Editing Photophone (section)

==Further development==
[[File:Ernst Ruhmer, Technical World cover (1905).jpg|thumb|200px|right|Ernst Ruhmer at his "photo-electric" optical telephone system station. (1905)<ref>[https://babel.hathitrust.org/cgi/pt?id=mdp.39015057148481;view=1up;seq=13 Cover page] ''Technical World'', March 1905.</ref>]]
Although Bell Telephone researchers made several modest incremental improvements on Bell and Tainter's design, [[Guglielmo Marconi#Radio work|Marconi's radio transmissions]] started to far surpass the maximum range of the photophone as early as 1897<ref name="Mims 1982, p. 14"/> and further development of the photophone was largely arrested until German-Austrian experiments began at the turn of the 20th century.

The German physicist [[Ernst Ruhmer]] believed that the increased sensitivity of his improved selenium cells, combined with the superior receiving capabilities of professor H. T. Simon's "speaking arc", would make the photophone practical over longer signalling distances. Ruhmer carried out a series of experimental transmissions along the [[Havel|Havel river]] and on Lake Wannsee from 1901 to 1902. He reported achieving sending distances under good conditions of 15 kilometers (9 miles),<ref>[https://babel.hathitrust.org/cgi/pt?id=nyp.33433090837562;view=1up;seq=133 "Correspondence: Wireless Telephony"] (October 30, 1902 letter from Ernst Ruhmer), ''The Electrician'', November 7, 1902, page 111.</ref> with equal success during the day and at night. He continued his experiments around Berlin through 1904, in conjunction with the German Navy, which supplied high-powered searchlights for use in the transmissions.<ref>[https://babel.hathitrust.org/cgi/pt?id=uc1.$b33006;view=1up;seq=73 ''Wireless Telephony In Theory and Practice''] by Ernst Ruhmer, 1908, pages 55–59.</ref>

The German [[Siemens & Halske|Siemens & Halske Company]] boosted the photophone's range by utilizing current-modulated carbon arc lamps which provided a useful range of approximately {{convert|8|km|mi}}. They produced units commercially for the [[German Navy]], which were further adapted to increase their range to {{convert|11|km|mi}} using voice-modulated ship [[searchlight]]s.<ref name="Groth"/>

[[British Admiralty]] research during WWI resulted in the development of a vibrating mirror modulator in 1916. More sensitive [[molybdenite]] receiver cells, which also had greater sensitivity to infra-red radiation, replaced the older selenium cells in 1917.<ref name="Groth"/> The United States and German governments also worked on technical improvements to Bell's system.<ref>Mims 1982, pp. 14–17.</ref>

By 1935 the German [[Carl Zeiss AG|Carl Zeiss Company]] had started producing infra-red photophones for the [[German Army (Wehrmacht)|German Army]]'s tank battalions, employing tungsten lamps with infra-red filters which were modulated by vibrating mirrors or prisms. These also used receivers which employed [[lead sulfide]] detector cells and amplifiers, boosting their range to {{convert|14|km|mi}} under optimal conditions. The Japanese and Italian armies also attempted similar development of lightwave telecommunications before 1945.<ref name="Groth"/>

Several military laboratories, including those in the United States, continued R&D efforts on the photophone into the 1950s, experimenting with high-pressure vapour and mercury arc lamps of between 500 and 2,000 watts power.<ref name="Groth"/>