Abstract

In the year 1880, Alexander Graham Bell patented a device with which a human voice could be transmitted by using a beam of intensity modulated light. He considered this instrument, the photophone, to be his greatest invention, “…even greater than the telephone.” The original photophone used a thin silvered glass plate to reflect a beam of sunlight. The voice of someone speaking near the plate caused it to vibrate, which in turn induced slight variations in the divergence of the reflected beam. The receiver used a photoresistive detector made of pure selenium connected to a Bell telephone earpiece. Bell and his assistant managed to communicate over a distance of 240 m with their photophone. We have built a working model of the photophone that respects the principles of operation of the original device. Instead of a beam of sunlight, we use an ordinary flashlight as a source. The modulating diaphragm is virtually identical to that used by Bell. The receiver uses an inexpensive CdS detector, a cousin of Bell's selenium detector. It is easy to understand how the demonstration works, and it is fun to see how a flashlight can be used to talk to someone across the room. Although the photophone never found practical applications, it is one of the earliest examples of optical communication technology.

© 1992 Optical Society of America