Colorimeters and densitometers

Karen Potje <kpotje [at] cca__qc__ca> writes

>I would like to know if many institutions have turned from
>densitometry to colorimetry to monitor the effects of exhibition on
>photographs.

At the US Holocaust Memorial Museum, we are working with a
spectrophotometer to take color measurements of objects before and
after exhibition.  The spectrometer can provide L* a* b* readings as
a colorimeter  does.  I have found many sources of error that makes
measuring color changes difficult.  These sources of error include
returning to the exact same location, having the probe the same
distance from the surface each time, and eliminating all extraneous
light from reaching the detector. One technique I have been using is
to  averaging readings taken on several different days because I
have found  differences between  readings  when the spectrometer has
been turned off in between them.

Comparing readings from two colorimeter would have multiple
problems. The two colorimeters might not measure the same size area.
The sensors may be located on a different places on the
colorimeters, so marking the location to be measured would be
difficult, The colorimeter might have different sources of
illumination or different geometry to the surface.

In between any two colorimeters, there would be some difference in
the color readings.  If a color standard was sent with the artifact,
you could evaluate how similar the readings from your colorimeter
and the other institution's colorimeter is.  It would be even better
to have standards that are several different colors.

Definitely using two colorimeters would not detect as small a color
change as using  the same colorimeter would, because more error
would be introduced.  The main question would be if the two
colorimeters could detect an acceptably small change in color.

Allison McGuire Olson
Paper Conservator
US Holocaust Memorial Museum



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                  Conservation DistList Instance 15:6
                  Distributed: Tuesday, June 26, 2001
                        Message Id: cdl-15-6-003
                                  ***