ChargeCoupled Devices CCD

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Charge-Coupled Devices (CCD)

• C.2.7 - C.2.11
• by Mic Chan and AnZhi Zhang

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C.2.7

• Two points on an object may be just resolved
  (seen as two separate points) on a CCD if the
  images of the points are at least two pixels
  apart.

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C.2.7

• Two points on an object may be just resolved
  (seen as two separate points) on a CCD if the
  images of the points are at least two pixels
  apart.
• Consider the situation below where a star gives
  rise to an image on a CCD
• The following graph shows the charge (number of
  electrons) in the pixels viewed across the image 
  The graph below shows the charge (number of
  electrons) in the pixels viewed across the image
• If the 2nd star is close to the first, then it
  produces a similar charge distribution across the
  pixels

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If the 2nd star falls on adjacent pixels, the
charge distribution would result as below.
As can be seen, there is little evidence from the
resulting charge distribution that there are two
stars.  The charge distribution would suggest
just one larger and brighter star.
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If the image of the second star is two pixels
away from the first image then the charge
distribution below will result.
In this diagram it can be seen that the two
images will be able to be resolved.
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C.2.8 (quantum efficiency)

• The effect of quantum efficiency on a CCD is
  making it a efficient detector of light
• Many photoelectric materials have a quantum
  efficiency of 10-20
• A CCD has a quantum efficiency of around 80
• This makes it a good detector 
• This enables smaller exposure time than with
  photographic film, creating sharper images of
  moving objects, and more images captured in a
  given amount of time when it is used with
  astronomical telescope.

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C.2.8 (magnification)

• Magnification of an image produces a closer look
  at whatever was captured in the actual image
• With lower quality images (such as those produced
  by inexpensive digital cameras that take lower
  megapixel images), the more you magnify the
  image, the grainier the image gets
• This is called pixelation
• With higher quality images, magnifying, to a
  certain point, won't produce a noticeable change
  in the quality of the image shown

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C.2.8 (resolution)

• Measured in megapixels
• more megapixels gt sharper images and more
  detailed images the pictures your digital camera
  can take
• an image with more megapixels can be enlarged and
  printed without sacrificing image quality

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C.2.9 (practical uses)

• Practical uses of CCDs
• Digital video and still camera
• Telescopes
• X-ray
• Thermal imaging 

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C.2.9 (advantages of CCDs)

• Advantages of using CCDs
• Reusable
• CCDs can be used numerous times
• Greater sensitivity
• CCDs are more sensitive to human eye
• Greater color response
• CCDs responded to more electromagnetic radiation
  wavelength
• Linear response
• Output voltage of CCD is proportional to charges
  collected by each pixels

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C.2.10

• A CCD can be thought of as an array of shift
  registers
• A picture is read out of the device by a
  succession of shifts through the imaging section,
  with all of the rows simultaneously moving one
  space at a time along the columns of the body of
  the device

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C.2.10 (continued)

• This diagram represents a small segment near the
  edge of the device, with the output shift
  register being shown at the top

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C.2.10 (continued)

• In order to understand how the image is converted
  into a digital signal, consider the following
  series of diagrams 
• In the first diagram the section of the CCD has
  been exposed to light and each electron in each
  potential well is represented by

•  

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C.2.10 (continued)

• On the first shift, the voltage level of the next
  barrier towards the output shift register is
  lowered to the same level as the well
• The electrons then divide between the two wells,
  as shown in the following diagram

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C.2.10 (continued)

• Finally, the voltage level of the original well
  is raised so that it becomes a barrier
• The effect of this operation is to move the
  electrons one third of a pixel upwards, as shown
  in this diagram

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C.2.10 (continued)

• This process is then repeated for each successive
  shift within the CCD
• The following diagram shows the position of the
  electrons after the next shift

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C.2.10 (continued)

• On the next shift the information passes out of
  the imaging section through an isolating region
  called a transfer gate into the output shift
  register, as shown in the following diagram
• Electrons from pixels further in the body of the
  CCD now enter at the bottom of the diagram

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C.2.10 (continued)

• The same technique is now used to move the
  electrons along the output shift register and the
  next diagram shows the electrons moved towards
  the left

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C.2.10 (continued)

• An amplifier at the end of the output register
  measures each charge packet in turn and gives a
  corresponding voltage output.  The process is
  repeated until the entire chip has been emptied
  of information

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C.2.11

• Example
• A CCD has a linear magnification of 3 times. 
  When viewed through a digital microscope, the
  length of the image of the flea on the CCD is
  6mm.  What is the original size of the flea?
•                                             Image
  Height                    6
• Linear Magnification   -------------------- gt 
  3 ---  gt
•                                           
   Object Height                    X
• 63(X)   gt   X6/3  gt  X2mm

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Bibliography

• http//islandphysics.wikidot.com/ib
• http//www.scribd.com/doc/13750977/IB-Physics-Core
  -Syllabus-Summary-2009-Draft1