Markers
OptiTrack motion capture systems can use both passive and active markers as indicators for 3D position and orientation. An appropriate marker setup is essential for both tracking the quality and reliability of captured data. All markers must be properly placed and must remain securely attached to surfaces throughout the capture. If any markers are taken off or moved, they will become unlabeled from the Marker Set and will stop contributing to the tracking of the attached object. In addition to marker placements, marker counts and specifications (sizes, circularity, and reflectivity) also influence the tracking quality. Passive (retroreflective) markers need to have well-maintained retroreflective surfaces in order to fully reflect the IR light back to the camera. Active (LED) markers must be properly configured and synchronized with the system.
Retroreflective Markers
Marker Size
The size of markers affects visibility. Larger markers stand out in the camera view and can be tracked at longer distances, but they are less suitable for tracking fine movements or small objects. In contrast, smaller markers are beneficial for precise tracking (e.g. facial tracking and microvolume tracking), but have difficulty being tracked at long distances or in restricted settings and are more likely to be occluded during capture. Choose appropriate marker sizes to optimize the tracking for different applications.
Circularity
Worn markers
Custom Markers
OptiTrack cameras can track any surface covered with retro-reflective material. For best results, markers should be completely spherical with a smooth and clean surface. Hemispherical or flat markers (e.g. retro-reflective tape on a flat surface) can be tracked effectively from straight on, but when viewed from an angle, they will produce a less accurate centroid calculation. Hence, non-spherical markers will have a less trackable range of motion when compared to tracking fully spherical markers.
Active Markers
OptiTrack Active Markers
Custom Active Markers
Active (LED) markers can also be tracked with OptiTrack cameras when properly configured. We recommend using OptiTrack’s Ultra Wide Angle 850nm LEDs for active LED tracking applications. If third-party LEDs are used, their illumination wavelength should be at 850nm for best results. Otherwise, light from the LED will be filtered by the band-pass filter.
Place a spherical diffuser around each LED marker to increase the illumination angle. This will improve the tracking since bare LED bulbs have limited illumination angles due to their narrow beamwidth. Even with wide-angle LEDs, the lighting coverage of bare LED bulbs will be insufficient for the cameras to track the markers at an angle.
Many applications that require active LEDs for tracking (e.g. very large setups with long distances from a camera to a marker) will also require active LEDs during calibration to ensure sufficient overlap in-camera samples during the wanding process. We recommend using OptiTrack’s Wireless Active LED Calibration Wand for best results in these types of applications. Please contact one of our Sales Engineers to order this calibration accessory.
Marker Placement
Asymmetry
Asymmetry is the key to avoiding the congruency for tracking multiple Marker Sets. When there are more than one similar marker arrangements in the volume, marker labels may be confused. Thus, it is beneficial to place segment makers — joint markers must always be placed on anatomical landmarks — in asymmetrical positions for similar Rigid Bodies and Skeletal segments. This provides a clear distinction between two similar arrangements. Furthermore, avoid placing markers in a symmetrical shape within the segment as well. For example, a perfect square marker arrangement will have ambiguous orientation and frequent mislabels may occur throughout the capture. Instead, follow the rule of thumb of placing the less critical markers in asymmetrical arrangements.
Marker Bases and Adhesives
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