UV and glazing

We use the Thomson UV specification when providing performance
requirements for glazing for use in museums.  The Thomson UV
specification is:

    Transmission at 400nm should be less than 50% of that at 550nm,
    and the transmission at 380nm and 320nm should be less than 1%
    of that at 550nm.

For new build projects it is desirable to achieve the UV performance
by using laminated glazing with PVB interlayers (as opposed to films
such as the MT range from Sun-X).  I have discussed this UV criteria
with a number of glazing manufacturers and it can be achieved with
multiple layers of 0.38mm PVB interlayers.  A colour rendering index
of > 96 is also achieved with low iron glass.  However, all samples
provided by the manufacturers have a yellow/brown tint, which in
some cases is an unacceptable visual appearance.  This yellow/brown
tint is primarily due to the low transmission at 400nm.  By relaxing
the criteria at 400nm the colour appearance is improved.

ISO 9050 defines the UV range as 280nm to 380nm, the IESNA anything
below 400nm.  My question is what materials are susceptible to
damage by radiation in the 380-400nm range?  Does relaxing the
criteria at 400nm whilst maintaining it at 380nm compromise the
protection of exhibits from damage by UV radiation?

Arfon Davies
ArupLighting
London


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                  Conservation DistList Instance 17:23
                  Distributed: Friday, August 22, 2003
                       Message Id: cdl-17-23-004
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