How does optical tracking work?

The object whose position shall be tracked is equipped with markers. Markers can be light reflectors („passive markers“, e.g. retroreflectors) or light emitters („active markers“, e.g. LEDs). To also measure the orientation of a body, several (>= 3) of these markers have to be arranged at a known geometry. 

Tracking cameras scan a certain volume and detect the light that comes from the markers. Their images are processed to identify and calculate potential marker positions (in image coordinates, 2DOF) with high accuracy; a mean accuracy of 0.04 pixels is standard in ART tracking systems

These 2DOF data are combined to calculate 3DOF positions of single markers or 6DOF poses of rigid arrangements of several markers („rigid bodies“ or „targets“). Some additional information about the tracking system is necessary for this, which has to be collected in calibration processes beforehand: position and orientation of the tracking cameras, as well as the geometry of rigid bodies (i.e. the positions of the markers within a body). 

The result of each measurement are coordinates that describe the position of the markers, and hence the position and orientation of the body carrying the markers. 

The ART setup

ART setup

ART tracking cameras are equipped with CCD or CMOS image sensors, working in the near infrared light spectrum. An infrared light flash (not visible for the human eye) illuminates the measurement volume periodically.

The figure below shows the principles of the system setup: At least two tracking cameras are connected with the central unit („ARTTRACK Controller“ or ATC) via ethernet or firewire, depending on the camera type.
The ATC supplies a synchronization signal for the tracking cameras; it allows the input of an external synchronization signal (rectangular or video sync signal), or is able to generate an internal sync signal. All sync signals are transferred via BNC connections or twisted pair cables to the cameras, depending on the camera type. Note that in SMARTTRACK tracking systems two cameras and the central unit are combined within a common housing.

Two central units might be connected to form a larger tracking system („Cascaded System“, refer to the TRACKPACK System for details). In this way, it is possible to use up to 16 tracking cameras per system.

The central unit is also equipped with an Ethernet port for system control and data output. The tracking software is managed by DTrack2 software, which can be installed on the PC of your choice; it's available for Windows® and Linux systems.


Step by step: How do ART tracking systems work?

Step 1:
Measurement volume is illuminated by an IR flash and camera images are taken.

Step 2:
The image processing recognizes the markers and calculates the marker positions in image coordinates (2DOF) with high accuracy.

Step 3:
3DOF marker positions and/or 6DOF target poses are calculated within the ARTTRACK Controller.

Step 4:
The ARTTRACK Controller transfers 3DOF and 6DOF data via Ethernet to an application software. For a list of graphics software applications that are running with ART tracking systems please click here.