From the literature I've gathered in and experiments I have done during the course of my research on the light fading of artwork, my personal take on the UV filtering issue is that it's a complex issue, but we can indeed make some generalizations:
1) Outdoor sunlight is rich in UV content with what scientists call UVC energy in the 260-280nm wavelength range, UVB energy in the 280 to 315nm wavelength range (which is the primary cause of skin sunburn), and UVA energy from 315-390nm band. Ordinary glass used for picture glazing and house windows is a soda lime composition. The degree of greenish tint in it comes from other trace level impurities in the glass so better quality picture frame glass is often selected to have less green tint than typical window glass, but nevertheless the bulk soda lime composition acts as a strong cut-off filter for UV energy below 340 nm wavelength. Hence, ordinary glass effectively filters UVC and UVB energy which is why we don't get sunburned sitting behind glass but most of the UVA energy gets transmitted. Standard acrylic becomes sharply cutting at about 360 nm, enough to block about 90% of the UV energy if you count all the UVC, UVB and UVC energy in the calculation. All of the "UV blocking" types of glazing such as OP3 plexiglass, or AR coated "museum" glass cut sharply at 390-400 nm, hence achieving 98-99% UV blocking efficiency. To cut off so strongly right at the UVA-visible transition of 400nm, UVA blocking acrylic like OP3 plexi also begins to lose its highest visible energy transmission characteristics at slightly longer wavelenghs such as 420-450nm. Hence, this type of acrylic starts to impart a slight yellow tint to the artwork (because it's also absorbing proportionately more blue light than green or red). Many find the slight yellow tint unsatisfactory. The AR coated glass and AR coated acrylic (very expensive as others have noted but it is available) can be fine tuned to do better on this UV-Vis transmission issue, but even so, when you view the artwork off angle, these thin AR films start to impart various tints as well. Which brings us back to standard acrylic as arguably the best UV-visible wavelength transmission compromise in terms of eliminating UV energy without imparting a visual tint.
2). The UV or no UV issue gets more complicated for framers due to the widespread use of optical brighteners (OBAs) in modern papers. OBAs generally have peak absorptions at 370nm, Thus if that peak wavelength region is fully blocked by the cover glazing, the OBAs won't fluoresce well at all, and images made on "bright-white" papers will lose their bright white appearance under the glazing. Standard acrylic which still allows a significant part of the 370nm energy to get through will enable the OBAs to still perform their function albeit not as well as ordinary glass which transmits more 370nm radiation than acrylic. Fully UV filtered glazing will essentially shut down the functional fluorescence of the OBAs (yet ironically help to preserve its fluorescing properties longer should the artwork later be removed from that framing package).
3). Now for the pragmatic part of the debate: What is the total practical benefit of filtering or not filtering UV on the actual fade rate of the artwork?
In numerous technical studies, it has been found that full UV filtering can under the most UV-rich indoor display conditions like direct sunlight streaming through a window typically help to reduce the fade rate by a factor of two to three. Some people wrongly believe that if you filter the UV completely, the artwork won't ever show any light-induced fading. However, visible energy (what we call light), especially the blue wavelength region, is still potent enough photon per photon and abundant enough in indoor display situations to cause light-induced fading of pigments and dyes.
A factor of two to three is indeed significant which is why most museum conservators and picture framers consider UV filtered glazing a worthy endeavor, but here's the big fallacy in the UV blocking argument. If you don't also pay attention to the total illumination level, the benefit of conservation framing can very quickly be squandered! For example, a homeowner can move a print on the wall a few feet closer to a window and unknowingly increase the average illumination level ten or one hundred fold, thus increasing the fade rate proportionately and thus overwhelming the practical benefit of the UV filtered glazing. Take a light meter and walk around your indoor environment and you will quickly see what I mean. The bottom line is that if you took two identical prints, one framed with the best UV blocking glazing and the other under ordinary glass, and place them a few feet apart on an interior wall, you might find accidentally in time that the one with ordinary glass is fading less than the one that received all the first-class conservation framing techniques. It is simply because average indoor illumination levels can easily vary by three orders of magnitude throughout the interior of a typical building. It is the choice of total illumination level that the end-user should therefore be most concerned about. The UV filtering benefit is no where near as important as the choice of illumination level on the print, but unfortunately the typical consumer usually has no first-hand knowledge of this fact and has often been lulled into a false sense of security by purchasing the conservation framing.
kind regards,
Mark
http://www.aardenburg-imaging.com
--- In DigitalBlackandWhiteThePrint@yahoogroups.com, Ernst Dinkla <edinkla@...> wrote:
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>
> mrgs1001 schreef:
> > Yes, just plain old glass has some uv filter, just not 98% like the filter stuff has, but I have not seen hard numbers as to how much.
> >
> > --- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Paul" <paulmwhiting@> wrote:
> >> Thank you! By that you mean just "ordinary" glass or acrylic, not the more expensive variety that is advertised as having special UV filtering?
>
> The UV cutting of plain window glass is at about 380 Nm where typical UV
> blocking glass tries to get that done at 400 Nm.
>
> There is a PDF here about the spectral transmission of several grades of
> framing glass, acrylics and foils.
>
> http://www.icn.nl/getasset.aspx?id=2604
>
> In Dutch but with the spectral plots for each type
>
>
> --
> Met vriendelijke groeten, Ernst
>
> Try: http://tech.groups.yahoo.com/group/Wide_Inkjet_Printers/
>
> | Dinkla Grafische Techniek |
> | www.pigment-print.com |
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>