Retroreflectors: Common uses, uncommon applications and the Lunar Laser Ranging Experiment

Throughout our World Space Week series, we’ve talked about satellites, lasers, telescopes, software, and many other applications of our optical systems. We’ve also discussed the years of work and research and development that goes into creating them.

Now, we’re turning our lens on a simple optical component that’s used in many of the devices that you use every day. It’s one that you’ve likely never noticed or considered—the retroreflector.

Safety first
You know, that plastic reflector you see on your bike, or the “cats’ eye” embedded in the road, or the shiny elements on your running shoes? They’re actually very interesting  pieces of optical design.

A retroreflector, or corner cube prism as we sometimes call them, is a device or surface that reflects light back to its source with a minimum of scattering. What does that actually mean? Well, think about it. A mirror, lens, or prism scatters or refracts light in different directions with little focus.

How retroreflectors work
Here’s how a corner reflector works. Think of an inverted triangle—three symmetrical planes form the inside corner of a cube. When a ray of light reflects from the first side, it’s reversed to the next side, and then transferred to the last plane. It’s then sent back to the source.

In addition to corner reflectors, there are also cats’ eye reflectors and phase-conjugate mirrors. Both operate on similar principals, however with variations.

How retroreflectors measure the moon
As a piece of common safety equipment, retroreflectors have saved countless lives. But that’s just one small application. There are bigger ones. How big? Well, how big is the moon—and more importantly, how close is the moon? Retroreflectors are helping us find out.

Now, we covered a portion of this topic in our lasers blogpost, but we want go over it again. For those of you who don't think astronauts from the United States landed on the moon, here's some empirical evidence.

Part of the Apollo program, specifically the Apollo 11, 14, and 15 missions,  involved something called the Lunar Laser Ranging Experiment, where astronauts set up retroreflectors that can reflect a laser beam sent from Earth. The purpose, was to calculate distance between Earth and the moon. These were no ordinary retroreflectors. The Apollo 11 retroreflector array, contains 100 corner-cube prisms in a 10×10 array. Each corner cube is made of fused silica and is 3.8 cm in diameter and the total housing is just under 18 inches. 

Now imagine aiming from earth and hitting a target on the moon that is less than 18 inches. You'd have to be an unbelievable marksman. Scientists liken the task to using a rifle to hit a moving dime at 1.9 miles away. However, the bright minds at NASA once again make technological breakthroughs with even the simplest of technologies, like a retroreflector. And the best part? The program is still active today. So, yes, not only did we land on the moon, several times, but we’re also actively firing lasers at it, and having them bounce back to us. How cool is that?

Now, we love this stuff, and are proud to create a range of components, from windows, mirrors, and prisms, to custom optical arrays. We’re honored to provide equipment for scientists, hobbyists, and explorers. Just about the only thing we don’t make is bicycle parts.