Write:

1
Write IB Physics 4 Life! in binary
2
8. Digital Technology

• Chapter 8.2 Digital imaging with charge-coupled
  devices

3
Capacitance

• Any two conductors that are separated by either a
  vacuum or an insulator are called a capacitor.
• This might include two parallel plates a certain
  distance apart, two conducting spheres in a
  vacuum a certain distance apart or even a single
  conducting sphere isolated from the earth by an
  insulating stand.

4
Capacitance

• Consider two parallel plates a distance d apart
  as shown below.

• The plates are connected to a source of potential
  difference V, provided by a battery.

• When the switch S is closed, a current will flow
  for a short time and then stop.
• The current will flow in a anticlockwise
  direction (the electrons will move clockwise).

• The negative charge will accumulate on the bottom
  plate, leaving behind an equal amount (in
  magnitude) of positive charge on the top plate.

5
Capacitance

• The amount of charge that can accumulate on
  either plate, given the p.d. of the battery is
  determined by a property known as the capacitance
  of the parallel plates.
• The amount of charge Q that can accumulate on the
  plates is directly proportional to the potential
  difference V between the plates.
• The constant of proportionality in this relation
  is called the capacitance C of the plates.

6
Capacitor
7
Capacitance

• Capacitance is the charge per unit potential
  difference that can accumulate on a conductor.
  The SI unit of capacitance if the farad (F), with
  one farad (1F) being a capacitance of one coulomb
  per volt (1CV-1)

• The farad is a large capacitance and smaller
  multiple units are used the microfarad (?F),
  nanofarad (nF) and picofarad (pF).
• The capacitance of parallel plates depends on the
  surface area of the plates, their distance apart
  and the material between the plates.

8
The charge-coupled device

• The charge-couple device (CCD) was invented in
  1969 and has revolutionized image acquisition in
  astronomy by providing images of high resolution,
  in digital form, that can be easily manipulated
  and processed.
• These images can be obtained in a fraction of the
  time required using conventional means such as
  photographic film, and can be used to obtain
  images of very faint objects.

9
The charge-coupled device

• The CCD is a silicon chip varying in surface
  dimension from 20mm x 20mm to 60mm x 60mm.
• The surface is covered with light-sensitive
  elements called pixels (picture elements), whose
  size varies from 5x10-6m to 25x10-6m.
• Each pixel releases electrons when light is
  incident on it by a process known as
  photoelectric effect (strictly electron-hole
  production in a capacitor).

10
The charge-coupled device

• We may think of each pixel as a small capacitor.
• The electrons released in the pixel constitute a
  certain amount of electric charge Q and therefore
  a potential difference V develops at the ends of
  the pixel equal to V Q/C, where C is the
  capacitance of the pixel.
• This p.d. can be measured with electrodes
  attached to the pixel.
• The energy carried by a single photon of light of
  frequency f is given by

where h 6.63x10-34 Js
11
The charge-coupled device

• Imaging with a CCD is then made possible by the
  following fact

The number of electrons released when light is
incident on a pixel is proportional to the
intensity of light. This means that the charge
and so the potential difference across a pixel
are also proportional to the intensity of light
in that pixel.
12
The charge-coupled device

• When a CCD surface is exposed to light for a
  certain period of time (by opening a shutter),
  charge and hence voltage begins to build up in
  each pixel.

• After the shutter closes, a p.d. is applied to
  each row of pixels in order to force the charge
  stored in each pixel to move to the row below.
• This is the origin of the name charge-coupled
  as the charges in one row are coupled to those in
  the row below.

13
The charge-coupled device

• Starting from the bottom row, the charge of each
  pixel is moved vertically down into the register.
• From here, one by one, the charge is mode
  horizontally, where the voltage is amplified,
  measured and passed through an analogue-to-digital
  converter (ADC) until the charge in the entire
  row is read.
• The computer that is processing all this now has
  two pieces of information stored.
• The first is the value of the voltage in each
  pixel and the second is the position of each
  pixel.

