Exhibition lighting standards

The short answer to Tom Dixon's question, "where did the 'new
standard' of 10 microwatts per lumen come from?" is that it appears
as one of the conclusions in David Saunders' paper 'Ultra-violet
filters for artificial light sources' in the National Gallery
Technical Bulletin, Vol. 13 (1989), 61-68.

The problem with unreliable measurements of UV content at low light
levels arises because we are trying to measure the ratio of two
small quantities, the UV power divided by the visible illuminance. A
small error in measuring either quantity can cause a large
difference in their ratio.  This error could be caused by small
differences between the photodiodes which measure the visible and UV
components, and between the filters used to exclude visible light
from the UV sensor and to give the photopic curve response of the
visible light sensor.  A more accurate result might be obtained by
measuring the two quantities separately and then dividing them.

However, the whole matter of the UV content of museum lighting is
worth looking at in some detail.  The first reference to the
proportion of UV in different light sources that I am aware of is in
Garry Thomson's paper "A new look at colour rendering, level of
illumination, and protection from ultraviolet radiation in museum
lighting", Studies in Conservation 6 (1961), 49-70.  His Table 1
gives the amount of UV in tungsten light as about 1%, and in
fluorescent light as about 3 to 7%; however, it is not clear what
units the visible and UV light were measured in.

By 1967, in his paper "Calibration and use of a UV monitor", in the
proceedings of the IIC London Conference on Museum Climatology,
159-172, Thomson has developed this idea to give a table of UV
content or UVC, which he defines as 'UV content divided by
illumination value'.  Normal tungsten incandescent lamps have a UVC
of 3 to 5, while fluorescent lamps have a UVC of 2 to 15.  A
footnote on page 171 reads 'To convert UVC to microwatts of UV
radiation per lumen multiply by 16', so we have the UV content of
tungsten lamps is 48 to 80 microwatts per lumen, while the UV
content of fluorescent tubes is 32 to 240 microwatts per lumen.

By the time the first edition of 'The Museum Environment' appeared
in 1978, these figures had been refined to normal tungsten, 60 * 80
microwatts per lumen, and fluorescent lamps, 40 * 250 microwatts per
lumen.

Some time ago I did some rough and ready measurements of the UV
emission of ordinary GLS tungsten lamps, and found that the UV
content varied between about 25 microwatts per lumen for a 150W
lamp, down to about 12 microwatts per lumen for a 15W lamp.  This
demonstrates that the UV content depends on the power and filament
temperature of a lamp.  It also shows that, other things being
equal, it is better to use a lower wattage lamp closer to an object
in order to achieve a certain level of illumination than to use a
higher wattage lamp at a greater distance, because the UV power will
be less for the lower wattage lamp although the illuminance is the
same.

It has seemed to me for some time that it might be more satisfactory
to give up quoting the ratio of UV power to visible illuminance, and
instead just talk about the UV dose, in the same way that many
institutions now measure the annual dose of visible light
(lux-hours) to objects, without being too prescriptive about light
levels.

If we take the 'standard' levels of 50 lux at 75 microwatts per
lumen for more sensitive objects, and 200 lux at 75 microwatts per
lumen for less sensitive objects, we get maximum permissible UV
power densities of 0.375 microwatts per square centimetre (3.75
milliwatts per square metre) and 1.5 microwatts per square
centimetre (15 milliwatts per square metre) respectively.  If we
also take the 'standard' opening hours of 7 hours per day, 6 days
per week, 52 weeks per year (2184 hours per year), we get an annual
maximum permissible UV dose of about 3 joules per square centimetre
for the more sensitive objects, and about 12 joules per square
centimetre for the less sensitive.  As always, these are maximum
figures, and one should aim to keep the annual dose as low as
possible.

Given the availability of instruments that enable one to measure
lux, UV power and UV content, this may be a reasonable option for
assessing risk to objects from UV exposure.

Dr Barry Knight
Senior Conservation Scientist
English Heritage
+44 20 7973 3000
Fax: +44 20 7973 3001


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                  Conservation DistList Instance 15:43
                 Distributed: Monday, December 17, 2001
                       Message Id: cdl-15-43-001
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