CCD Basics and operation

1. Introduction
• All imaging systems are characterized by a few simple
parameters:
1. Size of the sensitive area
2. Number of picture elements (pixels)
3. The range of light levels that the detector can work with.
• There is always a high light level above which the
detector is saturated and a low light level below which a
useful image cannot be obtained.
• Domestic TV cameras have their sensitivity defined in
terms of the lux.
• For quantitative work the lux is a very unsatisfactory unit
since it is defined with a broad spectrum light source.

2. Introduction (Contd.)
• Very roughly, low cost monochrome TV
cameras that use CCDs are often able to
work down to 0.1 lux or better.
• This corresponds to photon arrival rates of
about 1010 photons/second/mm2.
• Color CCD cameras have sensitivity limits
in the region of 10 lux, which are very high
light levels.

3. Sensitivity limits of light detectors

4. Unintensified Imaging Detectors
1-Unintensified Vidicons
• These are a form of TV camera tube where the image that
falls on the target progressively discharges the target.
• A scanned electron beam recharges the target each time
the image is ‘read out’.
• The resolution of vidicons is generally poor.
• In addition, the target recharging mechanism is highly non-
linear, so it is difficult to use vidicons for accurate
photometric work.
• The low light level sensitivity limit is set by internally
generated noise sources which occur even in the absence
of an input signal (dark current and read-out noise).
• This limits vidicon sensitivity to around 10-3 lux at best.

5. 2- Unintensified CCD TV cameras
• They consist of a slice of silicon that absorbs photons and directly
generates a charge (one electron per absorbed photon).
• The generated charges are held in place by an array of electrodes
and the signal is read out by transferring the charge in each pixel to
the device output, one pixel at a time.
• At low signal level the performance of a CCD TV camera is limited by
the internally generated dark current and by the noise generated in
the first stage signal amplifier that is built as part of the CCD chip.
• The best monochrome CCD TV cameras will work down to 10-3 lux.
The CCD response is highly linear, although CCD cameras are often
designed with an intentional electronic non-linearity that gives a
cosmetically more appealing picture on a TV monitor.
• Their resolution is largely set by the pixel size and their sensitivity is
very high.
• By virtue of being solid state integrated circuits they are rugged and
highly resistant to damage from light overload.

6. 3- Unintensified Photodiode Arrays
• They are generally one-dimensional devices, often
with long, thin pixels (2.5 mm by 25 microns is
common).
• They have similar advantages to CCDs in that they
are solid state devices and are therefore rugged.
• The large area of their pixels gives a high dark
current at room temperatures.
• Even if the device is cooled and in the absence of
signal or dark current, the readout structure
generates a rather high readout noise.

7. Intensified Imaging Detectors
• In trying to achieve low light level performance with any of
the above systems we find we are limited by sources of
noise that are generated internally by the detector itself.
• If we can amplify the signal we wish to detect and couple
that intensified signal to the detector then we should be able
to work at lower light levels.
• This can be achieved by an image intensifier which is
placed in front of the detector and consists of a
photocathode which emits many photons or electrons for
each incident photon.
• Image intensifiers produce an improvement in sensitivity
which can be as high as a factor of 1000, or even more.
• However, the intensifiers have a number of consequences
on the overall performance of the system.

8. Intensifiers Consequences on System Performance
1. The light detection efficiency of the photocathode is often poorer
than that of the detector to which it is coupled.
2. The overall luminous gain of the intensifier plus the image
coupling to the detector has to be carefully selected so that no
saturation will result for relatively low input signal.
3. An intensifier reduces the dynamic range of the detector (the ratio
of the strongest detectable signal to the weakest detectable
signal in a single image)
4. With all intensifiers there is a lot of signal-induced background
which limits the dynamic range to a level that can be as low as
100:1.
• Clearly these effects reduce the overall imaging quality of the
system, and imperfections in the intensifier itself (photocathode
non-uniformities, geometric distortions and resolution) will be
added to those inherent in the unintensified detector.
• However, intensifiers are generally capable of making much
lower light levels accessible to the scientist, albeit at a
considerable increase in overall system cost.

