The frameless laser harp is the most difficult harp to build and the highest goal. It is also the most exciting one beacuse you dont see it when its off. So you can surprise your audience
It works as follows: The photo cell or photo resistor is located inside the harp. When the hand is inside a beam, it reflects the laser light which can be detected by the photo cell. The only way to make a frameless harp work is using scanners and a powerful laser (at least >50 mw for completely dark rooms, better more). The power of a weak laser is not enough for reflection. To explain the frameless harp, I use Sebastien Pamart’s documentation and illustration:
The beam goes through a hole in a mirror and is scanned by the stepper motor. When the player puts his hand in the beam, the light is reflected by the hand back to the mirror on the step motor. From there, the light is reflected again to another mirror, leading the light through a lens to a photo cell. The photo cell detects the light and changes its state. The electronics knows the position of the mirror (the motor) and can send the corresponding note to MIDI. The scanning algorithm of the electronics is simliar to Jarre’s harp software.
Once again, this is the most difficult harp to build. I strongly recommend trying the framed harp first to earn some experience with lasers, optics and electronics. Remember, especially with the frameless harp, you are dealing with high power lasers. They can and will damage the eye and in case of very high power lasers the skin as well!