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Light Amplifier

Wireless World, June 1, 1939.
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The principle of regeneration applied to a photo-electric cell in conjunction with an electron multiplier.

The point at which incident light is first converted into signal current by photo-electric action is one of the weakest links in the chain of television. The Iconoscope tube, with its 'storage' screen of mosaic cells, goes some way towards restoring the balance, whilst the electron-multiplier offers still further possibilities. Another promising scheme is to apply the principle of reaction to 'boost' the output from a photo-electric cell. For instance, the electrons liberated by the impact of light upon a photo-sensitive cathode can be focused upon a fluorescent screen, and the light from the latter 'fed back' on to the sensitive cathode, where it serves to increase the strength of the original stream.

The accompanying drawing (from Patent 499661) shows a 'back-coupled' photo-electric cell of this kind used in combination with an electron-multiplier, the latter serving to emphasise the regenerative effect. The cell contains a photoelectric cathode C which is arranged opposite to a fluorescent screen F, an electron-multiplier M being placed midway between the two and in line with a transparent screening partition P. Light falling on the cathode C liberates primary electrons, which are attracted by the positive voltage on the first electrode of the multiplier M and pass from it through a series of permeable target-electrodes, each biased more positively than the last.

The emerging stream, now considerably amplified by secondary emission, produces a more intense light than usual on the fluorescent screen F. This light 'reacts' back on the sensitive cathode C to liberate more electrons, and so build up the current strength still further until it is taken off from the last or output electrode of the multiplier M. To prevent the current from building-up to saturation a 'quenching' frequency is applied to the screen electrode P, together with an EMF of carrier frequency.

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