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What is a Phototube?

By D. Monda Dill
Updated May 16, 2024
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A phototube, also known as a photoelectric tube, is a light-sensitive electronic device. It is designed to emit an electric current when exposed to light or another form of electromagnetic radiation. The phototube has earned the moniker "electric eye," because of its use in a wide range of light-sensing applications.

There are three categories of phototubes: gas, vacuum, and electron multipliers. The basic phototube is made up of a gas-filled or vacuum electron tube in which two electrodes, a cathode and anode, are mounted. A gas phototube utilizes argon, or some other type of inert gas, at a very low pressure.

The electron multiplier is a modification on the basic gas and vacuum phototube. In addition to two electrodes, this type of phototube is also equipped with a series of metal disks called dynodes. The dynodes are strategically arranged inside the electron tube to amplify the emission of electric current.

The surface of the cathode in a phototube is typically coated in cesium or some other type of photo-emissive material. This coating is extremely sensitive to light in the ultraviolet, near-infrared, and visible ranges of the electromagnetic spectrum. A phototube works by means of a process called the photoelectric effect, whereby light falling onto a photo-emissive surface causes electrons to be released from that surface. Through this process, when the surface of the phototube's cathode is exposed to light, it emits electrons. These electrons are attracted to the positively-charged anode, creating an electric current.

Phototubes can be divided into three general categories of use: control, measurement, and audio-visual reproduction. Under control come light-sensing applications such as burglar alarm systems, automatic door openers, and traffic light controls. Measurement functions include their use in color temperature pyrometers, while audio-visual reproduction applications include theater sound systems and pinball games.

In a burglar alarm system for example, a phototube and a beam of light are used as part of an electric circuit. The beam of light is directed onto the phototube, which is installed some distance away. When the beam of light is interrupted, as when a door or window is opened, the circuit is broken. This may happen, for example, if a burglar jimmies open a door. The broken circuit will, in turn, cause a relay to close, sounding the burglar alarm.

In recent years, photoelectric cells have been developed, which use employ semi-conductors rather than an electron tube. This has rendered phototubes obsolete in many cases. Electron multipliers, however, are still in use for a number of applications.

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Discussion Comments

By miriam98 — On Jan 08, 2012

@everetra - The article mentions the electron multiplier as an existing application for the technology. So I suppose any application where you would need to increase the electric current resulting from the photoelectric effect would need the electron multiplier, and thus phototube technology.

I am not an engineer but I am thinking perhaps some applications with spectrometry inside of a laboratory might need it. You certainly wouldn’t need it for a basic burglar alarm. All you need there is enough light to trip a relay so that the alarm is set off. There is no need to increase the current.

By everetra — On Jan 07, 2012

I used to use photoelectric cells when I built circuits way back when. I never used phototubes, nor was I aware that such things existed. However it appears the principle is the same.

I wonder when you would still prefer the tube over the photoelectric cell which seems to have taken its place? I am guessing that with some application that uses and requires gas, you would probably need the phototube.

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