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Light sensors can be divided into two categories based on output.one sensor which converts light energy to electrical energy and those which change their electrical properties in some way such as photo-resistors or conductors. According to the output the sensors can be subdivided into following devices.
Photo-emissive Cells
The sensors which release free electrons from a light sensitive material such as Caesium when introduction but the photon of sufficient energy. Higher the frequency higher will be the energy that photon possess.
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Photoelectric sensor has basically 5 components, light source, light detector, lenses, logic circuit and output. However in this topic we are focusing on the light source and the light detector. Light source basically consists of a LED or a laser diode.
a. Light source
Due to the cheap price and availability, most of the devices use LED as the light source. LED (Light emitting Diode) is a semiconductor that produces light when current is passed through them.
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However, currently laser diodes are common in use due to its unique characteristics which include consistent wavelength of the emitted light, small beam diameter, and long range.
b. Light detector
The light detector is the component used to detect the light produced by the light source. They are commonly known as photodiode or phototransistorphoto transistor. Due to its capability of sensing the different pulses of light they are known as receiver. There best application is in the receiver end of the optical fiber data transmission. The transponder connected with it converts the light sensed by these detectors to electrical pulses.
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For example, photocell or photoresistor photo resistor are used in consumer products to measure the intensity of light. A burglar alarm or a lift doors have the light sensors in them. These sensors sends the light from one sensor to the other, if the light is interrupted, the alarm is on or the lift door opens.
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Telecommunication use the light emiiting emitting diode to convert the electrical pulses into light pulses and send it to the optical fiber. At the receiver end the photo diode senses the light pulse and converts that to the electrical signal. This fiber-optics technology has made the data transmission fastest beyond the thinking of human some years ago.
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