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The priciple is simple. It was invented by people from the Stone Age. You make a signal that is visible from distance. First used method was a smoke from a fire. As it lacked an usable bit rate, Mr. Chap invented his Optical Telegraph. The bit rate increased, latency decreased, but it was still remarkably slow.
Military men sometimes use so called heliograph which is a box that contains a strong light source and a bunch of flaps that can be shut close or jerked open. Using this stuff you can communicate over very long distances in the night. But the baud rate is not great, about that you reach when use hand-keying of Morse Code.
NASA has placed a retroreflector on the Moon and uses it and a laser beam to measure the distance between Moon and Earth. The signal is entirely optical.
Ronja emits ifrared (Ronja 115 Loopipe, 875 nm) or visible (future Ronja 10M Bithazard, 650nm) beam of light that is permanently aimed at the target. The target contains a refractor telescope (actually a very cheap one, based on a cheap loupe ;-) ) to focus the beam onto a sillicon detector from which the signal is directly fed into a sensitive, low-noise, wideband amplifier. Behind the amplifier, there is a comparator that decides what is one and what is zero. After that, you decode the binary signal into bit sequence and receive it in your PC.
The trasmitter beam is generated by one LED and a glass lens (Ronja 115 Loopipe) or a laser diode from a laser pointer and old projector lens (Ronja 10M Bithazard). The infrared beam has the centre wavelength of 875 nm and the visible one 650 nm (red). The infrared beam is not visible and it is not recommended to stare deliberately into the transmitter, especially in the night, because it's like you stared into a table lamp and your eyes didn't accomodate the light because infrared is not visible. However, it is not eye-hazardeous, because the same LED's are used in notebooks and palmtops for IrDA communications at the same powers, and the lens only magnifies the source, but doesn't increase the brightness (actually slightly decreases). So the energy concentration on the retina remains the same. The beam is 90mm wide (Loopipe) or elliptical, with the major axis 52.5mm (Bithazard).
The 650nm is a child's toy: it' visible in the night with streetlamps on for about 700 meters, so alignment is simple using a retroreflector installed on the other side. Only the holder must be strong enough not to move even the tiniest bit int the strongest wind.
At Loopipe, the misalignment may be as big as about half an angular degree and still no great performance decrease is observable. The light is not visible so aligment is more difficult than in case of Bithazard.
The holders for Loopipe must be firm and attached to some hard, solid, non-moving surface. It's not a general problem because most houses are not moving more than half an angular degree. Usual house moving more than half an angular degree cracks and falls apart ;-) This is the great advantage against laser systems that suffer from wooden building movements and so they must be realigned avery month almost everywhere. Concrete buildings are OK for laser devices (Bithazard).
In case of unrestricted visibility, the device operates reliably with constant bit error ratio dependent on the distance. Only distances up to 400 meters are usable for Loopipe, greater distances cause annoying dropouts due to weakening signal.
The modulation in Loopipe is 3/16 IrDA SIR modulation in case of Ronja 115 Loopipe or Ethernet modulation interspered with 1MHz square wave in case of Bithazard. The square wave is there for keeping the receiver accomodated to the right level of light. It is ignored by connected network card.
The reciever in Loopipe is sensitive only to infrared (It's a HSDL5420). The receiver in Bithazard is a SFH 203, which is sensitive both to infrared and visible spectrum.