Deterioration of UV filters

On the question of the deterioration of the UV-filtering power of various
UV shields, does anyone have data--either from laboratory measurements or
from measurements in use--related to UV transmittance?  Since many of
the plastic carrier materials yellow with age and therefore absorb more at
the blue end of the spectrum, I understood that UV transmittance can
increase rather than decrease.  A previous search for data got me some
data from a manufacturer of, I think, the Plexiglas version, which showed
only very minor changes over several years of exposure to full sunlight.
I would appreciate any hard data on this subject.

B. Appelbaum

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                  Conservation DistList Instance 10:72
                Distributed: Thursday, February 13, 1997
                       Message Id: cdl-10-72-012
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