The document discusses capacitance and how charge accumulates on parallel plates when a potential difference is applied. It then summarizes how charge-coupled devices (CCDs) work, noting that each pixel acts as a small capacitor that releases electrons proportional to the intensity of light hitting it. The charges are transferred to a register and converted to a digital image. Key characteristics of CCDs discussed include quantum efficiency, magnification, and resolution. Medical uses of CCDs in endoscopy and X-ray detection are also summarized.

1. Digital
Technology
Digital imaging with charge-coupled devices

2. 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.

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

4. 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.
CVQ

5. Capacitor

6. 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.

7. 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.

8. 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).

9. 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
hf
hfE where h = 6.63x10-34 Js

10. 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.

11. The charge-coupled device
• 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.
•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.

12. 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.

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

14. The charge-coupled device

15. 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

16. CCD imaging characteristics
• 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
Quantum efficiency of a pixel is the ratio of the number of
emitted electrons to the number of incident photons
%100
photonsofNumber
ronsphotoelectofNumber
efficiencyQuantum

17. 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.

18. 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.
O
I
heightobject
heightimage
ionmagnificatlinear
Object
O
lens CCD
I
Image
©IKES07

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.
O
I
heightobject
heightimage
ionmagnificatlinear
Object
O
lens CCD
I
Image
©IKES07

20. 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.

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

22. 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.

23. 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.