A phototube, also known as a photoelectric tube, is a light-sensitive electronic device designed to emit an electric current when exposed to light or other forms of electromagnetic radiation. Commonly referred to as the “electric eye,” phototubes find applications in various light-sensing scenarios. Here’s an overview of the different types of phototubes and their functioning:
Categories of Phototubes
Gas Phototubes: Utilize inert gases like argon at low pressure. Vacuum Phototubes: Employ a vacuum electron tube with a cathode and anode. Electron Multipliers: Modification of gas and vacuum phototubes with dynodes to amplify electric current emission.
Structure of Basic Phototube
Consists of a gas-filled or vacuum electron tube with a cathode and anode. Photo-emissive material, such as cesium, coats the cathode’s surface, making it sensitive to light in the UV, near-infrared, and visible spectra.
Working Principle – Photoelectric Effect
When light falls on the photo-emissive surface, electrons are released, initiating the photoelectric effect. Electrons emitted from the cathode move toward the positively charged anode, generating an electric current.
Applications of Phototubes
- Burglar Alarm Systems: Phototubes create an electric circuit with a light beam; circuit interruption triggers the alarm.
- Automatic Door Openers: Used for detecting the presence of individuals.
- Traffic Light Controls: Employed in traffic management systems.
- Color Temperature Pyrometers: Measure high temperatures based on the color of emitted light.
- Theater Sound Systems: Used in sound detection and reproduction.
- Pinball Games: Employed for various game functions.
Burglar Alarm System
A phototube and a light beam form part of an electric circuit. The beam is directed onto the phototube, which is placed at a distance. When the beam is interrupted (e.g., by opening a door), the circuit breaks. A broken circuit triggers a relay, activating the burglar alarm. Phototubes play a major role in diverse applications, ranging from security systems to audio-visual reproduction, leveraging their ability to detect and respond to light stimuli.