Active Markers

Marker comparison

Some optical tracking systems use coded black-and-white labels as passive markers, however these require pretty time consuming image processing and the quality of tracking is usually sensitive to changes of ambient light conditions. Therefore the passive markers used in A.R.T. tracking systems (as well as in most competing systems) are retro reflectors. I.e., these markers are backreflecting the incoming IR radiation into the direction of the incoming light. More precise: the IR radiation is backreflected into a narrow range of angles around the (opposite) direction of the incoming light. These markers are mostly spheres covered with retro reflecting foils, but can also be stickers made from retro reflecting material.

Retro reflecting sheets or foils available on the market can be based on two different optical principles:

 

1. Three mirrors, arranged in 90° angles to each other, are reflecting light in the described way. Mostly foils with arrangements of lots of very small mirrors in a plane are used.
2. Glass balls (with a proper refraction index) are focussing incoming light approximately to the opposite surface of the ball. A layer of microscopic glass balls, carried by a reflecting material, acts as a retro reflector. These foils can be fabricated on a flexible carrier material, thus they are widely used for equipping spherical markers with retro reflecting surfaces.