Transcript Charge Couple Devices

Charge Couple Devices
• Charge Couple Devices, or CCDs operate in the charge domain,
rather than the current domain, which speeds up their response
time.
• They are made up of an array of metal-oxide-semiconductor
(MOS) capacitors.
• Individual discrete detectors are made in a line array or area array.
• Small pixel size of the CCD ~5 mm, with a provision for optical
input, make the device suitable for high-density imaging.
• CCDs are used for almost all digital imaging devices today.
Charge Couple Devices
A CCD shown as an array of MOS capacitors
Charge Couple Devices
• CCDs rely upon the charge generation in the depletion region of
the MOS capacitor.
• An applied voltage, which is pulsed, holds the charge created by
the photons generating current in the photodiode.
• The potential in the capacitor is linearly dependent on the voltage
applied to the surface electrode.
• The charges are confined laterally, since the neighbouring MOS
capacitor is at a lower potential, so the charges do not leave the
potential well.
Charge Couple Devices
Potential well (p-channel) in a surface of CCD.
Charge Couple Devices
• The amount of charge generated depends on the light intensity and
the period during which light is allowed to fall on the CCD.
• The charges are then shifted out serially from the array using a
shift registry.
• The stored charges in the pulses mutually repel each other,
although thermal diffusion and self-induced drift can blur
individual pulses.
Charge Couple Devices
A CCD array imager.
Charge Couple Devices
• A pixel represents a single point in an image, and for a CCD camera,
it's often defined by the optics of a much-reduced image area.
• A pixel comprises of three to four MOS capacitors, or a size on the
order of 10 microns, so the charge collection can have a higher
potential (see next page).
• 1024 x 1024 dot-matrix arrays are common with 4096 x 4096 now
readily available for commercial cameras.
• Good film has a resolution of only 2024 x 2024, high-quality film has
4048 x 4048 and very expensive film has a slightly higher than this.
• Film’s collection is non-linear, especially at low and high light
intensity so underexposure or overexposure is common.
• CCD signal collection is linear so what you see is what you get.
• Thus, modern CCDs are much better than film.
Charge Couple Devices
A four phase CCD, showing four MOS capacitors/pixel.
• Often 3-5 capacitors are used per pixel to increase the signal and
to vary the resolution.
• Higher resolution imaging requires the use of more pixels, which
requires using less capacitors/pixel.
• High resolution imaging uses 3 capacitors/pixel, medium
resolution imaging uses 4 capacitors/pixel and low resolution
imaging uses 5 capacitors per pixel.
Charge Couple Devices
Color imaging is accomplished by the use of filters.
Color CCDs have less spatial resolution than black-white CCDs are
more individual CCDs per pixel must used (4x).
CCD Dynamic Range
Dynamic range is the gray-scale that is presented in an image.
Film has a gray-scale of ~90 dB.
Currently, good commercial CCDs have a gray-scale of ~80 dB.
Experimental CCDs being developed have been reported to have 117
dB.
1 ms
10 ms
Dynamic range is increased by the CCD
elements, which represent one pixel,
collecting photons for different
lengths of time.
For high intensity light (sunlight), a
group of CCDS will collect photons
for a microsecond.
For dark conditions (night), a group of
CCDs will collect photons for 100s
50 ms
100 ms
of microseconds.
Charge Couple Devices
• A CCD is not a digital device.
• A CCD is a sampled data device, which means that it continuously
collects and processes data. It is an analog device.
Why is an image collected by a CCD camera considered digital?
• The amplitude of the signal depends linearly on the collection, which
is very useful for taking measurements of light intensity, i.e., the light
signal can be quantified. This also makes it easy to digitize the signal
by an A/D converter.
Charge Couple Devices
Because the CCD collects light linearly, it has a simple transfer
function, Ta, ideally given as,
Ta  I o  KI
where k and Io are constants and I is the intensity of the light on the
detector, which is the forcing function of a 1st order measurement
system having a ramp input.
Charge Couple Devices
• Since CCDs collect light linearly, they are very useful in quantitative
studies where the number of photons, or intensity has physical
meaning.
• This fact makes them very useful for electron microscopy or
holography studies where the gray scale has physical meaning.
• We’ll be looking at this more closely later in the course.