14
The charge-coupled device

• The process is now repeated with the next row,
  until the voltage in each pixel in each row has
  been measured, converted and stored.

• The charge, and hence voltage, in each pixel is
  proportional to the intensity of light incident
  on the pixel.
• A digital copy of the image is then stored since
  the intensity of light in each pixel is now
  known.
• The process so described would result in a
  black-and-white image.
• It can then be displayed on a computer screen or
  an LCD screen in general.

15
The charge-coupled device
16
The charge-coupled device

• To form a coloured image, the pixels are arranged
  in groups of 4 with green filters on two of them
  (as the eye is most sensitive at green) and one
  blue and one red for the other two.
• The intensity of light in pixels of the same
  colour, say green, is measured as outlined above.
• A computer program is then used to find the
  intensity of green light in each pixel by
  interpolation based on the intensity in
  neighbouring green pixels.
• In this way one has the intensity in each pixel
  for each of the three colours green, red and
  blue.
• Combining the different intensities for different
  colours gives a coloured image

17
CCD imaging characteristics
Quantum efficiency

• Not every photon incident on a pixel will result
  in an electron being released.
• Some may be reflected and others may simply go
  through the pixel.

Quantum efficiency of a pixel is the ratio of the
number of emitted electrons to the number of
incident photons
18
CCD imaging characteristics

• One of the great advantages of CCDs is their very
  high quantum efficiency. It ranges between 70
  and 80.
• This is to be compared to 4 for the best quality
  photographic film and 1 for the human eye.
• However, quantum efficiency is not constant for
  all wavelengths.
• CCDs are now routinely used to measure the
  apparent brightness of stars, which is typically
  of order of 10-12 W m-2.

19
CCD imaging characteristics
Magnification
Magnification of a CCD is the ratio of the length
of the image as it is formed on the CCD to the
actual length of the object.
20
CCD imaging characteristics
Magnification
Magnification of a CCD is the ratio of the length
of the image as it is formed on the CCD to the
actual length of the object.
21
CCD imaging characteristics
Resolution

• A very important characteristic of a CCD is its
  ability to resolve two closely spaced points on
  the object whose image we see, that is, to see
  them as distinct.
• A rough measure of the resolution ability is that
  the images (on the CCD) of the two points do not
  fall on the same pixel. This means that the
  images must be at least one pixel length apart.
• A safer and more conservative measure is to
  demand that the images of two points are two
  pixels length apart.
• In this way, we are sure to resolve the points
  without ambiguities.

22
CCD imaging characteristics
Resolution
Two points are resolved if their images are more
than two pixels length apart

• The resolution is clearly better with a high
  pixel density (that is, number of pixels per unit
  area).
• An image of high resolution is of better quality
  since the image includes more detail than an
  image of low resolution.
• A higher quantum efficiency means that the image
  will require less time to form if the incident
  light intensity is very low and is therefore of
  special importance in astronomical images

23
Medical uses of CCDs

• In medicine the CCD has had a major impact in
  endoscopy an endoscope is a device (a thin tube)
  that can be inserted into a patient to make
  observation of internal organs possible.
• CCDs are now used in endoscopes so that real-time
  images can be obtained.

24
Medical uses of CCDs

• Driven by the needs of X-rays astronomers,
  special CCDs have been developed in which X-rays
  can be detected.
• These devices have been adapted by medical
  imaging researchers for medical use.
• For X-rays with energies below 150keV (which is
  the case with most medical applications of
  X-rays), photons incident on a silicon pixel
  produce electrons via the photoelectric effect,
  as does visible light.
• The X-ray CCD can then act as a detector of
  X-rays, replacing the old X-ray pick-up tube.
• One extra advantage is that the sensitivity of
  the CCD allows for shorter exposure times, with
  an obvious benefit to the patient.
• The negative side is that these devices are still
  expensive.

25
(No Transcript)