Roger Hamlett
Guest
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 Posted:
Tue Apr 05, 2005 6:10 pm Post subject:
Re: Increasing Sensitivity of CMOS Image Sensor. |
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"david cowan" <david@cowan44.freeserve.co.uk> wrote in message
news:f33da58b.0504050155.737b49a4@posting.google.com...
| Quote: | Hi,
I'm engaged in a low light level application and I'm looking to
increase the sensitivity of an image sensor by:
1. Slowing the clock radically to allow more charge to accumulate.
2. Obtaining a direct ANALOGUE signal from the sensor to allow me to
digitise it with much higher resolution.
Remember that the 'point' of CMOS sensors, compared to CCD's, is that the |
complete camera can be built much more cheaply because major parts of the
electronics can be included in the CMOS chip. Hence it is rare for them to
provide access to the analogue signals. Also the presence of these
components increases the thermal noise inside the chip.
Realistically, what you want to do, is easier to do with a CCD. The Sony
CCD's, are 'favoured', because they have some of the lowest dark current
levels for any chip. Refrigerate the chip, and you can integrate the
signal for minutes (hours!...).
The cheapest 'mass production' camera on the market of this sort, is the
Meade DSI. This has a 16bit ADC, instead of the normal webcam 8/10bit
unit, and will accept exposure times of several minutes if required. It is
limited by the fact that the CCD is not directly coooled, so thermal noise
on the chip, builds relatively quickly. However for exposures up to a few
minutes, it is the cheapest solution.
Going slightly 'up market', Starlight Express in the UK, do a number of
astronomical cameras based on the larger versions of this chip, and some
industrial cameras based on the same chassis. These use unregulated
Peltier cooling to keep the thermal noise better controlled. Some of the
mono models o these have quantum efficiencies well over 60%. SBIG, do
cameras based on the Kodak CCD's (which at room temperature, exhibit more
dark current than the Sony CCD's), with multi-stage Peltier coolers, which
allow the chips to be taken to around 45C below the ambient, and
temperature regulated. Some models of their cameras, reach QE levels over
85%, across quite a large piece of the visible spectrum, and are only
bettered by rear illuminated CCD's costing vastly more.
| Quote: | In principal these devices are incredibly sensitive but unfortunately
(for me )the tend to be aimed at high speed applications. I need only
moderate resolution.
Any thoughts or anyone know of such a device???
Thanks again,
David Cowan
Look at the DSI. It may well be useable, if not the CCD is readily |
available, and sounds as if it might be usable as the basis of such a
unit.
Best Wishes
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