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Charged Coupled Device

Charged Coupled Device.  Many image sensors: Infrared, gamma ray, x-rays etc.  Focus on sensors for visible light (slightly into infrared and uv light). CCD and CMOS sensors are normally used for visible light Have good natural sensitivity in near infrared, usually removes that by filters

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Charged Coupled Device

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  1. Charged Coupled Device

  2.  Many image sensors: Infrared, gamma ray, x-rays etc.  Focus on sensors for visible light (slightly into infrared and uv light) CCD and CMOS sensors are normally used for visible light Have good natural sensitivity in near infrared, usually removes that by filters Can be specially adapted for other parts of the spectrum e.g. X- rays Michael Thomas, TU Berlin, 2010 Processing Digital Camera Images, WS 2010/2011, Alexa/Eitz 2

  3. The beginnings  Video camera tube sensors in the 1930s .1969,George Smith،  First , CCD , Willard Boyl  Fairchild's effort, led by ex-Bell researcher Gil Amelio, was the first with commercial devices, and by 1974 had a linear 500-element device and a 2-D 100 x 100 pixel device.  The first KH-11 KENNAN reconnaissance satellite equipped with charge- coupled device array technology for imaging was launched in December 1976.[6]  Under the leadership of Kazuo Iwama, Sony also started a big development effort on CCDs involving a significant investment. Eventually, Sonymanaged to mass produce CCDs for their camcorders. Before this happened, Iwama died in August 1982; subsequently, a CCD chip was placed on his tombstone to acknowledge his contribution.[7] Michael Thomas, TU Berlin, 2010 Processing Digital Camera Images, WS 2010/2011, Alexa/Eitz 3

  4. How to convert light to electric charge?  Inner photoelectric-effect at a photodiode:  Photon excites electron creating a free electron and a hole  The hole moves towards the anode, the electron towards the cathode Now we have our charge! Michael Thomas, TU Berlin, 2010 Processing Digital Camera Images, WS 2010/2011, Alexa/Eitz 4

  5. Charge-Coupled Device (CCD) •  Integrated circuit •  Array of connected capacitors (Shift register) •  Charge of capacitor is transfered to neighbour capacitor •  At the end of chain, charge is converted into voltage by charge amplifier •  Transfer stepped by Clock-Signal • CCD has photosites, arranged in a matrix. • Each comprises a photodiode which converts light into charge and a charge holding region • The charges are shifted out of the sensor as a bucket brigade • The A/D conversion is done at the edge of the circuit •  Serial charge processing Michael Thomas, TU Berlin, 2010 Processing Digital Camera Images, WS 2010/2011, Alexa/Eitz 5

  6.  Each capacitor is coupled with a photodiode  All capacitors are charged parallelly  Charges are transferred serially

  7. CCD-Sensor  But how to prevent light to charge up the capacitors while transferring?  Mechanical shutter  Buffer of capacitors that store the charge until it is transferred  Loss of resolution or larger sensor → more expensive

  8. One-or three-chip camerathree-chip is usually at least 3 times as expensive The color filter matrix for one-chip, usually ”Bayer mosaic” Reduces color resolution to about half Also reduces light collection efficiency Anisotropic in x and y A new method invented by Foveon uses “vertical filters” with less resolution loss

  9.  What happens, if too much light hits the sensor?  Capacitors overload  Charge “spills“ over to neighbour capacitors  Blooming effect occurs

  10. CCD vs. CMOS CCD: Pro:  No rolling shutter  Lower noise  Good image quality Con:  Analog device!  Blooming effect CMOS: Pro:  No blooming  Cheaper  Lower power consumption  Faster readout Con:  Rolling shutter  Variations in brightness (per Pixel) Michael Thomas, TU Berlin, 2010 Processing Digital Camera Images, WS 2010/2011, Alexa/Eitz 12

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