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Subject: UV monitors

UV monitors

From: Doug Nishimura <dwnpph>
Date: Sunday, September 8, 1991
Yes, there are photo-sensitive papers for UV, although they are not
really the best method to use now.  During the early days of UV
research, they were about the only way of monitoring UV radiation, but
now there are many methods that are faster, easier and more precise.

Photographic plates may be used, although most are too fast and not UV
specific enough for your purposes.  A pure silver chloride plate is most
specific to UV (although only the near UV), while bromide plates are
sensitive in the blue region.  Gelatin in unfortunately a good absorber
of UV at shorter than 2800 A.  Below about 2000 A, gelatin is virtually
opaque to UV.  Thus the photographic plate or paper is only sensitive up
to about 2500 A.  It is possible to make salted papers much as they did
in the 1840's, but the normal sizing additive to the salting solution is
gelatin and another additive would have to be used (another protein or a
starch.)  In addition, these papers would have to be changed fairly
often -- on sunny days, possibly more than once per hour.  Also, if your
city is quite polluted, they are sensitive to many chemicals in the air,
especially sulfur compounds. One approach to extending the UV range of
these papers is to add a UV fluorescent chemical such as
dihydrocollidine ethyl carboxylate.  Another test paper used was made
from purified sulfite pulp and dyed with Benzoazurine G.

Many of the other chemical detectors require an analytical lab to
quantify. Such reactions include the bleaching of a solution of acetone
methyelene blue or the decomposition of oxalic acid/uranyl sulfate.  In
the latter case, a lab is required to quantify the remaining oxalic
acid.  Other color reactions include (colorless) derivatives of
triphenyl-methane dyes, and zinc sulfide in lead acetate (which goes

Most museums use two devices.  The first is a lux meter for measuring
illuminance.  This device is non-specific to UV.  The second device is a
UV monitor (most commonly, the Crawford UV Monitor), which measures the
UV content of light in microwatts per lumen.  The lumen (lm) is the SI
unit for luminous flux in candela-steradians while the lux (lx) is the
SI unit for illuminance in lumens per square meter. (None of this is
really important.) Thomson (in The Museum Environment) says that both
measures should be considered independently.   Sources with a UV content
of 75 microwatts per lumen or more should be filtered.

Beyond these devices are any number of radiometric devices (including
thermopiles, bolometers and radiometers) and photoelectric devices
(including phototubes, photovoltaic cells and photoresistance cells).
The radiometric devices have no long wave cut-off limit and read right
into the IR end.  They are also non-spectral sensitive.  The
photoelectric devices have long wave limits from the UV on into near IR.
Photoelectric devices also vary in response depending on wavelength.
Because of the long range wavelengths that any of these devices are
sensitive to, appropriate filtration, monochromator or diffraction
grating should be used (depending on application.) (Monochromators and
diffraction gratings allow single or narrow bandwidths to be monitored.)

My recommendation is that you follow what other museums do and use a
Crawford. However, I have very recently heard that some people who did
not like the Crawford although no one was specific about why.


                  Conservation DistList Instance 5:19
                 Distributed: Sunday, September 8, 1991
                        Message Id: cdl-5-19-004
Received on Sunday, 8 September, 1991

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