A photocell is any of a wide range of sensors that react based on the presence of light or electromagnetic energy. They can be found in many different forms, from solar panels that use light to generate electricity, to photoresistors, which shift in resistance based on how much light is present. This device is one of the most important modern advancements, allowing for a whole range of new technologies to be created.
One of the most common types of photocell is the photoresistor, a device constructed out of a semiconductor of high resistance. When enough light hits the semiconductor, it absorbs photons so that the electrons already present have enough energy to conduct, lowering resistance. A photoresistor may be either intrinsic, such as those made of silicon, in which case it will require a fair amount of light to lower the resistance, or extrinsic, with a dopant added to lower the amount of energy needed to trigger a reaction, making them highly sensitive.
Photoresistors are used in all sorts of things that need to react to light in some way. Some displays, such as those in clocks, may include a photoresistor to keep the face readable or to light up when dark. Many street lights include them to make sure they automatically turn on when the light gets low enough. Nightlights, such as those many people keep in their bathroom, also contain this type of sensor so that they are only on when it is dark. The light meter found in most modern cameras is also usually a photoresistor, helping to measure how much light is reaching the lens to help the photographer calculate what shutter speed and aperture they should be using.
Another widely-seen form of photocell is known as a photovoltaic cell, or solar cell. These utilize the photovoltaic effect, where electrons are released when electromagnetic radiation strikes the surface, to generate electricity. This electricity is then generally stored in a battery system and can be used as a source of renewable energy.
Some other types focus more on measuring specific types of energy. Optical detectors, for example, act as advanced thermometers: they take in electromagnetic radiation and react to the increase in energy in set ways that can determine exactly the increase in ambient temperature, or can simply measure how much light is striking the surface. Others, known as cryogenic detectors, are so sensitive that they can be used to detect the presence of a single tiny bit of electromagnetic radiation, such as a single X-ray or a single infrared photon.
One of the most basic types is a chemical photocell. This type is found in a non-digital camera in the form of a photographic plate, where light striking the plate causes a silver halide molecule to split into both a halogen atom and a metallic silver atom, which can then be used to create an image. Modern cameras use charge-coupled devices instead, which are another type of photocell, to achieve much the same result by transferring electric charge instead of splitting silver.