Digital BW, The Print

----- Original Message ----- 
From: "Mark" <mark@...>
To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
Sent: Monday, August 30, 2010 10:27 AM
Subject: [Digital BW] Re: no glass vs. glass vs. plexi: which shows 
carbon print at its best?











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:
>
> 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        |
> |                 ( unvollendet )                 |
>



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