As a player places a finger on the string the beam is shortened and light is reflected back to photo elements at the bridge
If you've tried to digitally capture a guitar performance or turn your six-string into a powerful MIDI synth using an interface or special pickup, you'll no doubt be familiar with signal lag and pitch accuracy issues. As you pluck a string, there is a very short delay before it reaches its final frequency - it might only be a few milliseconds but it can be enough to cause latency angst. Germany's M3i Technologies has developed an optical system for accurately detecting the pitch of a string - even before a player has plucked it. The Laser Pitch Detection (LPD) Pickup system runs a beam under each string, from the bridge to the end of the fingerboard, and monitors its return to photosensitive receptors in the unit. When a player's finger touches a string, the beam is shortened and the system quickly measures and calculates the exact pitch.