RONJA

Single high-brightness LED with a cheap loupe lens creates a bright narrow beam that can stream DVD-quality video over neighbourhoods. The red beam is invisible when observed outside of its unobstructed path.
Twibright Ronja with 130-millimetre (5.1 in) diameter lenses, operating on a 1,205-metre (1,318 yd) link using visible red light, max. range 1,300 metres (1,400 yd), with HPWT-BD00-E4000 transmitter LED. Installed on a rooftop, with its user posing to the right, in Czech Republic.
Three bolts preloaded with pink rubber blocks facilitate fine adjustment of the optical head direction with a gear ratio 1:300. The bolt on the right side is a part of a rough adjustment mechanism which allows pointing the optical head in needed direction.
Artificially enhanced picture of fog interfering with a RONJA beam, compromising the connection by introducing interference

RONJA (Reasonable Optical Near Joint Access) is a free-space optical communication system developed in the Czech Republic by Karel Kulhavý of Twibright Labs. Released in 2001. It transmits data wirelessly using beams of light. Ronja can be used to create a 10 Mbit/s full duplex Ethernet point-to-point link. It has been estimated that 1,000 to 2,000 links have been built worldwide.

The basic configuration has a range of 1.4 km (0.87 mi). The device consists of a receiver and transmitter pipe (optical head) mounted on a sturdy adjustable holder. Two coaxial cables are used to connect the rooftop installation with a protocol translator installed in the house near a computer or switch. By increasing the diameter of the lens and transmitter pipe diameter, the range can be extended to 1.9 km (1.2 mi).

Building instructions, blueprints, and schematics are published under the GNU Free Documentation License, with development using only free software tools. The author calls this approach "User Controlled Technology", emphasising their view on the importance of open-source and user-driven software and innovation

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