Photodetectors are devices capable of sensing electromagnetic energy, typically light, which contains photon particles that 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, to generate voltage, to amplify an existing current, and to record images. Plants even use a form of photodetection to guide their growth, as their cells react to the light and grow towards it. Whether natural or artificial, all photodetectors share a common principle: a reaction triggered by the presence of light.
An example of a mechanical sensor of this sort would be a laser security system that detects the presence of laser light, and its interruption, to determine intrusions and trigger an alarm. The most commonly 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 onto its surface. Photographers develop their film in darkrooms to avoid ruining it, should the film have a chemical reaction to the light.
With a wide range of uses, photodetectors appear everywhere from particle-detecting telescopes to the Large Hadron Collider to UV-sensitive sunglasses. The majority of photodetectors are calibrated to detect light and radiation on a very specific spectrum, ranging from ultra-violet 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 a button is pressed on a television remote control, the remote control emits an infrared signal on a wavelength invisible to the human eye. The television intercepts and interprets the signal as a command to turn the volume down, change the channel, or turn power on or off.
Depending on their purpose, photodetectors can have a variety of other functions. For example, semiconductors and semiconducting circuits use photodetectors to conduct an electrical current by changing light into electricity. When the semiconductor is exposed to light in its target spectrum, the semiconductor material absorbs photons that act on electrons to separate electron-hole pairs and create electrons in an excited state. This outcome allows the electrons to travel freely as a conductive medium, which creates a photocurrent. This conductive action makes semiconductors a key base component of virtually all modern electronics.