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Charge Coupled Devices


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CCDs for Mr Stead

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Charge Coupled Devices

  1. 1. CHARGE COUPLED DEVICES What are they? How do they? Why do they do that?
  2. 2. WHAT ARE THEY?  CCDs are used to convert light into a digital value  Capture high quality image data in a wide range of applications
  3. 3. HOW DO THEY?  They transfer incoming photons into an electrical charge in the CCD chip  The photons are received in a photoactive region of silicon  A lens is used to focus the light onto the silicon  A charge is accumulated in the capacitors proportional to the light intensity  The charge is stored in capacitors known as “bins”  The charge is transferred between “bins” moving from one bin to its neighbour until it reaches the last capacitor  The charge is then dumped to a charge amplifier which converts his charge into a voltage  By repeating this process for many rows the charge values for the whole array can be digitized and stored in memory
  4. 4. WHAT IS A BAYER FILTER?  A Bayer filter is put over the image sensor  It has three colours: green, red and blue. These are arranged in squares in the ratio 50% green, 25% red and blue  The reason green is more common than the others is to imitate the biology of a human eye, as the eye is more sensitive to green light  It produces a colour image while only using a single chip digital sensor such as in a digital camera or a scanner One Pixel
  6. 6. BAYER PATTERN IMAGES  The image produced by a Bayer filter is known as a Bayer pattern image  This cannot be used as the final image because it only shows three colours  A computer can be used to perform an algorithm on the Bayer pattern image  This is image will look more as we would expect to see it such as below
  7. 7. WHY DO THEY DO THAT? CCDs are used in a great number of applications such as:  Digital cameras  Telescopes  Spectroscopy
  8. 8. PROBLEMS  There may be what is known as “dead pixels” and “hot pixels”, these would reduce the usefulness of the image produced  When there is no light on the CCD there is still a current produced known as the “dark current”  The “dark current” changes the image because it adds charge to the values so the light appears to be brighter than it in fact was  To combat this the dark current charge values are taken from those of the image produced resulting in a real image