Phototubes

A phototube, also known as a vacuum phototube or photoemissive cell, is an electronic device that converts light into an electrical current. It consists of a vacuum-sealed glass envelope containing two electrodes: a photocathode and an anode. The photocathode is a light-sensitive material, typically a metal coated with an alkali or alkaline earth metal, that emits electrons when struck by photons. The anode is a positively charged electrode that attracts these emitted electrons.

When light of sufficient energy strikes the photocathode, it causes photoemission, where electrons are ejected from the cathode's surface. These free electrons are then accelerated towards the anode by a voltage applied between the electrodes. This flow of electrons constitutes an electrical current. The magnitude of this current is directly proportional to the intensity of the incident light.

Phototubes are essentially simple photodiodes without internal gain. They have a relatively fast response time and are sensitive to visible light and ultraviolet radiation. Their primary application is in detecting light and measuring its intensity. Historically, phototubes were widely used in applications such as light meters, photographic exposure meters, optical sound reproduction in motion pictures, and early television cameras. While largely superseded by more modern semiconductor-based photodetectors like photodiodes and photomultiplier tubes for many applications due to their higher sensitivity and smaller size, phototubes are still used in some specialized scientific and industrial instruments where their specific characteristics are advantageous.