Farnsworth’s Image Dissector

The optical scene becomes an electron image.

Visible light from the scene was focused onto a specially coated photosensitive detector at the front of the evacuated tube. Brighter regions released more electrons than darker regions, so the emitted charge distribution preserved the spatial brightness pattern.

The resulting “electron image” existed inside the tube rather than as a mechanical pattern of holes. An anode established the field needed to collect and direct the emitted electrons.

A small aperture samples one area at a time.

The tube measured only a small part of the electron image at any instant. Deflection fields moved the electron distribution across a fixed aperture so successive image locations contributed to the output current.

Repeated horizontal and vertical scanning converted the whole image into a time-varying electrical signal. The method performed the same space-to-time conversion as Nipkow's disk but without rotating the image scanner.

The camera becomes fully electronic.

Farnsworth's 1927 design demonstrated the central principle of an electronic television camera: convert photons into electrons, scan electrically and produce a serial video signal. Later camera tubes improved sensitivity by storing charge between scans, but the image dissector was a decisive step away from mechanical television.