What technology makes a whiteboard interactive?

Interactive whiteboards use a
variety of technologies to provide
the interface between user and
screen. Infrared whiteboards use
infrared wavelengths directed
across the surface of the board.
When a pen touches the board, it
interferes with the light, allowing
the point of contact to be
calculated. Similarly, ultrasound
whiteboards use the defl ection of
ultrasound waves to detect a
stylus’s movements.
Some whiteboards, like the
eBeam made by Luidia pictured
here, do not actually need an
electronic board at all, but use a
detector which is mounted to the
side of a traditional whiteboard.
The eBeam system actually
combines both infrared and
ultrasound. By using the two
together, differences in the time
taken for the waves of light and
sound to travel across the board
enable the location and direction
of the pen to be pinpointed. This
works in a similar way to
predicting the distance of a storm
based on the time delay between
seeing the lightning strike and
then hearing the thunder.