Whatever method you currently use for visualising your images,
a cooled CCD imaging system from PixCellent will give
you a much better picture.
Whether you use a standard camera which produces TV signals at
TV frame rates or an integrating camera with a grabber board,
whether you simply "eyeball it" or use an instant film
camera or even other forms of CCD camera, you will find the image
quality from an PixCellent system allows
you to obtain much greater value from your images in a number
of significant ways that are probably critical for your application.
Other systems suffer from one or more of the problems which are all addressed by PixCellent's cooled CCD imaging systems. The problems often encountered with other systems are:
PixCellent's cooled CCD imaging systems
utilise CCDs with high quantum efficiency and which are cooled
to reduce the dark current noise, making them extremely sensitive
in low light and ultra-low light imaging situations. This dark
current is the internally generated signal that can swamp low
light signals. It is reduced by a factor of 10 for every 20oC
reduction in CCD operating temperature. Low temperatures allow
longer exposure times, up to many hours for some applications.
PixCellent's cooled CCD imaging systems
digitise the images with up to 65536 grey levels per pixel making
them uniquely able to view extremely low contrast features even
in the presence of very bright regions in the image. The cooling
allows much lower signals to be detected without affecting the
level of the strongest signal that can be handled.
PixCellent's cooled CCD imaging systems generate digital data directly without the need for a grabber board and the data are of very high linearity and precision. The measurements you make with an PixCellent system will be more accurate and quantitative compared with those from systems that grab from analogue signals.
This leaflet acts as an overview and the following sections lead
you through the principal choices that you need to make in selecting
your system. Remember, we do not expect you to make the best selection
without our help, and this we are always happy to give.
PixCellent has two main camera families
that differ principally in terms of the read-out rates at which
they operate:
The Capella family (Click
here for Capella Data Sheet)operates at pixel rates between
500 kHz and 8MHz. It has 12-bit (4096 grey level) or 14-bit (16384
grey level) dynamic range and must be used with CCDs suitable
for such high speed use, mainly supplied by Kodak and by EEV Frame
Transfer CCDs. The Capella allows much
faster set-up (such as focusing and alignment), than the other
systems. Users may then select a slower read out speed if they
wish, in order to optimise the read-out noise which may be as
low as 8 electrons RMS.
The Antares controller
(Click here for Antares Data Sheet)reads
out the image from the CCD detector at relatively slow rates (up
to 167 kHz) in order to achieve the very lowest noise (as low
as 2 electrons RMS) and the widest dynamic range. The Antares
system uses precision analogue to digital converters (ADC), runs
at up to 167 kHz and gives 16-bit data (over 65,000 grey levels).
The trade-off when selecting a controller is therefore between
the read-out speed of the Capella and the exceptional
dynamic range, low noise and read-out format flexibility of the
Antares family at somewhat slower read-out
rates. Remember, larger CCDs take longer to read out, though if
you expect your exposures to be long (many seconds) the CCD read-out
time for the Antares series may
be insignificant.
With all PixCellent's imaging systems the time taken to read out an image can be reduced dramatically by:
The Antares family has
sophisticated on-chip binning and multi-region read-out capabilities
so that much faster frame rates may be obtained while preserving
the best system noise performance.
The Capella family has simpler sub-array
and on-chip binning facilities than the Antares
family, more appropriate to the faster read-out capabilities of
the system.
In selecting the CCD you want, remember that the larger the
area of a CCD, the more pixels it contains, and the higher the
CCD grade you select the higher will be the price of your CCD.
Generally choose the smallest CCD that will do the job you have.
Not only is this the cheaper approach but smaller CCDs are faster
to read out, and subsequent image display, processing and archiving
are all proportionally faster. It is usually possible to upgrade
a system CCD at a later date should you need more resolution or
sensitivity.
If you have selected a Capella fast read-out
camera then select from the Kodak range of CCDs or the EEV frame-transfer
only.
If you need the highest sensitivity to light then select a back-illuminated
CCD for the highest quantum efficiency. Back-illuminated (thinned)
CCDs are relatively expensive as they are made in small volumes.
Good thinned devices are now available from EEV and SITe.
If you need blue sensitivity (400nm or shorter) choose a thinned
CCD (expensive) or an luimogen coating (much cheaper but having
lower sensitivity to blue light yet higher sensitivity in the
ultra-violet, out to 90 nm). The luimogen coating is available
on CCDs from EEV and Kodak.
The EEV CCDs have an excellent combination of low read-out
noise and dark current, good dynamic range and represent good
value for money. Their large pixels (13 to 27 microns square)
have good maximum charge capacity. Many of the EEV CCDs, including
frame transfer devices, are now available thinned for higher quantum
efficiency, especially in the blue.
The Kodak CCDs offer a wider range of pixel sizes (6.8
to 24 microns) and pixel numbers (from 768 x 512 up to 3072 x
2048). These are cosmetically excellent. However, at the slowest
read-out rates they have higher read-out noise than the EEV CCDs.
The SITe CCDs offer large pixel sizes and exceptionally
high quantum efficiency in the blue and red with thinned (back
illuminated) devices.
