Quantification of a systematic source of error affecting optoelectronic stereophotogrammetric measurements: the camera occlusion artefact

We present a systematic source of error affecting the measurement of optoelectronic stereophogrammetric sys-tems (OSS), hereinafter called the camera occlusion arte-fact (COA). The COA takes place when the view of a mark-er is partially or completely occluded to one or more cam-eras. In these conditions, the set of rays tracking the marker suddenly changes, introducing a discontinuity in the re-constructed marker position and a consequent apparent marker displacement. A Vicon OSS is investigated in two cameras configurations, one maximizes accuracy (Measure Setup) and the other typical of gait analysis (Gait Setup). In the experiments, the COA resulted in a maximum apparent marker displacement of 6.67 mm and 5.70 mm for the Measure and Gait Setup, respectively. Although smaller than the soft tissue artefact (STA), the COA may signifi-cantly impact on the results of motion analysis.