Photo telegraphy is a method of transmitting photographs or images between two locations using radio or telegraph systems. In this process, a photographic print is wrapped around a cylinder and scanned by a focused light spot. The reflected light is converted into a stream of electrical signals, which are sent to a receiving station. There, the signals control an exposing light source that recreates the image on photosensitive material wrapped on a similar cylinder.
This technique, an early form of facsimile or "fax" transmission, was historically important for news agencies and remote communication of images. Photo telegraphy paved the way for modern electronic image transmission, combining principles of photography, optics, and telecommunications.