C2 Data Capture and Ccds

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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Qu / Capacitance / Charge / Potential Difference
1 / 100µF / 25V
2 / 4mC / 10V
3 / 2200µ / 26mC
4 / 27nF / 50V
5 / 5µC / 20V
6 / 470nF / 24µC
7 / 680pF / 15V
8 / 0.04µC / 120V
9 / 0.47F / 5V
10 / 2.2F / 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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3). Complete the table below.

Qu / EM radiation / Wavelength m / Frequency Hz / Energy of photon J
1 / Infra-red
2 / Red light
3 / Green light
4 / Violet light
5 / Ultraviolet

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. / Input voltage V / ADC output
1 / 0.25 / 2
2 / 0.00
3 / 0.68
4 / 0.95
5 / 1.36
6 / 1.50

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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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(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) 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.

(b)

(c)

Qu / Capacitance / Charge / Potential Difference
1 / 100µF / 2.5mC / 25V
2 / 400mF / 4mC / 10V
3 / 2200µ / 26mC / 11.8V
4 / 27nF / 1.35mC / 50V
5 / 250nF / 5µC / 20V
6 / 470nF / 24µC / 51V
7 / 680pF / 10.2nC / 15V
8 / 333pF / 0.04µC / 120V
9 / 0.47F / 2.35C / 5V
10 / 2.2F / 7.5C / 3.4V

2). (a) The smallest individual light sensing element that is used to sense/form the image.

(b) Charge Coupled Device – an analogue storage system – an analogue shift register.

3).

Qu / EM radiation / Wavelength m / Frequency Hz / Energy of photon J
1 / Infra-red / 10-6 / 3x1014 / 1.98x10-19
2 / Red light / 6.5x10-7 / 4.6x1014 / 3.04x10-19
3 / Green light / 5x10-7 / 6x1014 / 3.96x10-19
4 / Violet light / 4.5x10-7 / 6.7x1014 / 4.42x10-19
5 / Ultraviolet / 3x10-7 / 1015 / 6.6x10-19

4). (a) See the Support booklet.

(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.

5). (a) Converts analogue data to digital data.

(b) (i) 1.5 / 15 = 0.1V

(ii)

Qu. / Input voltage V / ADC output
1 / 0.25 / 2
2 / 0.00 / 0
3 / 0.68 / 6
4 / 0.95 / 9
5 / 1.36 / 13
6 / 1.50 / 15

(c) Errors resulting from an analogue voltage being converted into a digital value when the resolution of the ADC is low.

6). (a) The ratio of the number of photoelectrons emitted to the number of photons incident on the pixel expressed as a percentage.

(b) »90%

(d) 1/20

7). (a)

(b) 6 / 2 = 3

8). (a) How close two objects can be together and still form separate images that are at least two pixels apart.

(b) See Support booklet, page 5.

where is the angular separation.

=> θ = 2x10-6/3 = 6.7x10-7 radians = 0.000038°

(d) Advantages of CCDs:- e.g. more sensitive, sensitive to a greater range of wavelengths, immediate results, etc.

Disadvantage:- e.g. more noise, “grainier”, etc

9). (a) (i) Assume each pixel is square => each pixel has a side of 10-5m.

=> CCD is 3mm square

(ii) Area is 9mm2

(b) 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.

10). Produce an illustrated summary of the use of CCDs and summarise their advantages over other imaging technologies.