Boris Landoni from Open Electronics writes about an open source project laser reflection barrier:
It allows us to detect the presence and passage of an object thanks to the combination of a laser diode which emits a light ray and a phototransistor which detects reflected light.
Robotics applications and industrial control systems normally make use of optical systems in order to detect proximity and passing of objects, taking advantage of light interruption or light reflection on a surface of the object to be detected. In this article, we want to show you the project of a laser barrier: however this is not an interruption-type barrier, which needs the object to be detected to pass through a meter and a photodetector, in fact, this is a reflection barrier: in our circuit, a laser projects a ray of focused and infinitely-collimated light and any object passing in front of it will reflect a portion of it, which will be intercepted by a lens on its way back and focused on the sensible surface of a photo-sensible component.
Full details at Open Electronics project page.