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Markers |
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NAVIGATOR: Back - Home > Adi > Services > Support > Tutorials > Gait > Chapter3 : |
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Markers
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3.4 Markers
Which kinds of markers are recommended?Shape - APAS/Gait uses passive markers covered by retroreflective adhesive sheet. Common shapes are disks of spheres or semispheres. Spheres are preferred to semisphere because the measured coordinates should be independent of the line-of-sight angle. Weight - Of course, low weight (low inertia) is preferred for minimal skin slippage. The lighter, the better. Size - Small size are better because of low weight, less chance of merging effect, and increased accuracy. But too small sized markers are difficult to track automatically. For APAS/Gait, which examines lower body (pelvis and both legs), markers with 1.4 (for children) - 1.8 (for adults) cm diameter are good. The volume for examination is around 4m*1.5m*1.5m. It is not impossible to use 1.8 cm sized markers for children with 70 - 80 cm of height or to use less than 1cm of diameter. The excellent automatic tracking ability of APAS enables to use wide range of marker size. To minimize the merging effect, 1cm or less would be good for heel markers. (Fig: photo of markers) How do you throw light on these markers?Because APAS/Gait uses retroreflective markers, light sources should be used to maximize the contrast between the markers and background structures for excellent automatic tracking. One light source should be assigned for each camera. The direction of the beam from each light source should be aligned to the line of sight angle of assigned camera. The intensity of light does not have to be so high. The higher is the intensity, the more is the subject overwhelmed and the more is the view of opposite camera interfered. (figure: marker with / without
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