Photo Sensor,Photo emission cell & Photoconductive cells

As-Salaam-Alaikum, Welcome to my blogger "World of Knowledge".Here we discuss the topic Photo sensors, photo emission cells & photoconductive cells.

Points to be discuss in this Article are given below:

  • Photo Sensors
  • Working Principle
  • Types of Photo Sensors
  • Construction of photovoltaic cells
  • Advantages & Disadvantages of photovotaic cells
  • Photo emission cells
  • Advantages & Disadvantages of photo emission cells
  • Photoconductive cells
  • Construction of photoconductive cells
Photo Sensor:


 A photo sensor is an electronic component that detects the presence of visible light, infrared transmission (IR), and/or ultraviolet (UV) energy. Most photo sensors consist of semiconductor having a property called photoconductivity , in which the electrical conductance varies depending on the intensity of radiation striking the material.

 Working Principle:
  • Conversion of Light energy into electrical energy
  • When light falls on photosensitive element, electric current is generated.
  • Photoelectric effect is the ejection of electrons from a metal or semiconductor when illuminated by light or any radiation of suitable wavelength.
Types of Photo sensors/ Transducers:

 Passive transducers:
  • Photo emissive
  • Photo conductive


Active Transducers:
Photovoltaic (made of semiconductor material like selenium deposited on iron base, silver or
gold covering this material act as a collecting electrode).


Construction Photo voltaic cell:


Advantages and Disadvantages of Photo Voltaic Cells:

               Advantages

 •       Robust in construction
•       Need no external Power supply
•       It produces photo current stronger than the photo sensitive material
                          Disadvantages

 •       Selenium has low internal resistance. Hence amplification is difficult.
•       Very sensitive galvanometers have to be used for measurement.

Photo emissive cells:
Anode and cathode mounted in glass tube  filled with vacuum.
Cathode is a curved metal plate made of photo sensitive material like cesium or oxidised 
silver
Anode is o nickel or platinum
When radiation of frequency above the threshold frequency falls on cathode, electrons are 
emitted, causes I to flow.



Advantages and Disadvantages of Photo emissive cells:

                       Advantages
  • Emission is instantaneous
  • Maximum current is proportional to the intensity of radiation.
  • Increased sensitivity
                     Disadvantages
  •  Generates extremely small current
  • Direct power supply required for photo multiplier.
  • More expensive

Photoconductive cells:
  • Also called LDRs( Light Dependant Resistors)
  • Resistivity of photosensitive material decreases with the intensity of the incident light
  • Major disadvantage is that they show hysteresis effect.

Construction of Photoconductive cells:

  • When light of varying intensity falls on photo conductive film, current flows through the circuit containing a galvanometer and a battery.
  • Current is proportional to the intensity of light












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