# Re: SANE & exposure times

Andreas Rick (rickand@gemse.fr)
Fri, 30 Jul 1999 12:18:01 +0200

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Didier Carlier wrote:
>
> ewald@pobox.com said:
> > The main use is for scanning negatives. For negatives, density of blue
> > is ~3x that of red, and that of green is ~2x that of red (hence the
> > orange color of the negative mask). So if you scan with 1:1:1 exposure
> > time, the green channel will only use 1/2 of it's full range. In other
> > words, you have lost one bit of what is basically your luminance. The
> > blue channel is filled for only 1/3 and this poor blue definition can
> > also be very visible to the human eye.
> >
> > All this can be solved by scanning with a 1:2:3 exposure so that the
> > full range of all channels is utilized.
>
> If I look at the RGB values after a scan of a negative, the values of all
> channels go from 0 to 255, so I'm not quite sure of what you are trying to do.
>
> Putting the multiple scan issue aside you can't get more than 8 bits of
> resolution per channel so that you don't loose definition on the blue channel,
> you loose dynamic range.
> I.e. you do not extract the clearer blue details, but you do keep the full 8 bit
> color resolution for the dynamic range that was scanned.
> Am I missing something here ?

I think you are right: When scanning negatives with the Coolscan scanners
the backend tells the scanner it is doing a negatives and the
scanner adapts its exposure range to the right levels for negatives.
That's why you don't get the same image when scanning a negative
using "positive" as a backend-option and inverting the image yourself.
I havn't tested the quality of this exposure adaptation throughout
a significant image base of negatives yet, but I didn't have any problems with
it either.
Perhabs this is different for the HP-Photosmart?

So without doing multiple scanning there is no way of getting more
than 8-Bit out of the LS-20, 10 bit out of the LS-30 or 12 bit out of the
LS-2000 and there doesn't seem to be much neccessity to change
exposure values to get these 8/10/12 bit.
Can the other users of these Scanners confirm this hypothesis?

Apart from that I still think it is a neat idea to have a
calibrated system where you can figure out the optical density
from the RGB values (but I know it is difficult to achieve).
When scanning color images, optical density allone is not enough,
we need to know what color (wavelength) was used to do the
scanning and what is the detector efficiency as a function of
wavelength.
We may need to say what is "standard Red" what is Green and what is Blue
and try to do a color space conversion from the scanners
color to the standard space.
I don't know too much about the subject, but I think there must be
some standards allready defined that we can use.
Who does know more about this.

Andreas

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- Next message: Stephen Williams: "Re: SANE & exposure times"
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- Maybe in reply to: Ewald R. de Wit: "SANE & exposure times"
- Next in thread: Stephen Williams: "Re: SANE & exposure times"
- Reply: Stephen Williams: "Re: SANE & exposure times"
- Reply: Ewald R. de Wit: "Re: SANE & exposure times"