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How did Lee De Forest’s Phonofilm system record sound directly onto film?

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Zazu Pitts, Gibson Gowland, and Hughie Mack in a dramatic scene from the silent film Greed (1924); Gowland stands center, supported by Pitts and Mack on either side. — Silent film era (From left) Zasu Pitts, Gibson Gowland, and Hughie Mack in the silent film *Greed* (1924), directed by Erich von Stroheim.

Lee De Forest’s Phonofilm system recorded sound directly onto film using an optical recording process. To capture the audio, the system utilized a gas-filled glow discharge lamp operated by a telephone transmitter. This lamp produced a beam of light that was modulated by the electrical sound waves being recorded. This modulated light was then focused onto the edge of the film, creating a photographic sound track.12

During projection, the sound was reproduced by directing a beam of light through this recorded sound track. The light passing through the track was detected by a light-sensitive photocell, which converted the variations in light intensity back into electrical signals. These signals were then electronically amplified to drive loudspeakers, allowing for synchronized audio playback.12