In December 2017, it was first suggested to me by my friend and esteemed colleague, Joseph Holmes, that my Sony α7R II’s (or more commonly written as a7R II) sensor might be crooked relative to its bayonet mount. Don’t laugh yet, it’s not a problem exclusive to Sony. I had previously known about the same problem with digital medium format backs from precisely the same person, who put out two extensive articles on the topic over a decade ago, here and here. Neither of us wanted to believe this initially. One would certainly like to assume that 35mm format camera manufacturers, making single-unit camera bodies rather than modular system digital backs, could hold the sensors to tight enough tolerances with today’s technology. Alas, we discovered that it was not quite good enough for what we wanted from our cameras.
A diffuse and complex story unfurls, necessary for the explanation of small details in great detail. For anyone wanting the very best out of their powerful, modern, wide-aperture, breathtakingly-sharp lenses, on equally modern, high-resolution (read: highly revealing of alignment errors) digital cameras, photographing the most optically demanding subject matter in the known universe – the starry night sky – you will want to sit up and pay attention. It’s entirely fine if one does not care about subtle differences between what’s in-focus and out-of-focus. I would not soon be forgetting how many failed to appreciate the camera-shake induced blurring from the original Sony a7R violent shutter mechanism. If one only wants photos displayed no larger than Instagram thumbnails, none of this is pertinent and you should stop reading now. Alright then, some good news! We have fortunately found reasonable solutions to the worst problems. I share them below freely.
This is the second installment to “The Framing Glass” article, which delves into new 99% UV filtering anti-reflective (AR) glass and comparisons of more AR framing glass from Schott, Guardian, Flabeg and Luxar, which were not included in the first article.
Since January 2016, picture framers have been blessed with two exciting new developments. Groglass announced a brand new 99% UV filtering AR glass, with no ripples in the optical coatings, called Artglass UV 99. Tru Vue also launched an updated Museum Glass product, which unlike the earlier product, also has ‘no ripple or “orange peel” effect on [the] glass surface’.
Interference vs Absorptive UV coatings
Both Artglass UV 99 and Museum Glass employ a UV absorptive coating on one side of the sheet, and then AR coatings are applied to both sides. Since the UV filtering coating is an absorptive type, it is effective at all angles of light entry. I consider these to be the only meaningful products to use if UV filtering is required.
Other products such as Artglass WW UV (>90% UV protection), Flabeg ArtControl UV90 (>90% UV protection) and Schott Mirogard Plus (~84% UV protection) use interference-based coatings to block UV light. There are three downsides to that. Firstly, they can only achieve a maximum 90% or so UV filtration between 300-380nm. This doesn’t sound too bad until the transmission band shift at oblique incident light angles is considered. So the second problem is the transmission band shifts to shorter wavelengths when light strikes the glass at shallower angles, allowing more UV light to pass through. Since in practice no artwork is illuminated solely straight on (the viewer’s shadow will be in the way), the added UV protection in the real world is not as significant as their manufacturer specifications would suggest. Thirdly, interference-based coatings tend to have a wide cut-off band, as seen in the spectral transmission graphs of Museum Glass and Artglass WW UV in the earlier article.
Artglass UV 99 and new Museum Glass
Both products have largely similar specifications: AR coatings resulting in less than 1% reflection, 99% UV filtration (from 280nm-380nm for Artglass UV 99 and from 300-380nm for Museum Glass) and similar glass thicknesses, the standard products are 2mm for Artglass and 2.5mm for Museum Glass. The base substrate is regular soda lime float glass rather than low iron glass. The UV absorbing coating is an orangey-brownish color so when combined with the aqua green color of regular glass, they balance each other out resulting in a visually more neutral transmission color. Using low iron glass does increase the overall transmission a little but since it is so neutral, adding the UV coating makes it look overly yellowish in tint. The transmission rating for both is ~97%, though AR glass with lower UV protection ratings tend to have better transmission ratings, up to >99%.
Both manufacturers also claim to have no rippling pattern in their coatings. The previous version of Museum Glass was plagued by quite serious ripples in the UV coatings, which became even more obvious under non-diffused light sources. Let us first investigate these claims.
9 Sep 2015 Changed spectral graphs to include UV spectrum, revised article language.
9 Nov 2016 Revised article language. I have since received samples from the other manufacturers of AR glass, and am awaiting samples of newer products such as Artglass UV99 and the new “ripple-free” Museum Glass. Some of my conclusions and recommendations will be revised in light of the performance of these new and updated products in a future article.
2 Jul 2017 Revised article language.
5 Aug 2020 Updated product names to reflect manufacturer changes. The illustrations still retain the old naming conventions, so watch out!
As a maker of finely crafted prints, I want them to look as good as possible, so it is rather bothersome when we cannot see them as well once they are framed behind glass, due to its obtrusive reflections and greenish tint. Framing without glazing is out of the question for fine prints in the interest of long-term preservation. It shields the art from physical damage, atmospheric pollutants and to varying extents, degradation from ultraviolet (UV) light. What we need then is highly transparent, anti-reflective glass.
Uncoated “regular” soda lime glass (and clear acrylic) reflects about 8% of light in total (4% off each side). The glass itself absorbs another ~2% or more of light. But the absorption is not uniform across the visible spectrum. Iron oxides naturally present in silica, the main ingredient of soda lime glass, is responsible for its green tint. This is most visible when looking at the edges of float glass. Low iron glass is manufactured from silica with very low iron content. This makes the glass almost completely neutral in colour and light absorption is reduced to about 0.5% or less. Clear acrylic has the advantage of having absolutely no colour tint and absorbing virtually no light at all.
To increase the transparency of glass still further, we need something else to reduce surface reflections—anti-reflection (AR) coatings.