(((bruces remarks: It's a fact that the Germans had video telephones in the 1930s, but if the Nazis had light- transmitting telephones, that would indeed be a rare and remarkable dead medium.)))
I saw a strange WWII German military optical telephone apparatus in an electronic surplus store in Sweden in the sixties. I cannot recall many details, but I think the transmitting unit was an infrared sound modulated lamp. and the receiving unit was a binocular with some photoelectrical detector. I also think that the user interface or I/O unit was a telephone handset.
Is there a Necronaut out there with more information?
Lars-Erik Astrom (firstname.lastname@example.org)