Webster IRP 4100 Installation Manual page 9

For the scanner data to accurately show the pen position on the
writing surface, the scanned laser beams are aligned in a pre-
determined pattern with respect to the writing surface, both in
azimuth and in pitch. Additional encoded retro-reflectors (calibration
strips) located at the corners of the writing surface provide feedback
for establishing and maintaining these alignments.
In addition to being a passive element that reflects the laser scanner
beams for positional information, the electronic pen is an active
source of information. The status of the pen tip-switch and the two
barrel button switches are transmitted via a separate IR link to a
separate IR receiver located behind the translucent writing surface.
There the signals are decoded and sent on to the Digital Signal
Processor, where this additional information is combined with the
pen position before being sent to the host computer.
OVERVIEW 7