>Date: Sun, 21 Sep 2003 09:50:53 +0800
>From: "C.H.Ling" <chling@xxxxxxxxxxxxx>
>Subject: Re: [OM] New E-1 (Effect of pixel size on dynamic range)
>
>In dimmer light, using longer exposure and low ISO will put the noise at
>very low level even with the C5050's tiny sensor without noise reduction
>enable.
Yes, I should have included that. The total number of photons or electrons
accumulated is the product of image intensity and time, just as it is for film.
Unlike film, CCDs don't really have ISO values. The "ISO" of a CCD is
basically a mathematical fiction used to allow the f-stop and shutter speed to
be set to achieve a specified average (18%) image brightness.
>- ----- Original Message -----
>From: "Joe Gwinn" <joegwinn@xxxxxxxxxxx>
>
> > The theoretical limit of the "Ideal Observer", achieved in scientific CCDs
>anyway, is to be limited only by shot noise in either the photon flux
>falling on the sensor, or in the electrons collected by that sensor. These
>both follow Poisson statistics, which I described some time ago. Basically,
>the standard deviation of the noise is equal to the square root of the
>average level. This is inescapable, arising from the physics of light and
>of electron devices. So, to achieve a given signal-to-noise level, one
>must collect a certain number of photons (or electrons) per pixel. In
>bright light, a small pixel can do this in a reasonable amount of time. In
>dimmer light, the pixel must be larger, or one must accept a more noisy
>image.
At 10:07 PM +0000 9/21/03, olympus-digest wrote:
>Date: Sun, 21 Sep 2003 00:30:16 -0400
>From: W Shumaker <om4t@xxxxxxxx>
>Subject: Re: [OM] New E-1 (Effect of pixel size on dynamic range)
>
>- --=====================_61689935==.ALT
>Content-Type: text/plain; charset="us-ascii"
>
>Don't fancy telescope cool their CCD's to reduce noise?
Yes, to reduce dark current. This is an important effect, but one unrelated to
the photon/electron statistics I have been talking about.
>I think one of the noise issues with the E-10 was due to how hot it got.
This is certainly plausible, but I don't know if it's true or not. Dark
current typically doubles for every 10 degrees centigrade of temperature rise,
so it would take a pretty hot chip for this to matter. More likely was that
the chip wasn't quite as good as it should have been.
Dark noise is just that, and varies with temperature but not with pixel size or
illumination level, and so has the largest effect when one makes long exposures
of dim targets. Astronomers of course do just that, which is why they always
cool their CCDs with liquid nitrogen.
Joe Gwinn
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