Markers: Active Markers
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- Big active Marker
Active markers are light (IR) emitting elements, mostly LEDs. In A.R.T. tracking systems three types of LED-based active markers are used, depending on the application:
- Single LEDs: can be covered with acryl protection sheet, gives simple and robust markers providing visibility up to high distances – however the angular range of visibility is limited to ca. +/- 60°.
- Single LEDs with diffusor sphere: for optimum angular range of visibility, however distance between marker and tracking camera is limited.
- Big active markers: several single LEDs per marker, covered with light scattering spheres. Provide visibility from all sides and up to very high distances, however pretty big and heavy.
All active markers provided by A.R.T. are controlled by a special pcboard (for synchronization, see below) and need power supply.
Synchronization of active markers:
Active markers could in principle be activated in CW mode (continuous light emission). However this would not be very clever, because tracking cameras have a very narrow time slot of sensitivity, i.e. most of the light emitted by the markers would be useless for tracking. As a consequence, maximum distance between cameras and marker would be very low, due to an upper limit of power dissipation allowed for each single LED. Therefore all active markers provided by A.R.T. are just emitting radiation when the tracking cameras are sensitive, thus have to be synchronized with the cameras. Synchronization can be done by a wired connection between the tracking system and the pcboard controlling the active markers, but can also be done in a wireless way: For wireless synchronization a coded IR flash is being sent out by a tracking camera. The active markers pcboard recognizes the coded flash and activates the LEDs.