The pixel size of a CCD is important. The smaller pixels allow
higher resolution to be obtained from a smaller CCD and the overall
size of the CCD is important in determining its cost. However
smaller pixels give smaller pixel charge capacity and smaller
pixels collect light over a smaller area, so the signal they see
will be lower. The pixel size is something that must be optimised
for the optical design of the experiment or set-up which is planned
to be used. Other sizes and makes can be used with PixCellent systems, and the range we
offer is continually updated. Should you prefer a device we have
not listed in our selection guide, we would be pleased to quote
for the configuration required.
A CCD detector operates by accumulating a pattern of electric
charge as a direct result of an optical image falling on the device.
However, charge also accumulates due to electrical leakage in
the device itself. This leakage is called dark current and its
magnitude depends strongly on the temperature of the device. Our
imaging systems use cooled CCDs in order to keep this unwanted
"dark current" to a minimum.
There is also an important CCD technology called multi-phase pinned
(MPP) that reduces the dark current further. Most CCDs are now
available in MPP versions, although such devices tend to have
lower peak signal capacity. Most EEV CCDs are offerred in a new
super-MPP model (AIMO) with the standard high full-well capacity
but with 100 to 1000 times lower dark current. The Kodak CCDs
only operate in MPP mode and the SITe CCDs are available mainly
in MPP versions. The SITe CCDs, though MPP, offer less substantial
improvement in dark current. Thinned CCDs (both EEV and SITe)
have higher dark currents than front illuminated CCDs by a factor
of three or four. The CCD datasheets give the dark current for
the various CCD types.
For nearly all applications thermo-electric (Peltier) coolers
are the best choice for convenience and give a satisfactory level
of dark current. Air cooled heat exchangers are operationally
most convenient and provide cooling to about 65oC. below ambient
temperature. Liquid cooling can reduce the temperature by a further
10oC. giving dark currents which are a factor of 3 lower than
with air cooling.
For the longest integration times without loss of full well capacity,
liquid nitrogen cooled vacuum dewars are available. Our new, high-performance,
unit is suitable for most. The older, standard sized unit is also
still available. Liquid nitrogen heads are less convenient since
you need a supply of liquid nitrogen as well as having occasional
access to a vacuum pump.
Your computer requirements are normally determined by the software
packages you wish to run.
The software specification sheetspecifies the minimum computer
requirements. We find that users need plenty of data storage capacity
because these systems can generate large amounts of image data.
Choose the fastest computer you can afford such as a fast Pentium
class machine with a PCI bus (for Capella
based systems). Both Capella and Antares
cameras need an ISA slot as well.. Allow plenty of computer memory
if you want to store several images in memory at once. However,
remember that computers are very easy to upgrade after purchase.
Do not overlook the issue of data archiving and back-up. Tapes
are cheap, with DAT drives offering capacities up to 8 Gigabytes
on a single tape. Re-writable optical drives are very convenient
and falling in price. Remember, computer systems are increasing
in performance rapidly and falling in price so the minimum configuration
specified here may be well below what you can afford.
Users generally find that they end up wanting to do much more
than they had imagined possible, andit is work specifying your
computer system relatively generously.
PixCellent offers a wide variety of
options such as fast liquid crystal shutters, filter wheels, gated
image intensifiers for ultra-fast imaging, specialist interfaces
to X-ray equipment, microscopes or electron microscopes, spectrographs,
advice on filters, advice on X-ray or other phosphors and scintillators,
polarisers, lenses and most other optical components. Please contact
us for help and advice on your application.
This is a relatively high light level application since the excitation light levels are often very high. The CCDs all have excellent red response which favour green or red fluorophores. Blue fluorophores may require your CCD to have a luimogen coating. Note that if you avoid blue or ultra-violet excitation the intrinsic fluorescence from optical components, chemicals and materials is reduced. Select thermoelectric coolers with either Capella or Antares controllers, depending on whether you are concerned about fast read-out speed for dynamic microscopy applications. Select whatever CCD matches your resolution requirements.
Often a very low-light level application with weak signals. Use a liquid-cooled thermoelectric head and a good MPP CCD or a liquid nitrogen head with a standard CCD. Expect long exposure times even with enhanced chemi-luminescence. Optimum light gathering efficiency is often critical and this can normally be achieved more easily with larger area CCDs that ones that have large pixels.
Use a phosphor screen lens coupled to the CCD. Large de-magnifications are inefficient and at lower X-ray energies the optical signal from the phosphor screen may be weak. The Antares family of controllers is often used unless fast read-out is needed as in tomographic applications when many images need to be captured in rapid succession. Usually a thermoelectric head is satisfactory. A separate application note on X-ray imaging is available.
Long exposures usually call for liquid nitrogen systems, and thinned large area CCDs are the usual choice, with the emphasis on the lowest read-out noise for spectroscopy. Customers usually specify the Antares family of controllers.
Standard MPP small area CCDs, such as the EEV CCD39 in a thermo electric cooler, are well suited for this application. The Antares controllers allow both acquisition and fast guiding to be handled by the same CCD system.