LIGHT WEDGE MODULATION:
Another scheme for modulating the light falling on the film on the cylinder, consists in placing a light wedge against the face of a lens which images the vibrating mirror (or light source) on the film.
As the light is constantly imaged on the film by the lens its slight displacement toward the dark end of the light wedge by the vibration of the mirror decreases the strength of the light falling on the film, while displacement toward the thin edge of the light wedge gives greater exposure on the film.
It is obvious, therefore, that the vibration of the mirror determines the exposure at successive positions on the film; and as these displacements follow the varying strength of the incoming electric current, and the latter in turn is determined by the light values of the picture at the sending station, it naturally follows that when the film is developed a duplicate of the distant picture results.