RONJA (Reasonable Optical Near Joint Access) is a free-space optical communication system originating in the Czech Republic, developed by Karel Kulhavý of Twibright Labs and 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 1000 to 2000 links have been built worldwide. The range of the basic configuration is . 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. The range can be extended to by . Building instructions, blueprints, and schematics are published under the GNU Free Documentation Licence. Only free software tools are used in the development. The author calls this level of freedom "User Controlled Technology". Ronja is a project of Twibright Labs. Manufacture The building instructions are written with an inexperienced builder in mind. Basic operations like drilling, soldering etc., are explained. Several techniques – drilling templates, detailed checks after soldering, testing procedures – are employed to minimize errors at critical places and help to speed up work. Printed circuit boards are downloadable ready for manufacture, with instructions for the fabhouse. People with no previous experience with building electronics have reported on the mailing list that the device ran on the first try. 154 installations worldwide have been registered into a gallery with technical data and pictures. Range With the brightest variant of Lumileds HPWT-BD00-F4000 LED and 130 mm diameter cheap magnifying glass lenses, the range is . The dimmer but more affordable E4000 variant of HPWT-BD00 yields . The speed is always 10 Mbit/s full duplex regardless of the distance. Models Ronja Tetrapolis: Range of , red visible light. Connect with 8P8C connector into a network card or switch. Ronja 10M Metropolis: Range of , red visible light. Connects to Attachment Unit Interface. Ronja Inferno: Range of , invisible infrared light. Ronja Benchpress: A measurement device for developers for physical measurement of lens/LED combination gain and calculation of range from that Ronja Lopipe: The original (discontinued) design using red visible light and a RS232 interface for a max 115 kbit/s PPP/SLIP link. Limitations By definition, clear visibility between the transmitter and receiver is essential. If the beam is obscured in any way, the