First, 15 years is a long time for UV filters.  They absorb UV light
which breaks bonds of most organic compounds, even UV absorbers.  The
absorbing component is destroyed by the function it is performing.  I'm
not saying the guarantee is bad, I'm just commenting on the nature of
the process.

UV light meters can be purchased from many Conservation equipment
suppliers such as Hanwell Instruments 516-624-2900 (my recommendation);
Talas (favorite of many Conservators) 212-219-0770 for meters that
calculate UV mW/lumen and UV mW output and "percent UV," respectively.
My source for a straight UVA (320-390 nm) meter, B40736A: $550, is
Extech Instruments 781-890-7440.

Tim Vitale
Preservation Associates
831-684-2731