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Light sensors creates an output signal indication the intensity of light by measuring the radiant energy that exists in a very narrow range of frequencies called light. It converts the light energy into an electrical signal output. They are also known as photoelectric devices or photo sensors as photons are converted to electrons.
Types of Light Sensor
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.
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.
Figure 1: A phototube works with the principle of photoemissive cells which gives of electrons when struck by light.
These sensors change their electrical resistance when the light is introduced. Photoconductivity results from light hitting as semiconductor material which controls the current flow through it. The most common photoconductive materials Cadmium Sulphide used in LDR photocells. Photoconductive light sensor doesn’t produce electricity but it changes its physical properties when introduced to the light. Photo resistor is the common devices used which changes its electrical resistance in response to change in light intensity.
Figure 2: Photo resisor. It changes resistance with the change in light intensity.
When the radiant light energy is received this device generates an emf and is similar in effect to photo conductivity. Light energy fall on to two semiconductor material linked together creating a voltage of 0.5V. Selenium is commonly used photovoltaic material in solar cells.
Figure 3: Photo-volatic cells or commonly known as solar cells
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.
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.
The light detector is the component used to detect the light produced by the light source. They are commonly known as photodiode or photo 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.
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.
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 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.
Figure 6: Barcode scanner. They use the light sensors.
Barcode scanner or the Quick Response(QR) code use the light sensors. The light from the scanner illuminates the barcode, which is read and decoded by a sensor.
Television use the remotes with infrared light sensors or infrared light technology.
Telecommunication use the light 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.
Hence there are numerous application of light sensors in the scientific applications and in the consumer devices. They have been playing a vital role to fulfill the dream of smart homes or smart systems imagined by the human kind.
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