UV monitors

Peter Lundskow <plundskow [at] dnr__state__in__us> writes

>I have a question that I have been researching and have not been
>able to find an answer to.  I know when it comes to general
>recommended UV readings for museums 10 microwatts per lumen is
>optimum with 75 microwatts per lumen being the high end of the
>'safe' range.  Also, that many museums use the Crawford type 760 UV
>monitor.
> ...
>Recently, we purchased a more accurate meter, the UVX Radiometer
>from UVP Ultra-Violet Products (this meter comes with certification
>of calibration)it takes readings in microwatts per cm squared.  I
>have not been able to find any information on how to convert one
>measurement to the other and what safe readings would be for museums
>when measured in microwatts per cm squared. Also, I would mention
>our new meter has two sensor heads, one in the 250nm range and the
>other in the 360nm range.  I have been told by the same source that
>readings with both sensors are needed to get a more complete picture
>of UV exposure.

The Crawford/Elsec UV Monitor is useful as a field instrument, but
less useful as a lab instrument. It can yield false or confusing
readings under extra-ordinary circumstances, especially when the
levels of light excitation are very low. For instance, you may get
elevated microwatt/lumen readings in the shade under your desk. But,
where light levels are abundant, the Elsec meter yields a pretty
reliable "bigger-than-a-breadbox" indication of UV levels. In any
case, the instrument was invented for the purpose of obtaining
meaningful, "quick and dirty" readings to test the effectiveness of
UV filters, and to test the UV content of various lighting sources.
Trying to get it to do more than that may leave you disappointed. As
such, the Elsec's major flaw is price. $1500 (USD) is a lot to fork
over for a meter that you may only need to use once or twice per
year.

Ordinary window-glass is opaque to wavelengths of UV that are less
than 340 nm, so unless you are arc-welding in the open gallery, or
displaying your collection outdoors, you are unlikely to be troubled
by UV in the 250 nm range that your meter is capable of
measuring--for your purposes, readings from the higher range will be
the most meaningful.

To do your conversion, remember that 1 foot-candle is about 10 Lux
(closer to 10.76). The term "Lux" means "lumens-per-square-meter".
Using a visible light meter, get a light reading in (or convert to)
Lux. There are 1,000 square centimeters in a square meter, so divide
the Lux number by 1,000. You will likely get a decimal number that
represents the "lumens-per-square-cm". Measuring your UV content
with your UVP/UVX Radiometer will give you a UV value in
"microwatts-per-square-cm". Compare the "lumens-per-sq-cm" to the
"microwatts-per-sq-cm" and cancel out the dimensional term. You will
then know the value in microwatts-per-lumen. The "safe range" you
mention (10 to 75 microwatts per lumen) would vary depending on the
level of illumination. If you were looking at a work on paper that
were illuminated at 50 lux, then the "safe range" you are seeking
would be 0.5 to 3.75 microwatts of UV per sq cm. At 100 lux, the
range would be 1.0 and 7.5, etc.

The term "safe" as regards UV is problematic. Since our eyes cannot
see UV, it can contribute nothing to rendering a work of art or
artifact, but it can still do lots of damage. Therefore no level of
UV is truly "safe". But the task of eliminating UV entirely lies
somewhere between extremely costly and impossible. The amount of UV
emitted by an ordinary tungsten-filament bulb (e.g. "GE SoftWhite")
was considered for years to be the practical threshold (ca. 75
microwatts per lumen), but even that is being rethought these
days--that's where the 10 microwatts per lumen number comes from.
It's most important to keep it all in perspective. Eliminate as much
UV as you practically can by using filters, shades and safer
lighting sources, but don't divert funds from fire-control, security
or maintenance in order to do it. Losses from fire, vandalism or
flood can be much more profound than years of exposure to 75
microwatts-per-lumen. Remember also that visible light is nearly as
damaging as UV. Proper and "safe" lighting is part of an over-all
environmental strategy. Minimizing UV is but one of the components.

Craig Oleszewski
New York City


                                  ***
                  Conservation DistList Instance 17:17
                  Distributed: Thursday, July 31, 2003
                       Message Id: cdl-17-17-001
                                  ***