Page "Video camera tube" Paragraph 15

The optical system of the image dissector focuses an image onto a photocathode mounted inside a high vacuum.

As light strikes the photocathode, electrons are emitted in proportion to the intensity of the light ( see photoelectric effect ).

The entire electron image is deflected and a scanning aperture permits only those electrons emanating from a very small area of the photocathode to be captured by the detector at any given time.

The output from the detector is an electric current whose magnitude is a measure of the brightness of the corresponding area of the image.

The electron image is periodically deflected horizontally and vertically (" raster scanning ") such that the entire image is read by the detector many times per second, producing an electrical signal that can be conveyed to a display device, such as a CRT monitor, to reproduce the image.