PixCellent is now able to offer a broad range of CCDs from different manufacturers for use with its camera systems. The physical characteristics of all the CCDs offered are basically very similar. However, the CCDs have different performance characteristics (such as spatial resolution, pixel size etc.) designed for specific imaging applications.
A charge-coupled device (CCD) consists of a set of polysilicon electrodes deposited on a silicon substrate and separated by an oxide insulation layer. The electrode size determines the pixel size and hence the spatial resolution of the device.
The electrodes are held at different potentials and incident
photons of light excite electrons in a doped depletion layer on
the silicon substrate. These electrons are captured and held in
position by the applied voltages on the electrodes.
This charge may then be transferred (or coupled) to adjacent electrodes by altering their relative potentials. In this way the charge pattern, corresponding to the pattern of incident light, may be moved along a column of pixels and into an output register for digitisation.
The Dynamic Range of a CCD is defined as the ratio of the largest signal which the CCD can handle to the read-out noise in a single exposure. Typical values for some of the latest Marconi (EEV) CCDs (e.g. CCD30-11) are 600,000.and 4 electrons respectively, giving a dynamic range in excess of 100,000:1. This wide dynamic range is achieved because CCDs are designed for use at high light levels and are normally operated at room temperatures with high dark currents. Cooling the CCD and reading it out slowly dramatically reduces the noise level but has no effect on the maximum signal that the CCD can store.
Top grade (Grade 1) CCDs are supplied as standard, although
PixCellent are able to supply lower
grade devices for price-sensitive applications. The grading of
a CCD relates to the number of defects present in its structure.
These defects occur during the manufacturing process and manifest
themselves as small blemishes in images recorded by the system.
Specially selected `super' grade CCDs which are as near cosmetically perfect as possible can also be offered as an option. Contact PixCellent for more details about these CCDs.
The accompanying table gives information regarding the quality of the CCDs offered by PixCellent. In the table, the defects are categorised as follows:
It is important to appreciate that each CCD manufacturer uses a different scheme for specifying and counting defects in a CCD so the figures given in the table are not quite as directly comparable as they might appear.
The table below lists many (but not all) of the CCDs offered by PixCellent. A great variety of pixel numbers, pixel sizes and sensitivities are available. Please refer to the Imaging system selection guide to help you choose the best device for your application.
The graphs with the CCD data sheets show the spectral response curves produced by a few of the CCDs offered by PixCellent. Quantum Efficiency is defined as the effectiveness of a CCD in generating electrons from the incident light falling on the CCD, as a function of wavelength. Standard CCDs all show similar spectral sensitivities between 400 and 1000nm. However, special techniques can be used to increase the spectral sensitivity of the CCDs. These are discussed in the next sections.
The spectral sensitivity of a CCD can be extended into the ultra-violet region by the addition of PixCellent's Luimogen coating. Deposited as a uniform layer, less than one micron thick on the front surface of the CCD, Luimogen coating extends the spectral sensitivity to around 180nm. If the camera is operated without the standard fused silica front window this sensitivity is further extended to 90nm. The spectral response longer than 500nm is virtually unaffected by the coating, exhibiting the same characteristics as an uncoated CCD. Lumiogen has been shown to be extremely stable over a wide temperature range (-150oC to +50oC) and is unaffected by temperature cycling. It is available for all types of Marconi (EEV) and Kodak CCDs.
The CCD is treated so that the side of the CCD away from the
electrodes is mechanically and chemically etched to an overall
thickness of only 10-15 microns. The CCD is then mounted so that
incident radiation falls on the rear surface of the CCD. This
means that the radiation does not need to pass through the covering
electrodes before entering the silicon substrate and producing
electrons. Consequently, sensitivity to wavelengths at the blue
end of the spectrum is enhanced as no light is absorbed before
interacting with the silicon. The sensitivity of the CCD to light
at other wavelengths is also improved, as illustrated in the spectral
Thinned CCDs may also be coated with Luimogen to further enhance their sensitivity in the ultra-violet region. The spectral response longer than 500nm is virtually unaffected, exhibiting the same characteristics as an uncoated thinned CCD. In some cases the response in the 350-450 nm region can be reduced by coating a thinned CCD.
The thermal energy of the electrons in the silicon substrate
layer allows some of the electrons to break away from the electron
lattice and become free to move through the silicon in just the
same way as electrons which are excited by external photons. These
electrons constitute the "dark current" and are seen
as a signal which is present even when there is no light falling
on the CCD. This signal is generated at all times: between exposures,
during an exposure and during read out.
MPP CCDs have an architecture which is capable of reducing the dark current dramatically. This architecture, called multi-phase pinning (MPP), can give a typical reduction in the dark current by a factor of 100 to 1000. As the dark current is reduced, so the full well capacity of the CCD is also reduced. At the lowest dark current levels, full well capacity may be considerably reduced. Most manufacturers use MPP structures now as standard. Advances in device structure means that the limited full well capacity of MPP CCDs is much less of a problem. The latest Marconi (EEV) CCDs, for example, combine low read-out noise of 4 electrons rms, low dark current of 0.01 electrons per pixel per second at -40oC. yet offer full well capacities of up to 600,000 electrons.
The benefits provided by MPP CCDs are substantial, especially for imaging applications where the exposure times are much longer than the read-out times. In such applications, TE cooled camera heads fitted with MPP CCDs can now be used where the low dark current performance of an LN cooled camera head is required along with the versatility of the smaller TE cooled camera heads.
Not all CCDs are suitable for operation by all the PixCellent controllers. The Imaging System Selection Guide gives more details. Briefly virtually all CCDs may be used with the Antares controller. Fewer CCDs may be run fast so the Capella controller family may be used with the full range of Kodak CCDs as well as the fast Marconi (EEV) frame-transfer CCDs such as the CCD39, CCD47 and CCD57.
Other makes and types of CCDs can also be used with PixCellent's cooled CCD camera system for specialist imaging applications.