That Dan Gelbart lecture where he also explains how retroreflection[0] is used to detect remote viewing, was it anything with a lens or even human eye is really good.
reply[0] straight link to where he explains retroflection https://youtu.be/0MtRxX0crjU?t=3019
Also many other videos where he explains how to solve hard problems are really good too.
e: There seems to be commercial product using retroreflection advertised Youtube