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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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Figure 1: A phototube works with the principle of photoemissive cells which gives of electrons when struck by light.
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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.
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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.
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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.
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