Light Sensors

LIGHT SENSORS

A light sensor is a device that is used to detect light. There are different types of light sensors such as photocell/photoresistor and photo diodes being used in manufacturing and other industrial applications.

Photoresistor

Photoresistor is also called as light dependent resistor (LDR). It has a resistor whose resistance decreases with increasing incident light intensity. It is made of a high resistance semiconductor material, cadmium sulfide (CdS). The resistance of a CdS photoresistor varies inversely to the amount of light incident upon it. Photoresistor follows the principle of photo conductivity which results from the generation of mobile carriers when photons are absorbed by the semiconductor material.

Figure shows the construction of a photo resistor. The CdS resistor coil is mounted on a ceramic substrate. This assembly is encapsulated by a resin material. The sensitive coil electrodes are connected to the control system though lead wires. On incidence of high intensity light on the electrodes, the resistance of resistor coil decreases which will be used further to generate the appropriate signal by the microprocessor via lead wires.

Photoresistors are used in science and in almost any branch of industry for control, safety, amusement, sound reproduction, inspection and measurement.

Applications of photo resistor

• Computers, wireless phones, and televisions, use ambient light sensors to automatically control the brightness of a screen

• Barcode scanners used in retailer locations work using light sensor technology

• In space and robotics: for controlled and guided motions of vehicles and robots. The light sensor enables a robot to detect light. Robots can be programmed to have a specific reaction if a certain amount of light is detected.

• Auto Flash for camera

• Industrial process control

Photo diodes

Photodiode is a solid‒state device which converts incident light into an electric current. It is made of Silicon. It consists of a shallow diffused p‒n junction, normally a p‒on‒n configuration. When photons of energy greater than 1.1 eV (the bandgap of silicon) fall on the device, they are absorbed and electron‒hole pairs are created. The depth at which the photons are absorbed depends upon their energy. The lower the energy of the photons, the deeper they are absorbed. Then the electron‒hole pairs drift apart. When the minority carriers reach the junction, they are swept across by the electric field and an electric current establishes.

Photodiodes are one of the types of photodetector, which convert light into either current or voltage. These are regular semiconductor diodes except that they may be either exposed to detect vacuum UV or X‒rays or packaged with a opening or optical fiber connection to allow light to reach the sensitive part of the device.