Page "Video camera tube" Paragraph 44

In the image store, light falls upon the photocathode which is a photosensitive plate at a very negative potential ( approx.

-600 V ), and is converted into an electron image ( a principle borrowed from the image dissector ).

This electron rain is then accelerated towards the target ( a very thin glass plate acting as a semi-isolator ) at ground potential ( 0 V ), and passes through a very fine wire mesh ( near 200 wires per cm ), very near ( a few hundredths of cm ) and parallel to the target, acting as a screen grid at a slightly positive voltage ( approx + 2 V ).

Once the image electrons reach the target, they cause a " splash " of electrons by the effect of secondary emission.

On average, each image electron ejects several " splash " electrons ( thus adding amplification by secondary emission ), and these excess electrons are soaked up by the positive mesh effectively removing electrons from the target and causing a positive charge on it in relation to the incident light in the photocathode.

The result is an image painted in positive charge, with the brightest portions having the largest positive charge.