Light Sensor

Definition

Light sensors are a type of photodetector (also called photosensors) that detect light. Different types of light sensors can be used to measure illuminance, respond to changes in the amount of light received, or convert light to electricity.

What are the different types of light sensors?

Common types of light sensors are photodiodes, photoresistors, phototransistors, and photovoltaic light sensors. These components can be used in applications such as light sensing in mobile devices, automatic outdoor lighting, proximity sensors, and renewable energy.

Photodiodes convert light into an electrical current. They are p-n junction devices that are similar to normal diodes. A p-n junction device consists of a p-type and an n-type semiconducting material. The “p” stands for “positive” due to the material’s excess of electron holes, and the “n” stands for “negative” due to an excess of electrons. This means that current can only flow in one direction through the boundary. In a photodiode, these electron hole pairs are formed when the energy from the incident light is absorbed by the device. Also see the related term avalanche photodiode.

Photoresistors (also known as light-dependent resistors or LDRs) are passive devices that decrease resistance in proportion to the amount of light received. Light forming electron hole pairs increases conductivity and therefore decreases resistivity.

Phototransistors switch or amplify signals similarly to regular transistors, with the current applied to the terminals being created from exposure to light.

Photovoltaic (or solar cells) convert light into electricity in a process known as energy harvesting. Voltage and electric current are generated by way of the photovoltaic effect exhibited by the cell's semiconducting components.

How do light sensors work?

Light sensors work by the photoelectric effect. Light can behave as a particle, referred to as a photon. When a photon hits the metal surface of the light sensor, the energy of the light is absorbed by the electrons, increasing their kinetic energy and allowing them to be emitted from the material. This movement of electrons, and therefore charge, is electrical current.

The photovoltaic effect is similar to the photoelectric effect in that the light is absorbed by electrons, causing them to be in a higher-energy state. In the photoelectric effect, the electrons are ejected from the material completely. In the photovoltaic effect, the electrons are excited from the valence band into the conduction band, but remain within the same material.