1. What is a retroreflector?

A retroreflector is an optical element that reflects parallal to incident light direction. A retroreflector can do this for a range of incident angles, unlike a normal reflection where reflected beam's orientation is a fucntion of the incident beam's angle with the plane normal. Retroreflector can perform this function while not requiring any precise alignment of the optical element.

2. How does a retroreflector work?

One of the most common retroreflector is a corner cube retroreflector. This works by reflecting light one by one from three mutually perpendicular reflecting surfaces. These surfaces could be reflecting due to either a metallic coating, or a HR dielectric coating, or could also be due to total internal reflection. For TIR, the cube needs to be solid, light needs to go from high refractive index material (eg glass) to lower refractive index material (air) for TIR to happen. For the other cases - metallic coating or HR dielectric coating, the cube can be hollow as well. The advantages of using a hollow cube for corner cube retroreflector is that there will be no chromatic dispersion as well as no absorption.

3. When would retroreflector fail?

In order for the light beam to compeletely reverse antiparalal to the incident direction, we need reflection from all three mutually perpendicular surfaces. If the incident beam is such that, for e.g. at extreme oblique angles or angle less than the TIR critical angle for TIR case, the light might not be able to hit all three surfaces.

4. Where are retroreflectors used?

Retroreflectors send light along the same path as the incoming beam. The reflected beam, in other words, is traversing along the same path (if not exactly the same!). Thus the time taken by the reflected beam to reach a sensor can be used to calculated distances.

Retroreflectors are also used in road and safety signs like on bicycles. They do not need their own light source. Light coming from headlights hit the retroreflector and is sent back to the car, making the object that has the retroreflector on it visible. This reflected beam is now in one direction instead of becoming diffuse or scattering, thereby increasing its intensity.