C2 Data capture and CCDs – Student Work Book
1).
(a)
Explain what is meant by the term “Capacitance”.
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(b)
State the relationship between Capacitance, Charge and Potential difference.
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(c)
Complete the table below. Ensure that you add the correct units.
Qu
1
2
3
4
5
6
7
8
9
10
Capacitance
100µF
2200µ
27nF
470nF
680pF
Charge
4mC
26mC
5µC
24µC
0.04µC
0.47F
2.2F
Potential Difference
25V
10V
50V
20V
15V
120V
5V
7.5C
For more examples go to www.ikes.freeserve.co.uk/homework/hwk.htm
2).
Explain, briefly, the meaning of the terms:(a)
Pixel
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(b)
CCD
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(c)
Photon
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3).
Complete the table below.
Qu
1
2
3
4
5
EM radiation
Infra-red
Red light
Green light
Violet light
Ultraviolet
Wavelength m
Frequency Hz Energy of photon J
For more examples go to www.ikes.freeserve.co.uk/homework/hwk.htm
4).
(a)
Draw a labeled diagram to represent the cross section of a CCD.
(b)
With reference to your diagram, explain what happens when photons of light enter the
thin silicon substrate.
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5).
(a)
What is the function of an ADC?
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(b)
A 4-bit ADC gives its maximum output with an input voltage of 1.5V.
(i)
What is the resolution of the ADC?
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(ii)
Complete the table below for the output from the ADC for the different input
voltages. The first one has been completed for you.
Qu.
1
2
3
4
5
6
(c)
Input voltage V
0.25
0.00
0.68
0.95
1.36
1.50
ADC output
2
Explain what is meant by Quantization errors.
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6).
(a)
Explain the meaning of the term ”Quantum Efficiency”.
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(b)
What is the typical Quantum Efficiency of a modern CCD.
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(c)
Explain why it is important for a CCD to have a high Quantum Efficiency.
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(d)
If a CCD has a Quantum Efficiency of 95%, what fraction of the incident photons
does not release electrons?
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7).
(a)
Define the term “Linear Magnification”.
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(b)
A flea is 2mm in long. When viewed through a digital microscope, the length of the
image of the flea on the CCD is 6mm. What is the linear magnification of the optical
system?
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8).
(a)
Explain what is meant by the “Resolution of a CCD”.
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(b)
Using diagrams, describe the conditions under which it is possible to see two stars
that are very close together when imaged using a CCD.
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(c)
The pixels on a CCD are spaced every 10µm. The CCD is placed at the focus of a
concave mirror with a focal length of 3m. Estimate the angular separation of
two stars that will just be resolved.
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(d)
Describe the advantages and disadvantages of using CCDs compared to
photographic emulsion to obtain images of stars from telescopes.
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9).
A CCD consists of a rectangular array of 300 by 300 pixels.
(a)
If each pixel has an area of 10-10 m2, estimate:(i)
the size of the CCD,
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(ii)
the area of the CCD.
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(b)
Each pixel consists of three potential wells, and it takes 1µs to move the charge from
one well to the next, estimate the maximum rate that pictures can be taken with this
CCD, stating any assumptions that you make.
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10).
The use of CCDs is extensive now that the price of manufacture has fallen and their resolution
has improved. Uses include the following:- Digital video and still cameras, Telescopes,
Thermal imagers, X-ray imagers etc.
Prepare a presentation on one use of CCDs, which you could deliver to the rest of your teaching
group. In this, you should explain what a CCD is, its application and why the use of CCD is
preferred to the previous method of imaging.
Attach a print out of your presentation to this booklet.
C2 Data capture and CCDs – Student Work Book – Responses
The following responses are intended to act as a guide for staff and students.
1).
(a)
(b)
Capacitance is measure of the quantity of charge that a conductor can hold. It is the
ratio of the charge to potential of the conductor.
ch arg e (Q )
capaci tan ce (C) 
potential ( V )
(c)
Qu
1
2
3
4
5
6
7
8
9
10
2).
Capacitance
100µF
400F
2200µ
27nF
250nF
470nF
680pF
333pF
0.47F
2.2F
Charge
2.5mC
4mC
26mC
1.35C
5µC
24µC
10.2nC
0.04µC
2.35C
7.5C
Potential Difference
25V
10V
11.8V
50V
20V
51V
15V
120V
5V
3.4V
(a)
(b)
(c)
The smallest individual light sensing element that is used to sense/form the image.
Charge Coupled Device – an analogue storage system – an analogue shift register.
The smallest quantity of e-m radiation – a “particle of light”.
Qu
1
2
3
4
5
EM radiation
Infra-red
Red light
Green light
Violet light
Ultraviolet
(a)
See the Support booklet.
electrons captured
3).
4).
-
Wavelength m
10-6
6.5x10-7
5x10-7
4.5x10-7
3x10-7
-
Frequency Hz
3x1014
4.6x1014
6x1014
6.7x1014
1015
Energy of photon J
1.98x10-19
3.04x10-19
3.96x10-19
4.42x10-19
6.6x10-19
silicon substrate
5).
(b)
When a photon strikes the silicon, it is very likely to interact with an electron in the thin
layer of silicon (approximately 8µm) and give sufficient energy to the electron to
displace it from the silicon lattice. This creates an electron-hole pair. The displaced
electron, or photoelectron, is collected in the nearest potential well while the hole
created by the loss of the electron is eventually filled by an electron from the silicon
substrate. The more photons which strike the silicon, the more electron-holes produced
and so the more electrons stored within each potential well.
(a)
(b)
Converts analogue data to digital data.
(i)
1.5 / 15 = 0.1V
(ii)
Qu.
1
2
3
4
5
6
6).
7).
8).
ADC output
2
0
6
9
13
15
(c)
Errors resulting from an analogue voltage being converted into a digital value when
the resolution of the ADC is low.
(a)
(b)
(c)
(d)
The ratio of the number of photoelectrons emitted to the number of photons incident on
the pixel expressed as a percentage.
90%
So that it is very sensitive in low level light without introducing noise.
1/20
(a)
linear magnificat ion 
(b)
6/2=3
(a)
How close two objects can be together and still form separate images that are at least
two pixels apart.
See Support booklet, page 5.
peaks separated by two pixels => peaks 20µm apart. => 20x10-6 = 3θ
where is the angular separation.
=> θ = 2x10-6/3 = 6.7x10-7 radians = 0.000038°
Advantages of CCDs:- e.g. more sensitive, sensitive to a greater range of wavelengths,
immediate results, etc.
Disadvantage:- e.g. more noise, “grainier”, etc
(b)
(c)
(d)
9).
Input voltage V
0.25
0.00
0.68
0.95
1.36
1.50
(a)
(i)
(ii)
image height
I

object height O
Assume each pixel is square => each pixel has a side of 10-5m.
=> CCD is 3mm square
Area is 9mm2
(b)
10).
It takes 3µs for the charge to move from one pixel to the next, so it takes 300µs to move
the charge along one column.
=> maximum rate is 1/300µs  3300 pictures per second,
assuming that the output shift register and ADC do not slow the process.
Produce an illustrated summary of the use of CCDs and summarise their advantages over other
imaging technologies.