Fluorescent tubes and UV

Gregor Trinkaus-Randall <gtrinkaus-randall<-a t->state< . >ma< . >us> writes

>Most of these institutions are lit with fluorescent tubes.  Most
>also do not have UV-filtering sleeves on these tubes.  However, many
>of them, the tubes that is, are located above the plastic shields
>that cover the bottom of the "boxes" in which the tubes are housed.
>Even so, I expected to get high UV readings below these lights. Much
>to my surprise, the readings are low - around 10uM/lumen or 2-3
>uM/M2.

In my experience all the plastic fluorescent tube diffusers which
actually cover the tubes as you described (as opposed to the
egg-crate type which are open and thus do not filter the light)
reduce UV transmission. I have recorded levels between 10-40
microwatts per lumen across my museums. Like Stefan I have also
always assumed that this is due to UV absorbers added by the
manufacturer to extend the life and to stop yellowing of the
polymer. This is a common practise for other types of plastic
materials (check the UV transmission of ordinary laminated glass for
example).

As a quick test I checked the light fittings above me now which are
cheap fluorescent tubes with a plastic opalescent filter and the
reading was 10 microwatts per lumen (the diffusers are at least 15
years old). As a comparison an unshielded tube was 200 microwatts
per lumen.

As for the life span of the diffusers, they tend to become very
brittle over time and often crack when removed for relamping.
However if the you wish to use this type of diffused light source
and the tested UV absorption meets your requirements then this could
save the expense and inconvenience of fitted UV absorbing sleeves to
each lamp.

Andrew Calver
Conservation Manager
Nottingham City Museums & Art Galleries
+44 115 935 0659

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                  Conservation DistList Instance 10:62
                 Distributed: Friday, January 10, 1997
                       Message Id: cdl-10-62-004
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