9. 1- Intensified Vidicons
• The most commonly used intensified vidicon is the Silicon
Intensified Target (SIT) vidicon which encapsulates a single
stage intensifier inside the vacuum envelope of a standard
vidicon.
• This gives a signal gain of about 2000 and allows work
down to 10-4 lux.
• As with all vidicons the linearity is poor.
• The resolution is reduced although it is possible to
purchase units with fairly high resolution.
• Further improvements in sensitivity are possible by gating
off the readout for several frames while letting the stored
image integrate up before reading it out.
• This cannot be used too much as the intensifier
photocathode itself suffers from dark current which limits
the sensitivity that may be achieved.

10. 2- Intensified CCD TV cameras
• As with the SIT camera, adding an intensifier
to a standard CCD TV camera gives a great
improvement in low light level sensitivity and
levels of 10-5 lux may be achieved with
careful component selection.
• As with the SIT camera the intensifier has
lower detection efficiency (photocathode
rather than silicon) and both resolution and
dynamic range are degraded.

11. 3- Intensified photodiode array
• Use of a high gain image intensifier allows
the high readout noise of the unintensified
photodiode array (PDA) to be overcome.
• Intensified PDAs are widely used for
spectroscopic applications where the
importance of the gain in sensitivity more
than offsets the lower detection efficiency
and poorer resolution of the system.

12. Video Signal Handling
• Both vidicons and CCD TV cameras (intensified or not)
produce standard video output signals.
• These consist of 525 or 625 lines of picture information
every 30 or 40 milliseconds.
• They have a big advantage in that image changes such as
motion may be displayed immediately on a standard TV
monitor.
• Computer cards known as frame grabbers allow a single TV
frame or a sequence of frames to be digitized and passed to
the computer software analysis package.
• This has several consequences:
• The high pixel rate (up to 10 MHz) means that the digitizing
is usually done to no more than 8 bits (256 levels)·
• The fast readout is often a source of noise limiting the useful
range of the data.

13. Video Signal Handling (Contd.)
• One method of improving the signal to noise that can be
achieved from a single video frame is to use a frame
grabber that allows a series of consecutive frames to be
co-added and averaged.
• However, most TV cameras are manufactured so that for
single frame operation the camera performs well. As
soon as many frames are averaged the summed image
can show other fixed pattern noise that cannot be
suppressed by averaging.
• Frame averaging will only work if truly random noise
limits the performance of the system. Video-rate
cameras usually suffer from many other noise sources
and these are not improved by frame-averaging.

14. Cooled CCD Systems
• The CCD is clearly attractive as an imaging detector
because it is rugged, compact, has good resolution,
excellent linearity and high detection efficiency.
• By cooling the CCD, by slowing down the read-out of the
CCD and by breaking free from the restrictions of TV
output format (analogue video signals), a much better
noise performance can be achieved, higher resolution
images can be obtained and dramatic sensitivity
improvements are possible, albeit at a longer cycle time
between images.
• This approach to achieving very low light level sensitivity
without using an image intensifier was originally
developed for astronomy.

15. Cooled CCD Systems
• At room temperatures, standard CCD TV cameras generate a dark
current that is very high - often hundreds or thousands of electrons
per pixel per second is typical.
• If the CCD is cooled, the dark current reduces roughly by a factor of
10 for every 20oC. Typical figures are 10 electrons per pixel per
second at -40oC, and less than one electron per pixel per hour at -
140oC.
• The next barrier to achieving very low light level performance is the
noise that is generated when the CCD is read out at video rates (i.e.
pixel rates of several megahertz).
• If the readout rate is reduced it becomes possible to use special
electronic signal processing procedures (called double correlated
sampling) to give a read-out noise of only a few electrons.
• This can to be compared with a read-out noise often several
hundred times higher at TV read-out rates.
• The net effect of using cooled CCDs and slower read-out rates is
that it is possible to achieve limiting sensitivities of 10-10 lux and
below at full image resolution.