UV filters

Just a short note on UV filters:

The perennial question, "how long do they last?" is difficult to
answer for two reasons:  the UV absorber is not always the same
(some age more than others), and the plastic carrier is not always
the same.

Case in point:

15 years ago we tested aged and new samples of an extruded rigid
tube type of filter and found a decrease in total UV transmission
along with an *increase* in higher frequency UV (?!).  The reason
for this was at first elusive but actually simple.  The plastic
extruded film was darkening (making it a better filter) but was also
crazing due to thermal expansion and contraction of the extruded
plastic.  The small cracks were parallel with the length of the tube
and could be easily seen if the tube was squeezed.  These cracks
allowed some essentially unfiltered light to pass through.

Moral?  Don't use rigid extruded filters? (The flexible mylar type
more common these days are probably OK?)  But more importantly, look
for signs of degradation of the plastic film (yellowing, crazing,
darkening) and let those symptoms signal a qualitative UV test.

John Burke

                                  ***
                  Conservation DistList Instance 9:36
                 Distributed: Sunday, October 22, 1995
                        Message Id: cdl-9-36-005
                                  ***