The photophone is a telecommunication device that allows transmission of speech on a beam of light. It was invented jointly by Alexander Graham Bell and his assistant Charles Sumner Tainter on February 19, 1880, at Bell’s laboratory in Washington, D.C. Bell believed the photophone was his most important invention. Of the 18 patents granted in Bell’s name alone, and the 12 he shared with his collaborators, four were for the photophone, which Bell referred to as his “greatest achievement”, telling a reporter shortly before his death that the photophone was “the greatest invention [I have] ever made, greater than the telephone”. The photophone was similar to a contemporary telephone, except that it used modulated light as a means of wireless transmission while the telephone relied on modulated electricity carried over a conductive wire circuit.
Bell’s own description of the light modulator:
“We have found that the simplest form of apparatus for producing the effect consists of a plane mirror of flexible material against the back of which the speaker’s voice is directed. Under the action of the voice the mirror becomes alternately convex and concave and thus alternately scatters and condenses the light.”
The photophone receiver used a simple selenium cell at the focus of a parabolic mirror. The cell’s electrical resistance (between about 100 and 300 ohms) varied inversely with the light falling upon it, i.e., its resistance was higher when dimly lit, lower when brightly lit.
The photophone was a precursor to the fiber-optic communication systems that achieved worldwide popular usage starting in the 1980s. The master patent for the photophone (U.S. Patent 235,199 Apparatus for Signaling and Communicating, called Photophone) was issued in December 1880, many decades before its principles came to have practical applications.
