Photodetectors detect electromagnetic energy, such as light, and can be mechanical, biological, or chemical. They can be used for various purposes, including as thermometers, to measure radiation, generate voltage, amplify current, and record images. Photodetectors are present in many devices, from telescopes to remote controls, and can have additional functions, such as conducting an electric current. All photodetectors share a common principle of reacting to the presence of light.
Photodetectors are devices that can detect electromagnetic energy, typically light, which contains photon particles which are a type of electromagnetic energy. Although there are many types, the most common are mechanical, biological, chemical. Photodetectors can also be used as thermometers, to measure radiation, generate voltage, amplify an existing current, and record images. Plants even use a form of photosensing to guide their growth, as their cells react to and grow towards light. Whether natural or man-made, all photodetectors share a common principle: a reaction triggered by the presence of light.
An example of such a mechanical sensor would be a laser security system that detects the presence and interruption of laser light to determine intrusion and trigger an alarm. The best known biological sensor is the eye, which detects and reacts to light to interpret optical signals, which it then sends to the brain as an image. Photographic film is one of the simplest forms of chemical sensors: it uses light to imprint an image on its surface. Photographers develop film in a darkroom to avoid damaging it should the film have a chemical reaction to light.
With a wide range of uses, photodetectors appear in everything from particle-detecting telescopes to the Large Hadron Collider to UV-sensitive sunglasses. Most photodetectors are calibrated to detect light and radiation in a very specific spectrum, ranging from ultraviolet to infrared. Infrared devices, such as heat sensors and television remote controls, use light on the infrared spectrum to transmit a signal, which is captured and interpreted by a detector. When you press a button on a television remote control, the remote emits an infrared signal on a wavelength invisible to the human eye. The TV intercepts and interprets the signal as a command to turn down the volume, change the channel, turn it on or off.
Depending on their purpose, photodetectors can have a variety of other functions. For example, semiconductors and semiconductor circuits use photodetectors to conduct an electric current by turning light into electricity. When the semiconductor is exposed to light in its target spectrum, the semiconductor material absorbs photons which act on electrons to separate electron-hole pairs and create electrons in an excited state. This result allows electrons to travel freely as a conductive medium, which creates a photocurrent. This conductive action makes semiconductors a fundamental building block of nearly all modern electronics.
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