Photophone is a device for transmission of sounds using light.
The photo was invented by Alexander Bell and his assistant Charles Tainter on February 19, 1880 Bell considered the photophone the most important of his invention. On June 3, 1880, Bell delivered the first wireless telephone message.
The action of the device is based on the property of the selenium to change the electrical conductivity under the effect of light rays reflecting from a mirror vibrating under the influence of sound.
Crystalline selenium cells are used as a recipient of the signal. The material changes the electrical conductivity under the effect of light rays, that is, the conductivity is higher when it is darker and lower when it is brighter. The principle of the photophone, thus, was modulating the light beam: as a result of different lighting, the receiver will create a corresponding different resistance in selenium cells, which is used to recreate the sounds received by the receiver. The light beam modulation was made by a vibrating mirror: the thin mirror was alternated between the concave and the convex shapes and thus the focus or dispersion of the light from the light source. The photophone works analogy with the phone only with the difference that the photophone uses light to provide information, while the phone uses electricity.
In Washington, during the experiment, Bell and Tainter conducted a confirmed transmission at a distance of about 213 meters, using ordinary sunlight as a light source. The recipient of the signal was a parabolic mirror with selenium cells at the focus point. Selenium cells changed the electric resistance from 300 to 100 Om.
Although the photophone was an important invention, it was many years before Bell's work was fully recognized. Bell's original photoophone couldn't protect the transmission from external interference like clouds that easily violated the transmission. Before the development of modern fiber optics, the lack of technology for reliable light delivery prevented the use of Bell's invention. Bella's photophone has been recognized as the predecessor of modern fiber optic communication lines, which are currently transmitted to 80 % of the world's telecommunication traffic.
