Eighty years ago, this well dressed gentleman was sending a message through this light beam transmitter, as described in the August 1941 issue of Popular Science.
The magazine was a bit prophetic, since there was more experimentation with the idea after Pearl Harbor. When hams were ordered off the air for the duration of the war, optical communications was one of the ways they continued to practice their hobby (along with carrier current communications.)
The idea is simplicity itself, and it’s an idea that I independently invented as a youngster 30 years later, and I know others did as well. The light beam of a flashlight is modulated with an audio signal by wiring the light in series with the secondary of a transformer. The primary of the transformer is wired with a buzzer and key to send Morse code. When I did the same thing, I hooked the transformer primary to the output of a transistor radio or other audio amplifier, allowing voice signals to be sent.
The receiver is an audio amplifier, with a photoelectric tube hooked to the input. In my later experiments, I used a solar cell hooked directly to the input of an amplifier, which didn’t seem to mind the small DC voltage.
The student looking for an interesting project for a science fair could easily duplicate this experiment with modern equipment, at very low cost. It’s an easy demonstration of a communications device dependent only upon light.