All UV radiation will cause damage to light sensitive materials.  There is NO
threshhold beneath which UV will not cause damage.  UV damage is cumulative
and non-reversable.  As a consequence, it is important to eliminate it to the
extent possible.  This is done through the selection of illumination sources
which are low in UV, the use of filtering materials, and illuminating your
objects only when necessary.

For many years museum conservators would specify 75 microwatts per lumen as
the proportion of UV acceptable for museum illumination.  However, that was
based on the most effective UV filter available.  At the present time, UV
filtering materials are available which take the proportion of UV down to 10
microwatts per lumen.  When looking for filtering materials, it is important
to receive a copy of the transmission spectra data from the manufacturer.  I
have attached a file that provides specifications for good filtering
materials.

For most museum/library applications, UV radiation is measured in microwatts
per lumen ( a porportional measurement) although conservators are also
interested in the total amount of ultraviolet radiation that falls upon an
object.  However, for most collections care/curatorial applications, I
believe that the proportional information is acceptable.  It will help you to
evaluate the efficacy of any filtering materials.

At the present time there are two excellent instruments available that are
useful for measuring UV.  Both are manufactured by Littlemore Scientific and
are the Elsec/Crawford 762 and the Elsec/Crawford 763.  Both are quite
expensive (ranging from $1100 - $1350 in US dollars).  Since I do not know
where you are writing from, I cannot refer you to a vendor.  In addition to
microwatts per lumen, the 763 will measure the total amount of UV as
microwatts per cubic meter.

Many of my clients share UV meters since it is not an instrument that needs
to be used daily.  Initially it should be used to check the UV emitted by all
light sources (daylight and artificial) and then annually to test the
efficacy of UV filtering materials.  In addition, it should be used to test
all newly purchased UV filtering materials because sometimes, just sometimes,
the wrong material is sent by the vendor.  (apologies to the vendors out
there, but it HAS happened!)

One caveat, however, these instruments have a glass lens which filters out
the extra high intensity, short wave UV that is emitted by halogen lamps.  As
a consequence, these meters cannot measure these very damaging wavelengths
and they must be filtered, even if the meters are not reading any UV.

I have greatly simplified this explanation, but I hope it is useful.
 Additional information can be obtained through Conservation On-Line, as
recommended by another Museum-L responder.

Please let me know if I can provide additional information.  Good luck!

Wendy Claire Jessup
President and Conservator
Wendy Jessup and Associates, Inc.
210 Little Falls St., Suite 203
Falls Church, VA  22046  USA
703-532-0788
FAX: 703-532-1661