Interpreting spectrophotometry data

We recently purchased a spectrophotometer and have just begun using
it to determine colour change in our photographs collections. With
the help of Roy Perkinson's article Statistics Without Anesthesia we
have set up a spreadsheet in Microsoft Excel which incorporates the
standard deviation calculation and the student 't' test, and which
tells us the delta E result before and after readings. We now have a
mechanism for determining if a statistically significant change has
occurred. My question is how important is that statistically
significant change?

We have had a loan request for ten 19th century photographs) for 4
venues,(approximately 300 days of exhibition).  The photo images are
in very good condition, and so, we have expressed concern about the
lengthy exposure time. We will take readings of the photographs
before and after each venue and have proposed that our museum
reserve the right to remove any photo based on the results of the
readings.

Are we right in thinking that if two readings show that a
statistically significant change has occurred that the photo is
beginning to change and that the object should not be exhibited at
the following venue? Or are we being too cautious and that just
because a change is indicated doesn't mean that the object has
changed perceptibly and that we should not immediately consider it
damaged? Should we be establishing for ourselves what we consider to
be an acceptable change? And if so, how would we quantify a change
if we don't rely entirely on the statistical change indicator?

I seem to be going around in circles but I would like to have a
clear notion of what the spectrophotometer is really telling us so
that I can transmit this info to our curators. Hopefully we'll all
be able to agree and to proceed based on fact rather than on
suspicion.

Margaret Morris
Senior Conservator
Canadian Centre for Architecture
1920, rue Baile
Montreal, Quebec H3H 2S6
514-939-7001ext.1239
Fax: 514-939-7020


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                  Conservation DistList Instance 17:56
                 Distributed: Friday, February 20, 2004
                       Message Id: cdl-17-56-018
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