Probably the best modern interpretations of the 1930’s free space optical communications device Lichtsprechgerät 80 are by Helge Fykse and by Dirk Rollema. Rollema’s article goes into excellent detail on the transmitter, receiver, optics and electronics. Here is a link to a recent youtube video on the Li Spr 80 photophone in operation.
These light-beam voice communications devices were in operation by Germany in 1935 and the range was 2-4km, depending on conditions. The Lichtsprechgerät 80 was developed and manufactured by Carl Zeiss. The RX used three RV2P800 pentodes and the TX used two RV2P800 pentodes, where the filament voltage was 2v and B+ was 60v. The filament batteries would last 20 hours and the B battery could last up to 100 operating hours.
The Li Spr 80 had a big brother, the “250/130” and it was said to have a range of >30km, again depending on conditions.
Remarkable for 1935. There are indications that these sets were made in large quantities, and I have yet to see one in person.
 Dirk W. Rollema, PA0SR, “Optical Communications – 1935 Style,” Electronics and Wireless World, p.46-49, August 1985. NB that his amateur radio call sign was PA0SE, not PA0SR. Looks like a typo. Unfortunately, Dirk is SK (2016).