Re: dynamic range and thermal noise

Roger N. Clark (change username to rnclark) wrote:
Marc Wossner wrote:
On 7 Okt., 15:38, Scott W <biph...@xxxxxxxxxxx> wrote:
Marc Wossner wrote:
dear ng,
I´m still into analog photography but have a keen interest in the
digital technique. As far as I understand it, the dynamic range of an
imaging sensor/camera combo is defined as the maximum signal divided
by the noise which is produced in the various stages. Does thermal
noise still play a vital role in this calculation (as far as longtime
exposures are concerned) or is it so successfully erased by the noise
reduction techniques that it doesn´t have to be be taken into account
in calculating dynamic range?
Best regards for your input!
Marc Wossner
I believe you are really talking about dark current, which is very
temperature depended, not thermal noise. If so then yes for long
exposures dark current can become an issue.

Scott


Well I thought dark current translates into thermal noise so that both
are indeed the same.
Isn´t that correct?

Marc

Yes, the square root of the dark current is the thermal noise.
Dark current is generally low in modern cameras, a fraction
of an electron per second, so it takes a while for it
to become a factor. It is higher with higher temperatures,
like 100F it could be several/second. The simple solution
is take multiple short exposures and add them together.
Effectively, with this technique, dark current is not an issue.

Roger

You might want to look at this
http://en.wikipedia.org/wiki/Thermal_noise

I have never heard anyone refer to dark current as thermal noise.

But forget that for the moment, in your solution of taking multiple short exposures you will still end up with the same ratio of dark current to signal current. What you can avoid is saturation due to dark current, but you will get the same noise from the dark current regardless of whether you break the one long exposure into a number of smaller ones or not.

Scott
.

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