It is common to use some form of targeting in close range photogrammetry as there are seldom enough points on the surface of an object with sufficient contrast. Targets which have been used include: light emitting diodes; black circles on a white background; retro-reflective film; projected laser beams; projected `white light' slides; feature encoded targets; and color targets. This paper discusses the characteristics of targets. In particular the established retro- reflective target and the promising projected laser target are considered as they both offer high signal-to-noise ratios together with optimum target sizes. The performance of the targets are analyzed by use of laboratory tests, for example: (1) a retro-reflective target was placed on a rotating mount with the center of the target located on the axis of rotation and the target monitored by a CCD camera under varying conditions; and (2) a laser target was analyzed by experiments which were designed to indicate the effect of speckle by moving a flat object in a direction perpendicular to the laser beam.
Timothy A. Clarke,
"Analysis of the properties of targets used in digital close-range photogrammetric measurement", Proc. SPIE 2350, Videometrics III, (6 October 1994); doi: 10.1117/12.189137; https://doi.org/10.1117/12.189137