Digital BW, The Print

Truman,

My experience through literally thousands of hours of design and testing
digital imaging systems is that all you do when you increase the exposure
time is simply shift the data values, it does not increase the dynamic range
unless the system has a limitation in the first place.  If this limit
exists, what you CAN do is take multiple exposures with differing exposure
times.  This WILL increase your dynamic range, if done properly...but, the
limitation, as always, is going to be noise.

In a correctly designed system, the data out of the A/D is limited by the
noise floor of the CCD, as the A/D is matched to the output noise of the
CCD.  This means that expanding the voltage into the A/D doesn't buy you
anything, as you will only be converting noise in the lower bits.

Point is, as a general rule, you can't say that increasing exposure time
increases dynamic range, it is very design dependant, and as you probably
know, dynamic range is limited by noise if the overall signal stays the
same.

Austin


> -----Original Message-----
> From: Truman Prevatt [mailto:tprevatt@...]
> Sent: Tuesday, May 27, 2003 12:57 PM
> To: DigitalBlackandWhiteThePrint@yahoogroups.com
> Subject: Re: [Digital BW] Re: Scanning
>
>
> A CCD will work as an integrator. That's the way they are used in
> wideband optical signal processing systems where you are using a laser
> to illuminate a (normally ) an accusto-optics device and the results
> focused on a detector array. Increased integration time will allow an
> expansion of the output voltage range (dynamic range) since if one area
> of the array is related to a area of very low transmission rate and
> another is a very high transmission rate the number of photons counted
> will between areas will expand with the integration time.
>
> This will get the output A/D's more signal to work with. In many cases
> coherent optical process can result into upwards to 90 dB spur free
> dymanic range and non-coherent up to 60 dB. I would expect the same
> thing could be realized in scanners, HOWEVER, there is a DC shift in the
> noise floor with this integration and it has to be accounted for in the
> process, and it probably isn't.
>
> Truman
>
> Austin Franklin wrote:
>
> >Hi Truman,
> >
> >
> >
> >>A CCD counts photons and produces a voltage as a function of the number
> >>of photons detected over a time window (integration time). So while it
> >>is probably a misnomer to call anything associated with CCD's analogue,
> >>the effect is similar if it is done correctly.
> >>
> >>
> >
> >Hum...  Gain is a multiplication factor, increasing exposure is a shift
> >(called an offset).  One will expand the range, the other will
> simply shift
> >it.  Right?  Basically, there are three things you can adjust in
> a signal.
> >Gain, offset and linearity.  Gain is a multiply, offset is an add, and
> >linearity is an add as well, but is on an individual value, not
> an overall
> >value, like a LUT (Look Up Table).
> >
> >
> >
> >>I would much prefer an integration time based on the film and then gain
> >>adjustments of the output voltage prior to the analog to digital
> >>converters - with some care taken as not to increase the background
> >>noise level.
> >>
> >>
> >
> >Correct.  That is exactly how I have done it, and noise is the operative
> >problem.
> >
> >Regards,
> >
> >Austin
> >
> >
> >
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Please visit the Group Homepage to check the Files, Bookmarks, Polls and
other resources as they are often being updated. The page is at:

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