In article , Richard Schtzl <Richard.Schaetzl@xxxxxxxxxxxxxxxxxxxx>
writes
>As far I know, the number of bits of an CCD device produce has nothing
>to do with the dynamical range of this device.
>They are just an indication how fine, exact, it measure the differences
>in brightness, reflection of an object (dynamical range, D), like meter,
>centimeter and milimeter on an ruler.
>For example, flatbed scaners are produced with 8, 10 and 12 bits, but
>there dynamical range is 1.8, slidescaner are also produced with 8, 10,
>and 12 bits, but with an dynamical range between 3.0 (Nikon) and 3.4
>(Polaroid Sprintscan Plus). Nikon has anounced an new slidescaner with
>an D of 3.6, the reason for this lies in the dynamical range of the
>slides, up to 3.8. If your scaner has an lower D as your orginal, you
>have to sacrifie either the highlights or the shadows.
>
Thought it might be better to discuss this off the mailing list since it
is getting a bit off the main topic, but I am quite happy to keep it
there if you wish.
I think you are using a different definition of dynamic range from me.
Normally it is defined in dB, as 20log(saturation limit/noise level)
with both levels expressed in volts, or 10log(.....) if levels are in
power or intensity. That definition certainly gives far higher figures
than the 1.8 - 3.8 you are quoting, which sounds more like the gamma
response than anything else I can think of. What are you defining as
dynamic range?
Whilst the number of bits used in the ADC is unrelated to the dynamic
range of a CCD, using less bits than the CCD is capable of simply limits
the dynamic range further, whilst increasing the bits simply quantises
noise - increasing cost without any increase in performance.
Also recall that the figures I gave were for two-dimensional CCD's,
where the storage capacity is limited by the space available under each
pixel. In the examples that you have quoted, a linear CCD is used where
the storage capacity can be much larger since it is ultimately limited
by the mean free path in the substrate if sufficiently large gates are
fabricated. Linear CCD's with storage capacities of 50-100 million
carriers are not unknown, giving a dynamic range up to 10,000 and
requiring at least 14 bits to fully quantise into the noise floor.
Of course, the time required to scan a frame with such devices makes
them totally impractical for use in a digital camera back - unless you
only want to make pictures with exposure times of the order of several
tens of seconds.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
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