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 systems (OSS), hereinafter called the camera occlusion artefact (COA). The COA takes place when the view of a marker is partially or completely occluded to one or more cameras. In these conditions, the set of rays tracking the marker suddenly changes, introducing a discontinuity in the reconstructed 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 significantly impact on the results of motion analysis.