What is a sphere-mounted retroreflector (SMR)?

A sphere-mounted retroreflector (SMR) is a corner cube accurately mounted in a sphere.

Corner cubes have the unique property that light sent into the corner cube returns directly along the same path.

SMRs are often used with laser trackers but are also implemented with AOM CGHs as highly accurate alignment targets. SMRs are used along with line focus (LF) alignment patterns to measure misalignment in a single degree of freedom.