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CCD sensor

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by

Parth Kotwal

on 11 May 2016

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Transcript of CCD sensor

An Imaging Semiconductor circuit - the CCD sensor
Introduction to CCDs
A charge-coupled device (CCD) is a light sensitive integrated circuit, invented by Willard Boyle and George E. Smith in 1969]
It is a silicon plate the size of a stamp, and holds millions of photosites [1]
It is used in cameras to store and display the data for an image in such a way that each pixel in the image is converted into an electrical charge. Each photosite corresponds to a single pixel in the final image [2]
The intensity of the charge is related to a particular colour in the spectrum of light
How does a CCD work?
The silicon chip is divided into thousands or millions of light sensitive squares (photosites)
Each photosite is surrounded by a non-conductive boundary which contains the charge that is collected during an exposure within the photosite [2]
Incident photons entering the lens of a camera hit the silicon plate and knock out electrons in the photocells
These electrons gather in the cells, and the larger the amount of light, the larger the number of electrons that fill these cells [3]
The photoelectric effect
Silicon is a metalloid and so it experiences the photoelectric effect
When a photon strikes an atom, it can elevate an electron to a higher energy level, in some cases freeing the electron from the atom, if the frequency of the incident photon is high enough
The energy of a single photon is transferred to a single electron. Of the energy delivered to the electron, some is used in getting the electron to the metal surface, some is used in getting the electron to escape, and the remainder is the kinetic energy of the electron
This happen within the silicon wafer of the CCD

We have charge...now what?
When a voltage is applied to the first capacitor in the CCD, its charge is transferred to its neighbour
This is done to each cell in succession, shifting the entire array by one location
The final cell in the array transfers its charge to a circuit, which puts the electrons into a capacitor, and measures and amplifies the voltage across it, them empties the capacitor
This gives a black and white image of how much light has fallen on each pixel [1]

Black and white?
The CCD itself produces an image in black and white
Various filters are used to obtain the image in colour
One kind of filter, that contains either red, green or blue, is placed over each photosite
Owing to the sensitivity of the human eye, the number of green pixels needs to be twice that of the blue or red pixels [3]
References
[1] Frank's Web Space, 11/05/2016
http://www.frankswebspace.org.uk/ScienceAndMaths/physics/physicsGCE/general.htm

[2] Starizona, 11/05/2016
http://starizona.com/acb/ccd/introimaginghow.aspx

[3] Nobel Prize Laureates, 11/05/016
http://www.nobelprize.org/nobel_prizes/physics/laureates/2009/popular-physicsprize2009.pdf
By Parth Kotwal
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