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How the Polaris Vicra System Works
3.11
Phantom Markers
Phantom markers are the result of the calculation that the Polaris Vicra System uses to determine the
position of a source of IR. They appear, and are reported as, markers but they do not actually exist;
they are phantom markers.
To determine the position of a source of IR, the Position Sensor calculates a line between the source
of IR and each sensor (displayed as dotted lines in
Polaris Vicra System calculates the line separation (the distance between the lines). If the line
separation at this point is less than a predefined limit, the Polaris Vicra System considers the point to
be a possible marker position.
Phantom markers are reported when the imaginary lines calculated from the sensors intersect in
more than one place with a line separation less than a predefined limit. This generally occurs when
two or more markers are in the same plane as the sensors. For example, in the case of two coplanar
markers, there will be four mathematical solutions, as illustrated in
marker locations and two are the phantom marker locations. One phantom marker will usually be
closer to the Position Sensor than the actual markers and the other phantom marker will be farther
away from the Position Sensor than the actual markers.
The number of phantom markers increases with the number of coplanar markers. When there are n
coplanar markers, there will be up to n • (n - 1) phantom markers.
When the stray marker reporting functionality is not enabled, the Polaris Vicra System can eliminate
phantom marker solutions. The system matches the marker position solutions to the known marker
geometry of the tools, as described in
unused solutions are phantom markers, reflections, or other IR sources, and are not reported by the
system.
When the stray marker reporting functionality is enabled, it is necessary to eliminate the reported
phantom markers using application software.
3.12
Passive Sphere Markers
NDI passive sphere markers have a retro-reflective coating that reflects IR light back to its source
instead of scattering it. The IR light from the Position Sensor illuminators reflects off the passive
markers directly back to the sensors.
The Polaris Vicra System can report the positions of passive markers individually, and calculate the
position and orientation of tools that incorporate them. The measured position is the centre of the
marker. When a passive sphere marker is attached to an NDI mounting post, the centre of the sphere
is located at the top of the mounting post.
38
Real Markers
Figure 3-8 Phantom Markers
"Marker Detection and Tool Tracking" on page
Figure
3-8). Where the lines cross each other, the
Figure
3-8. Two are the actual
Phantom Markers
Polaris Vicra User Guide - Revision 2
28. The
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