Re: dynamic range and thermal noise
- From: larry n. <spamless@xxxxxxxxxxxxxxxx>
- Date: Mon, 08 Oct 2007 02:32:46 GMT
On Sun, 07 Oct 2007 19:13:58 -0600, "Roger N. Clark (change username to
rnclark)" <username@xxxxxxxxx> wrote:
Scott W wrote:
Roger N. Clark (change username to rnclark) wrote:
Marc Wossner wrote:
On 7 Okt., 15:38, Scott W <biph...@xxxxxxxxxxx> wrote:Yes, the square root of the dark current is the thermal noise.
Marc Wossner wrote:
dear ng,I believe you are really talking about dark current, which is very
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
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
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.
A reference directly discussing electronic sensors:
http://learn.hamamatsu.com/articles/ccdsnr.html
Scroll down to dark current:
"Dark noise arises from statistical variation in the
number of electrons thermally generated within the silicon
structure of the CCD, which is independent of photon-induced
signal, but highly dependent on device temperature. The
generation rate of thermal electrons at a given CCD
temperature is referred to as dark current. In similarity
to photon noise, dark noise follows a Poisson relationship
to dark current, and is equivalent to the square-root of
the number of thermal electrons generated within the
time. Cooling the CCD reduces the dark current dramatically,
and in practice, high-performance cameras are usually cooled
to a temperature at which dark current is negligible over a
typical exposure interval."
And reduction of dynamic range is esxactly why to take many short
exposures and add them For example, if you had a gain of 1 on
your ADC (1 electron = 1 DN or ADU), then your 12-bit ADC
saturates at about 4000 electrons (including a small bias),
so if you had 1 electron/sec dark current, a 1 hour exposure
leaves tou with little dynamic range. Twelve 5-minute
exposure would maintain most of your dynamic range. That is
what I was referring to as not an issue, as the OP was asking
about dynamic range. You are correct that the noise from dark
current is always a factor, and the only way to reduce it is
to work at colder temperatures (e.g. winter nights are better
for night sly imaging than summer nights).
Roger
Displaying the pitfalls of a slightly-above-average I.Q. without logic and
common sense. Smarter people may be able to build bigger bridges, but they just
as easily dig deeper ditches for themselves, without even realizing it. With no
way out than one day relying on someone smarter and wiser than them to save them
from themselves. They rarely allow that, they're too smart for that. :-)
.
- References:
- dynamic range and thermal noise
- From: Marc Wossner
- Re: dynamic range and thermal noise
- From: Scott W
- Re: dynamic range and thermal noise
- From: Marc Wossner
- Re: dynamic range and thermal noise
- From: Roger N. Clark (change username to rnclark)
- Re: dynamic range and thermal noise
- From: Scott W
- Re: dynamic range and thermal noise
- From: Roger N. Clark (change username to rnclark)
- dynamic range and thermal noise
- Prev by Date: Re: Nikon vs Canon and the whole dSLR thing
- Next by Date: Re: Nikon vs Canon and the whole dSLR thing
- Previous by thread: Re: dynamic range and thermal noise
- Next by thread: Re: dynamic range and thermal noise
- Index(es):
Relevant Pages
|
