# Re: SANE & exposure times

Stephen Williams (steve@icarus.com)
Fri, 30 Jul 1999 18:03:20 -0700

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rickand@gemse.fr said:
> 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.

Not so simple. A red sensor, for example, is not just sensitive to a
single color that it defines as red. Instead it is typically sensitive
to colors ranging from ultraviolet to infrared. And if that were not
enough, the source of light has its own spectrum as well.

A red signal is generated when the frequencies that the sensor respond
to, the frequencies that the illuminator emit, and the frequencies that
subject reflect (or transmit, for transparencies) all match. It gets even
more complicated in some scanners, that have color filters in the optical
path. These are sometimes used to compensate for issues wrt illumination
or CCD sensitivity.

The closest thing there is to a calibration table for scanners is
an ICC color profile.

    --
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