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Digital Technology

Digital Technology. 14.1 Analogue and digital signals 14.2 Data capture; digital imaging using CCDs. 01101 01110 01111 10000 10001 10010 10011 10100 10101 10110 10111 11000 11001. 00000 00001 00010 00011 00100 00101 00110 00111 01000 01001 01010 01011 01100. Counting.

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Digital Technology

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  1. Digital Technology 14.1 Analogue and digital signals 14.2 Data capture; digital imaging using CCDs

  2. 01101 01110 01111 10000 10001 10010 10011 10100 10101 10110 10111 11000 11001 00000 00001 00010 00011 00100 00101 00110 00111 01000 01001 01010 01011 01100 Counting 0 1 2 3 4 5 6 7 8 9 10 11 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25

  3. Decimal Number

  4. Binary Number =10110010 Binary ↔Decimal

  5. Binary voltage pulse and reference pulse. 0 1 1 0 1 1 1 0 Reference pulse

  6. The Compact Disk (CD)

  7. 0.83μm 0.5μm 150nm Distance between tracks≈ 1.6μm Use the dimensions of the “bumps and flats to estimate the storage space of a CD.

  8. d Example: The laser of a typical DVD player has a frequency of 4.70 x 1014 Hz. Calculate the minimum height of the bumps (depth of pits) that must be etched onto the CD in order that the stored data can be read. Receiver/emitter

  9. Advantages of digital storage over analogue storage • Quality and Corruption • Reproducibility (accuracy) • Portability and high capacity • Manipulation

  10. Data Capture; Digital imaging using CCDs A charge-coupled device (CCD) is a type of complimentary metal oxide semiconductor (CMOS) used in digital imaging. When light (photons) are focused on the surface of a CCD, electron-hole pairs are produced in each pixel. The number of electron-hole pairs produced is proportional to the intensity of the incident light (photons). The free electrons migrate to relevant electrodes resulting in a change in potential across the pixel. The magnitude and position of the potential is converted to a digital signal. At a simple level each pixel acts as a capacitor storing specific charge, resulting in a specific voltage (pd).

  11. Things to remember. C = Capacitance (Farads F) Q = Charge (Coulombs C) V = Voltage or Potential Difference (volts = J/C = V) E = Energy (Joules J) f = frequency (hertz = 1/s) c = speed of light 3.0 x 108 m/s λ = wavelength (meters m) h = planks constant 6.63 x 10-34Js Energy of a photon

  12. pd pd pd pd pd pd _ _ _ _ _ _ + + + pixels silicon - - - - - - - - - photons photons photons photons photons

  13. Example: Suppose that a pixel has a capacitance of 40pF as a result of light incident on the pixel for a period of 30ms, the change in potential across the pixel is 0.24 mV. Calculate the rate at which photons are incident on the pixel.

  14. Quantum efficiencyMagnificationResolution

  15. Quantum efficiency: The percentage of photons in the incident light that produce electron-hole pairs. Typical values are 70-80%

  16. Magnification

  17. Resolution: The total number of pixels in the image collecting area of the CCD. 2500x2000 pixels = 5000000 = 5Megapixels (Mp) Resolution is also a function of the spacing between individual pixels

  18. Quality: The quality of the image is a function of the magnification and the resolution:

  19. Example: The collection area of CCD used in a particular digital camera has an area of 30mm x 30mm. Each pixel has an area of 2.2 x 10-10 m2. Estimate the resolution of the digital camera.

  20. Example: Light of wavelength 430nm and intensity 1.4MWm-2 is incident on a pixel of area 2.2 x 10-10 m2 for 20ms. The capacitance of the pixel is 25pF. Calculate the change in potential difference across the pixel if the quantum efficiency of the CCD is 70%

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