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.

Alexander Graham Bell patented the photophone on December 7, 1880, laying the fundamental groundwork for wireless, optical telecommunications. Bell considered this his greatest invention, stating it was even more significant than the telephone. The instrument functioned by projecting the voice toward a mirror, where the vibrations of the voice caused oscillations in the mirror's shape. This allowed for the transmission of speech on a beam of sunlight rather than through a physical wire.
​The topography of the Colorado Front Range provided an ideal environment for testing these emerging line-of-sight and wireless technologies.
​The Pikes Peak Signal Station
In 1873, the United States Army built a Signal Corps weather station on the summit of Pikes Peak. The original structure was constructed from native stone and served as the year-round quarters for two observers. While the primary mandate of the station was to monitor meteorological phenomenon for the newly established weather service, the altitude provided an unmatched vantage point for testing early signaling and communication methods across the region. The use of this specific high-altitude facility was discontinued around 1888.
​Crystal Park and Wireless Progression
The Manitou Springs region, specifically areas like Crystal Park, remained highly relevant as early radio and telephonic systems evolved. While local publications like Manitou Tidbits document early 1911 tests of wireless telephone services in the park, this aligns with a broader national trend during that decade where inventors were attempting to merge Bell's line-of-sight concepts with Hertzian radio waves.
​By the 1970s, the area was still utilized for communication system testing. Records indicate the continued strategic importance of the Crystal Park site through the testing of communication protocols and hardware, such as the LAP-2000 system. In military and telecommunications research, LAP historically refers to Laser (or Lightwave) Atmospheric Propagation, which is the direct technological descendant of Bell's 1880 photophone—using concentrated light pulses rather than physical infrastructure to transmit data across long distances.
​When reviewing these distinct eras of technological development—from 1880s mirrors to 1970s optical propagation—the historical record shows a continuous, highly documented progression of engineering driven by military and commercial communication needs. It is easy to look back at the dense, overlapping timelines of rapid technological advancement and perceive them as sudden anomalies or intentional structural traps. However, examining the primary source documents and patent filings reveals a steady, linear evolution of physics and material science, rather than manipulated time glitches or coordinated chronologic anomalies.