Telephone communication is possible by modulating a lightbeam. The Germans built such a system which can be seen as a predecessor of the fibreoptic technics of today. Along the Atlantikwall you find many Vf bunkers specially designed for these instruments. Communication with small islands for the coasts was made possible with a small lightbulb of 5 Watts. In the Dutch Scheldt estuary two sets placed in Tobruks were used. But of course the system is light depended, so heavy rain and snow causes interruptions. The maximum distance was 4 kilometers. To obscure the transmission at night an infrared filter could be placed. This reduced the range to 2.5 kilometers.
The optical unit.
An other specimen.
Amplifier and control unit. For communication two sets are needed. You can speak and listen at the same time. (2-way traffic)
The coverplate has been temporarily removed.