Imaging system selection guide


Why you need a cooled CCD system from PixCellent

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.



How is this possible?

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's families of imaging systems

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.


Area of Interest read out and Pixel Binning

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.


Selection of CCD make, size and pixel numbers

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.



Selection of the type of cooler you should use

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.


Things to consider when selecting your computer system

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.


Pix
Cellent
can supply many options and accessories for your application

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.


General summary of some popular applications



1. Fluorescent imaging including fluorescence microscopy

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.



2. Chemi-luminescent and bio-luminescent imaging microscopy

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.



3. X-ray imaging for medicine, non-destructive testing and diffraction studies

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.



4. Astronomical imaging, polarimetry and spectroscopy applications

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.



5. Astronomical acquisition, guiding and wavefront sensing

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.

 

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Site last updated: 12 April, 1999.