Reflective fiducial marks, or targets, permit the location of objects relative to a robot. Usually, object luminance falls off as the inverse square of the distance from the source. However, selected retroreflective marks illuminated by a source near the sensor can have luminances which are almost constant over a wide range of distances and which are also over one hundred times greater than those of diffuse reflectors. Retroreflective materials and illuminator design guidelines are discussed.
Gordon T. Uber, Gordon T. Uber,
"Constant-Luminance Retroreflective Targets For Robot Guidance", Proc. SPIE 0958, Automotive Displays and Industrial Illumination, (24 October 1988); doi: 10.1117/12.947732; https://doi.org/10.1117/12.947732