Relative whiteness of paper

Frank A. Reynolds <fr0c+ [at] andrew__cmu__edu> writes

>I am photographing
>watercolor paintings that where recently given to the Institute.
>The paintings are on what visually looks like "white" watercolor
>paper, but they have a slight blue cast to them in the photographs.
>There is Kodak color chart in the photographs that has a white patch
>that is "white" in the photographs which verifies that the blue cast
>is not a bias in the film, but a characteristic of the paper.  I
>suppose a densitometer can do this (?), but for those of us that do
>not have one it would be helpful if you could show someone visually
>that a paper has a warm or cold bias.

A reflection densitometer is what you need. If the Kodak color
chart's white patch appears white in the final print, it is a good
sign that everything is under control. If the patch appears white in
the final print but the painting does not, then the painting is not
really white, at least to the film.

A film only has one way to see colors. You eyes and your perception
tend to make all kinds of adjustments when you look at anything.
Films don't do that. They see things (e.g., infrared part of the
spectrum) your eyes won't be able to see. If your eyes are looking
at a slightly grey patch, next to a solid black area, you will think
that the "white" patch is bright white while it may be 0.20 grey.

The colorants selected in the manufacture of the MacBeth
ColorChecker are carefully selected to make sure they don't change
colors under a variety of lighting conditions, be it tungsten or
daylight. Colors in a painting do not behave that way however. Under
a warm light you'll have a warm effect, etc. Dentists have problems
with this when they replace people's teeth. If the replacement tooth
appears to match surrounding teeth in the patient's mouth in their
office, it will have a tint outdoor, or under tungsten lights, etc.
Those in the business of matching colors normally use a light source
that provides a standard 5,000 degree Kelvin.

You can get a reflection plaque (enameled) from MacBeth that has
both a solid black patch and a bright one, calibrated. Since it is
enameled it should last a long time and provide you with a reference
white that does not change. Don't use papers for this, because they
change quickly, because of brighteners, etc.

Luis Nadeau
Fredericton, New Brunswick, Canada

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                  Conservation DistList Instance 9:11
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