Editing Photophone (section)

==Design==
[[File:Photophony1.jpg|thumb|A photophone receiver and headset, one half of Bell and Tainter's optical telecommunication system of 1880]]

The photophone was similar to a contemporary telephone, except that it used [[free-space optical communication#Visible light communication|modulated light]] as a means of wireless transmission while the telephone relied on [[modulation|modulated]] [[electricity]] carried over a conductive [[two-wire circuit|wire circuit]].

Bell's own description of the light modulator:<ref name="Clark"/>

{{Blockquote
| 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 brightness of a reflected beam of light, as observed from the location of the receiver, therefore varied in accordance with the audio-frequency variations in air pressure—the sound waves—which acted upon the mirror.

In its initial form, the photophone receiver was also non-electronic, using the [[photoacoustic effect]]. Bell found that many substances could be used as direct light-to-sound transducers. [[Lampblack]] proved to be outstanding. Using a fully modulated beam of sunlight as a test signal, one experimental receiver design, employing only a deposit of lampblack, produced a tone that Bell described as "painfully loud" to an ear pressed close to the device.<ref>{{cite journal |last1=Bell |first1=Alexander Graham |title=The Production of Sound by Radiant Energy |journal=Science |date=28 May 1881 |volume=2 |issue=48 |pages=242–253 |doi=10.1126/science.os-2.49.242 |jstor=2900190 |pmid=17741736 |url=https://www.jstor.org/stable/2900190 |access-date=11 Oct 2022}}</ref>

In its ultimate electronic form, the photophone receiver used a simple [[selenium#Other uses|selenium cell]] [[photodetector]] at the [[focus (optics)|focus]] of a parabolic mirror.<ref name="Groth"/> The cell's [[electrical resistance]] (between about 100 and 300 [[ohm]]s) varied inversely with the light falling upon it, i.e., its resistance was higher when dimly lit, lower when brightly lit. The selenium cell took the place of a carbon microphone—also a variable-resistance device—in the circuit of what was otherwise essentially an ordinary telephone, consisting of a battery, an electromagnetic earphone, and the variable resistance, all connected in series. The selenium modulated the current flowing through the circuit, and the current was converted back into variations of air pressure—sound—by the earphone.

In his speech to the American Association for the Advancement of Science in August 1880, Bell gave credit for the first demonstration of speech transmission by light to Mr. A.C. Brown of London in the Fall of 1878.<ref name="Groth"/><ref name="Bell"/>

{{anchor|Radiophone}}Because the device used [[radiant energy]], the French scientist [[:fr:Ernest Mercadier|Ernest Mercadier]] suggested that the invention should not be named 'photophone', but 'radiophone', as its mirrors reflected the Sun's radiant energy in multiple bands including the invisible [[infrared|infrared band]].<ref name="Grosvenor & Wesson">Grosvenor and Wesson 1997, p. 104.</ref> Bell used the name for a while but it should not be confused with the later invention "[[radiophone]]" which used [[radio wave]]s.<ref>Ernest Victor Heyn, Fire of genius: inventors of the past century: based on the files of Popular Science Monthly since its founding in 1872, Anchor Press/Doubleday – 1976, page 74</ref>