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These devices use light to control the flow of electrons and holes across their PN-junction. These devices are mainly designed for detector application and light penetration with their spectral response tuned to the wavelength of incident light.
Physical principle of Photoelectric sensor sensing
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.
Figure 4: LED construction
LED consists of a state and p state. When the current is passed through the LED the electrons from one side gets excited and tries to go to the next state lose photon and comes back to the original state. This photon is the light produced by the LED. The color produced by the LED depends on the wavelength of the lights produced.
The greatest advantage of using the LED is its tendency to be turned on and off rapidly. This allows creating different pulses of the source. The amount of light produced by the LED depends on the current flowing through it. The more the current, the range of amount of light produced is high. However, more current might also destroy the LED due to the excessive heat produced by it.
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 phototransistor. 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.
Figure 5: Spectral Response of light detector
The spectral response of the light detector determines its sensitivity to different wavelengths in light spectrum. Thus, to increase the sensitivity, the spectral of source and the detector are matched.
Application
These light sensors have vast range of application according to it's type which works in slightly different way. However they are used to detect light. These kinds of sensors have different scientific applications, but are also encountered by persons in day to day life.They are used in computers, televisions and modern smartphones. These devices use ambient light sensors in order to change the brightness of the screen depending on the surrounding's light.
For example, photocell or photoresistor 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.
Figure 46: Barcode scanner. They use the light sensors.
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URL:http://www.wisegeek.org/what-is-a-light-sensor.htm [Online], Accessed 27.3.2014
5. Photo electric sensor construction, Rockwell Automation
URL:http://www.ab.com/en/epub/catalogs/12772/6543185/12041221/12041223/Photoelectric-Sensor-Construction.html [Online], Accessed 27.3.2014