Digital Technology

Digital imaging with charge-coupled devices

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.

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.

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

Capacitor

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.

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.

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).

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

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.

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.

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.

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.

The charge-coupled device

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

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

%100photonsofNumber

ronsphotoelectofNumberefficiencyQuantum

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.

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

heightimageionmagnificatlinear

Object

O

lens CCD

I

Image©IKES07

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

heightimageionmagnificatlinear

Object

O

lens CCD

I

Image©IKES07

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.

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

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.

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.