At 09:53 AM 1/12/01 +1000, Rob wrote:
>The only reason I can see that a greater number of bits would help is that
>when you are at the extremities of the CCD's range, more bits should help
>resolve meaningful data from noise, or by reducing the size of the steps,
>reduce the loss of image information which lies between the steps at a lower
>bit depth.
Not quite. There's no point going for extra bits,
without a corresponding decrease in overall system
noise. If the noise is equal to one LSB at 8 bits,
then it's 2 LSBs at 9 bits, 4 LSBs at 10 bits, etc.
If you do the math, you'll find that using a 14-bit
A/D on most CCD scanners is kind of silly; in such
cases, one LSB generally equates to about 10-50
microvolts of signal. A highly dynamic, time-
variant signal, at that.
One way to reduce the noise is averaging -- ie.,
scanning with several passes, and averaging the
results.
Alternatively, just scan more slowly, and accumulate
a larger charge in each CCD cell. Of course, the CCD
and the analog front end must allow for this without
saturation.
It's true that at the shoulder and toe of the film's
response curve, those extra bits -- if they were
real -- would be most useful and welcome.
rafe b.
