A retroreflector is an optical element that sends incoming light back toward its source. Unlike a mirror, which reflects light at an angle, a retroreflector returns the beam almost directly along the incoming path. Corner-cube designs are common because they work reliably from many directions.
In cyber security, retroreflectors matter in systems where light is used for measurement or control, such as laser ranging for satellites. They can reduce onboard attack surface because the object being measured may need no electronics, software, or radio link. The risk then shifts to the ground segment: timing, calibration, access control, and data integrity. If those systems are tampered with or misconfigured, the measurements can be wrong even when the reflector itself is untouched. In defense, retroreflectors are valued for passive, predictable behavior, but they still depend on a trustworthy measurement chain.