Light Sensor Basics: Types and Functionality

This page describes the basic function of a light-detecting sensor, often simply called a light sensor.

Definition: It detects the intensity of light and provides an output signal corresponding to that intensity.

Light sensors are commonly used for security purposes in offices and homes. They’re also a key component in automatic street light systems.

• The light sensor is a passive device.
• It converts light energy (in the visible or infrared spectrum) into an electrical signal, as illustrated in the figure above.

Light Sensor Types

There are three main types of light sensors:

• Photo Conductive Cells: These cells vary their electrical resistance depending on the amount of light they are exposed to.
• Photo Voltaic Cells: These cells generate an electromotive force (EMF) proportional to the light energy incident upon them. Selenium is a commonly used material in photovoltaic cells, and they are a core element in solar cells.
• Photo Junction Devices: These are true semiconductor devices, such as photodiodes or phototransistors. They use light to control the flow of electrons and holes across the PN junction.

Photoresistors are semiconductor devices that utilize light energy to control the flow of electrons, and consequently, the current passing through them. The most common type of photo-conductive cell is the Light Dependent Resistor (LDR).