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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.
- Photo-conductive Cells
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.
- Photo-voltaic Cells
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.
- Photo-junction Devices
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.
Photoconductive Cell
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.
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 4: 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.
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