UV and glazing

Arfon Davies <arfon.davies [at] arup__com> writes

>...  By relaxing
>the criteria at 400nm the colour appearance is improved.
>
>ISO 9050 defines the UV range as 280nm to 380nm, the IESNA anything
>below 400nm.  My question is what materials are susceptible to
>damage by radiation in the 380-400nm range?  ...

The barely visible 400 nm light is not as damaging as UV, but is far
more potent than the longer wavelengths that appear in the middle of
the visible spectrum. For starters, nearly every material that comes
from a plant or an animal would be damaged by prolonged exposure to
light that is rich in wavelengths measuring from 380nm to 400nm. So
eliminating the UV that's just below the visible threshold will
reduce the potential for damage. But UV is only part of the
equation. Visible light causes damage too and the shorter the
wavelength, the greater the potential for damage. The effect of
light on fugitive colors increases exponentially as the wavelength
gets smaller. In order for your filtering interlayer to provide
maximum benefit, it should do a significant amount of visible light
reduction too, preferably in the shorter wavelengths.

It is true that reducing the population of shorter wavelengths in
the mix of white light will yield a warmer tone, giving the light a
yellow or orange cast. This low color temperature light is less
damaging to the same artifacts that would have been harmed by the UV
that you are trying to filter in the first place.

Some studies have shown that our preference of color quality
actually changes with the intensity of light. This subjective
response follows what is known as the "Kruithof Curve". At high
intensity (greater than 500 lux), the eye prefers a color
temperature close to that of sunlight (above 6000K). At lower
intensities (below 200 lux) our eyes prefer a much lower color
temperature  close to that of a warm incandescent bulb (ca. 3000K),
so the yellow brown appearance of your filtered glass should
actually be favorable on all fronts, if you're talking about the
quality of the light coming through the glass  provided you can get
the intensity below 200 lux by the time it gets to the galleries
with light sensitive materials.

Steve Weintraub wrote an article on this subject for the WAAC
Newsletter in 2000:
    <URL:http://palimpsest.stanford.edu/waac/
        wn/wn21/wn21-3/wn21-308.html>

By "colour appearance" I don't know if you are referring to the look
of the glass in the windows, or the quality of light that comes
through them. It's important to remember that the glass and the
light that comes through it will "read" very differently once the
windows are in place. Typically, our eyes and brains compensate and
color correct, unless the color shift is extreme. Filtering
applications normally disappear unless there is an open window or
untreated pane of glass nearby that can serve as a reference.

    **** Moderator's comments: The above URL has been wrapped for
    email. There should be no newline.

Hope this helps,

Craig Oleszewski
New York City


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                  Conservation DistList Instance 17:24
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