead reflector, see Sean Michael Ragan’s projection screen project. This screen works best when the viewers are seated very close to the projector.
Plastic microprisms are excellent retroreflectors, but they require special equipment to make — until now. It should be possible to make arrays of microprisms using 3D printing with a transparent resin. Since such arrays wouldn’t be restricted to flat sheets as with mass-produced microprism arrays, there could be some intriguing applications for 3D arrays of microprisms.
Studies of eyeshine could make interesting science fair projects. For example, populations of spiders and moths that exhibit eyeshine can be surveyed with the help of a head-mounted light on a dark night. Spiders depend on insects for food, so their population is closely related to availability of insects. During a recent major drought where I live, the population of many insects plunged to nearly zero — and so did the spider count. Where there were once dozens of brightly glowing spider eyes each night, only one or two remained during the drought.