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Re: [OM] Sainte-Chapelle

Subject: Re: [OM] Sainte-Chapelle
From: Fernando Gonzalez Gentile <fgnzalez@xxxxxxxxxxxxx>
Date: Sun, 23 Aug 2009 00:43:10 -0300
Yes, they clip awfully specially in the red channel, green and blue a 
little less so; but they shouldn't.

Nonetheless, I was carefully matching what I was seeing in the 
transilluminated Ektachrome to what I was seeing in a supposedly 6500ºK CRT.
 From the consistent red - magenta shift ( and clipping ) I can only 
blame the lamp: it may have frequency peaks in the green spectrum, so 
when dominant color is blue, it shows magenta into the blue; and when 
dominant color is yellow, it shows red into the yellow.

I think that what may be *ideally* needed is a light source which has an 
even spectrum, that would properly be a pure white light source.

I certainly thought about what you had already advised me about using 
the monitor as the light source, but the same rules: you're seeing 
white, but in fact is a tristimulus (*) of red, green and blue. I ended 
in not using it because, at least in my monitor, luminance is too weak 
and colors shift away too bad. Worse than with my Philips lamp.

I cannot blame the Nikon on this one: if it has something wrong it could 
be a blue cast, which I am about to investigate

In short: having an uneven and too intense light source what I find to 
be the reason of my clipped scans with red-magenta hue. Naturally, this 
reason rules after I had learned and practiced a little bit with the 
scanner and photoshop, that is not a long time.

Did I make myself clear?

Thanks for answering (once more ... ), C.H.

Fernando.

(*) <http://en.wikipedia.org/wiki/CIE_1931>, 
<http://en.wikipedia.org/wiki/CIE_1931#The_CIE_standard_observer>
"In the CIE XYZ color space, the tristimulus values are not the /S/, 
/M/, and /L/ responses of the human eye, but rather a set of tristimulus 
values called /X/, /Y/, and /Z/, which are roughly red 
<http://en.wikipedia.org/wiki/Red>, green 
<http://en.wikipedia.org/wiki/Green> and blue 
<http://en.wikipedia.org/wiki/Blue>, respectively. (Note that the X,Y,Z 
values are not physically observed red, green, blue colors. Rather, they 
may be thought of as 'derived' parameters from the red, green, blue 
colors.) Two light sources, made up of different mixtures of various 
wavelengths, may appear to be the same color; this effect is called 
metamerism <http://en.wikipedia.org/wiki/Metamerism_%28color%29>. Two 
light sources have the same apparent color to an observer when they have 
the same tristimulus values, no matter what spectral distributions of 
light were used to produce them."

C.H.Ling wrote:
> Your images are poorly clipped, I never have a 4000ED scan like that. Don't 
> understand how the scanning workflow related to your projecting lamp. For 
> color adjustment of a scanned image I open a white box in PS and put the 
> slide over it for comparison.
>
> C.H.Ling
>
>   



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