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Photoemissive cell

A photoemissive cell is also known as a phototube that operates on the photoelectric effect. It consists of a photosensitive cathode(negative) from which electrons are emitted when hit by light. It also contains an anode(positive) that attracts these free electrons. Both cathode and anode are enclosed in glass or quartz. When the electrons are drawn from cathode to anode, an electric current is generated proportional to the fallen light.

Types of Photoemissive cell

Vacuum type cell

In a vacuum phototube, a rod anode and curvature cathode are housed in a vacuum glass. The current is generated immediately on light incidence and is proportional to the brightness of the light.

Gas-filled cell

A similar arrangement as the vacuum tube except, inert gas like argon is filled in the tube and emitted electrons by the light are accelerated by the electric field and cause ionization thereby increasing the current due to high collision.

Photomultiplier cell

It contains evacuated glass which is occupied by multiple dynodes in addition to anode and cathode. Each dynode has a higher electrical potential than the previous one. Electrons emitted from the cathode strike the first dynode that emits multiple electrons which in turn hits the next higher potential dynode and more electrons are emitted and is repeated forward. The anode is placed at the end and a large number of electrons reaching the anode results in a current pulse which is detected easily. It is highly sensitive and has a high-frequency response.

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