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14.2 Data Capture

14.2 Data Capture. Digital Imaging using CCDs. Capacitors. A capacitor is a device that stores charge. Def’n of Capacitance - The ratio of the charge Q on one of the plates to the potential difference V between the plates. C = Q/V Unit is the Farad (1F = 1Cv -1 ). How they work.

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14.2 Data Capture

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  1. 14.2 Data Capture Digital Imaging using CCDs

  2. Capacitors A capacitor is a device that stores charge. Def’n of Capacitance - The ratio of the charge Qon one of the plates to the potential difference Vbetween the plates. C= Q/V Unit is the Farad (1F = 1Cv-1)

  3. How they work

  4. Charge Coupled Devices (CCDs) • A silicon semi-conductor microchip that can be used to electronically record an image focused onto its surface. The surface is divided into a large number of small areas called pixels • http://electronics.howstuffworks.com/cameras-photography/digital/digital-camera2.htm

  5. Builiding up a digital imageusing a CCD The CCD is exposed to light energy (photons) These strike the CCD and the light energy is converted into electrical energy (via the PE Effect) The more incident photons on a pixel the more that pixel develops charge The charge is collected in different pixels The pixel behaves like a capacitor and stores the charge

  6. Continued… The p.d of each pixel is proportional to the charge built up during the exposure This p.d can be measured and converted into a digital signal using binary code. This information can be used to reconstruct the image as the information from each pixel has recorded the different light intensities in different parts of the image.

  7. Limiting factors/Image parameters Quantum Efficiency – A perfect CCD would emit one electron for every photon striking the surface. Practically, this is not achieved. QE = Ne/Np Actual CCDs operate around 20-90% efficiency. If QE is low not all photons striking the surface will register so it will reduce the quality of the image

  8. Continued… Magnification– Lenses must be used to focus the image of an object onto the CCD. If the image formed is small then only a small number of pixels will be illuminated cause poor image quality Magnification = Image Length / Object Length Resolution-The amount of detail the image contains Two points on an object may just be resolved if the points on the image are two pixels apart

  9. Uses of CCDs Digital Cameras Video Cameras Telescopes Medical x-ray imaging Scanners

  10. Advantages of CCDs Cheaper – doesn’t require film Traditional film has low QE (~10%) Easily enhanced and edited Viewing can take place virtually immediately Storage and archiving of a large number of photos is easy.

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