Digital Technology 14.1 Analogue and digital signals 14.2 Data capture; digital imaging using CCDs.
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Transcript of Digital Technology 14.1 Analogue and digital signals 14.2 Data capture; digital imaging using CCDs.
Digital Technology
14.1 Analogue and digital signals
14.2 Data capture; digital imaging using CCDs
Counting01101011100111110000100011001010011101001010110110101111100011001
00000000010001000011001000010100110001110100001001010100101101100
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0
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Decimal Number
012 108107101 178
Binary Number0
201
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21 178
2n 27 26 25 24 23 22 21 20
subtract 128 64 32 16 8 4 2 0
Left over
50 50 18 2 2 2 2 0
binary 1 0 1 1 0 0 1 0
=10110010
Binary ↔Decimal
Binary voltage pulse and reference pulse.
0 1 1 0 1 1 1 0
Reference pulse
-8.0
-6.0
-4.0
-2.0
0.0
2.0
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6.0
8.0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
sample
pd/V
The Compact Disk (CD)
Distance between tracks ≈ 1.6μm
150nm
0.83μm
0.5μm
Use the dimensions of the “bumps and flats to estimate the storage space of a CD.
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.
d
Receiver/emitter
Advantages of digital storage over analogue storage
• Quality and Corruption
• Reproducibility (accuracy)
• Portability and high capacity
• Manipulation
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).
Things to remember.
V
QC
C = Capacitance (Farads F)
Q = Charge (Coulombs C)
V = Voltage or Potential Difference (volts = J/C = V)
Energy of a photon
hc
hfE
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
silicon
pixels
_ _+ _ _+_ _+
- - - - - -- - -
pd pdpd pdpd pd
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.
Quantum efficiencyMagnification
Resolution
Quantum efficiency:
The percentage of photons in the incident light that produce electron-hole pairs. Typical values are 70-80%
Magnification
object of size
CCD on theobject of sizeM
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
Quality:The quality of the image is a function of the magnification and the resolution:
QR
QM
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.
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%