Re: colorsync-users digest, Vol 3 #315 - 4 msgs
Re: colorsync-users digest, Vol 3 #315 - 4 msgs
- Subject: Re: colorsync-users digest, Vol 3 #315 - 4 msgs
- From: "Tony Riley" <email@hidden>
- Date: Mon, 22 Jul 2002 09:20:10 +0100
Please bear with me folks, I'm trying to contribute! My purpose in posting
was to try and gather some information about problems list members might
have experienced in getting accurate color in painting copies. Can I remind
you of what I said originally. And thanks for the private emails I have
received. Some from museum/gallery photographers who have persuaded
employers to shell out for a line scan back, and found them not
color-accurate enough.
>
This is my first posting to the list, being an imaging science student
>
> preparing a dissertation on digital reproduction of paintings. > >
>
> Thanks
>
> Tony Riley (Kendal, UK)
>
> email@hidden
>
> www.tonyriley.dsl.pipex.com
>
Date: Sun, 21 Jul 2002 07:45:13 -0700
>
From: Dick Busher <email@hidden>
>
I thought all of these digital capture devices were nothing more than
photon counters. They
>
'see' the 'light' coming from the objects. They have no ability to 'know'
anything regarding
>
the physical/chemical composition of the object. The task of the device
software is to convert the analog signal generated by the device into a
digital file that results in an 'image' that matches what a 'standard
observer' sees when looking at the object. One of the difficulties is that
the device responds to more wavelengths than the human eye, and its response
curve does not correspond to ours. The problem is to force the device +
software to do so.
Silicon has a fundamentally biased response to wavelength, so the photosites
are filtered to optimise the ccd's response for its particular purpose. Even
at this level I feel its more than just a photon counter. What is the
purpose of a scanner? To reproduce the 'density' of cmy photographic dyes as
closely as possible. The signal can then be put through ADC and maybe DSP.
The purpose of the area array's response is different - its to reproduce the
spectral content of the original image ('colorimetric'). I feel this point's
been missed, with the camera system do you want to optimise for density
response or wavelength, you can't do both. cmy dye makers can avoid IR
reflectance, paint pigment makers aren't interested in the effect that
invisible relectance will have when a painting is scanned, so some pigments
reflect IR more (that value is included in the data). e.g. cobalt blue will
end up looking purplish. You can't just filter it out totally and perfectly
afterwards.
>
Date: Sun, 21 Jul 2002 12:58:50 -0400
>
From: Roger Breton <email@hidden>
>
In case you did not know, the London Art Gallery is using a special custom
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designed 7 channel color scanback to capture all its collection. The work
was conducted by Dr. Roy Burns. >
Yes, I am very familiar with Dr Berns publications, and the IBM research
ccd/camera built to get round the fundamental scan back (cmy) and area array
(rgb) problems I have described. Didn't know about the London AG though,
thanks. There are many research approaches to getting accurate color -
multispectral, and even hyperspectral being talked about (the balance of
wavelengths arriving at each sensor site are analysed). Our three channel
approach can't really be expected to give accurate color. Look how much
better spectrophotometric profiling is (typically 30 - 10nm bands) than
colorimetric profiling (just the three RGB bands).
>
Date: Sun, 21 Jul 2002 21:34:08 -0400
>
From: "Joseph A. Castay" <email@hidden>
>
>> There's no such thing as RGB pigments. RGB is additive color, CMY is
>
>> subtractive. To put another way, RGB is light and CMY is
pigment/paint/dye.
>
>>
I don't know what to say about this Joseph. I am stumped!
Tony Riley (Kendal, UK)
email@hidden
www.photosightdesign.co.uk
(very much under construction, sorry for slow image loading)
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