1.2 Existing optical systems
Realtime Tracking . Both of these systems are briefly described below, followed by a description of a system under development by a New Zealand research institute, Industrial Research Limited (IRL) .
The HiBall tracker
The HiBall optical tracker  began life at the University of North Carolina at Chapel Hill in the early 1990s. The project uses arrays of ceiling mounted infra-red LEDs as markers. The tracking sensor is a small device that the designers have termed a HiBall. This sensor uses Linear Effect Photo Diodes (LEPD) to detect the positions of the LEDs relative to the HiBall. These LEPD devices produce a current proportional to the position of the light that falls on the diodes. The HiBall has a complex optical design that allows the number of LEPDs to be minimised while still providing a large Field of View (FOV). The system uses
many hundreds of LEDs mounted precisely in ceiling panels. Using a Kalman Filter  based approach the tracker is able to accurately produce a pose estimate. The system can generate 2000 pose estimates per second with less than 0.5 mm and 0.02 degrees of position and orientation noise. This research aims to prototype a similar system to the HiBall tracker but it is intended to use cameras instead of LEPDs and to use fewer LEDs.
dvanced Realtime Tracking system
n example of an outside-looking-in tracking system is produced by a German company named dvanced Realtime Tracking. They produce tracking hardware and software named RTtrack and DTrack . In contrast to the HiBall tracker this system uses specially de- signed cameras that emit flashes of infra-red light. These cameras surround the object being tracked. Light from the cameras bounces off reflective spheres attached to the ob- ject being tracked and is reflected back to the cameras. The orientation and position of the spheres are known with respect to a reference point and this information is exploited to allow the object’s pose to be calculated. This system has the advantage that many objects
can be tracked concurrently as reflective markers are cheap and can be attached easily.
indication of the performance of this system could not be found on the company website.
Industrial Research Limited’s prototype system
Industrial Research Limited (IRL), a New Zealand research institution, has published pa- pers detailing their research into an optical scanning system [21, 22]. This system uses infra-red LEDs as does the HiBall tracker but uses cameras to sense the position of the LEDs instead of using LEPDs. The cameras are mounted in a fixed orientation and position and observe the infra-red LEDs placed on the outside of the system. Using the images from the cameras, the system is able to determine the position and orientation of the cameras.