Re: CMOS vs. CCD -- Link to Article


"Chris L Peterson" <clp@xxxxxxxxxxxxxxxxxx> wrote in message
news:0nj3p1p6v5k6qu3vqq2e6637eh6mf8kbv8@xxxxxxxxxx
> On Sat, 03 Dec 2005 16:42:06 GMT, "Roger Hamlett"
> <rogerspamignored@xxxxxxxxxxxxxxxxxxx> wrote:
>
>>Though this is somewhat 'misleading', since the reason the Canon cameras
>>give this performance is partially the internal processing they perform.
>>The chips contain a factory 'noise map', effectively a bias frame, and
>>data on the rate the noise grows with time for each pixel, and if asked
>>not to perform an 'auto-dark' (some models if asked to do long exposures
>>will perform an automatic dark subtraction), will instead synthesise a
>>'dark' from these numbers, and subtract it. The result is that the
>>dynamic
>>range of the image decreases with longer exposures, and this can be
>>measured and verified.
>
> There is no noise map. Such a thing is impossible. There is a dark
> frame, which is scaled for exposure time and subtracted from the image
> to remove the dark current signal. But the dark current noise remains-
> it cannot be removed by subtraction, only by filtering techniques that
> also destroy image information.
>
> The way that you measure the dark current with a Canon camera is to take
> a long exposure, measure the noise, and square that value. This yields
> the actual dark current signal for that exposure (which can't be
> directly measured because of being subtracted by the DIGIC processor).
> This is only an approximation, however, since it is likely that there is
> some internal filtering going on to reduce the noise.
>
> The Canon sensors have very low readout noise, which is why they perform
> so well for normal terrestrial imaging. Compared to most uncooled
> sensors, they also have low dark current, although this is still quite
> high compared with a cooled CCD.
You are right on the 'map', it is a dark current map, and calling it a
'noise map', is simply stupidity. However the reason I called it this, is
this was the term used by Canon in one of their original releases about
the camera...
They actually perform what is effectively a dark subtraction, but they
then scale the result. The way it is done, they throw away some of the
resolution of the ADC, and as a result the number of resolved steps in the
output declines. This has the 'side effect' of reducing the number of
noise 'steps', but leaves the SNR the same. However they then also perform
an averaging pass, which reduces the resolution, at the same time as
reducing the noise. If you try the experiment of photographing a fine line
grid, at a short exposure, then reducing the illumination, and lengthening
the exposure, you will find the resolution of the camera declines with
exposure time, as they strive to hide the noise.
They are filtering, and they increase the filtering with longer exposures.

Best Wishes


.

Relevant Pages

  • Re: In-camera noise FILTERING is a disaster, in most cases
    ... NR is "dark frame subtraction" and is used when exposure ... There is no heat build up for a long exposures, the noise is from dark ...
    (rec.photo.digital.slr-systems)
  • Re: CMOS vs. CCD -- Link to Article
    ... >data on the rate the noise grows with time for each pixel, ... >will perform an automatic dark subtraction), ... >'dark' from these numbers, and subtract it. ... a long exposure, measure the noise, and square that value. ...
    (sci.astro.amateur)
  • Re: too much light
    ... It's never truly dark out there. ... There is noise in a long exposure that is the same in successive frames. ... and subtract it (or a stack of blackframes ...
    (rec.photo.digital)
  • Re: Deep Sky Imager II Noise Reduction
    ... exposure taken at a certain temperature to a "dark" exposure taken at ... This ensures noise is minimized. ... chip in the DSI II has lower dark current noise than a DSI I chip. ...
    (sci.astro.amateur)
  • Re: NOT scanning negatives as positives
    ... The average noise equals the square root of exposure level. ... increases -- but not as much, so that effectively, if you vary the exposure *of the CCD*, the signal-to-noise ratio improves. ... lower CCD exposure levels, but those levels cannot be increased indefinitely by longer integration/exposure times because that would cause clipping of the most transparent film areas. ...
    (comp.periphs.scanners)