Effects of UV-protective glass on the colour rendering of images
Fine art paper and UV protection: a brief introduction
A digital photo only comes to life on the right high-quality fine art paper . Bright white fine art papers often have optical brighteners or Optical Brightening Agents (OBAs), which make the paper appear whiter thanks to certain additives. Optical brighteners absorb the UV light and reflect it into the visible spectrum, creating a higher, pure white of the paper. This effect, also known as fluorescence, appears to the human eye as a bluish, bright white. Over time, however, the effectiveness of the OBAs decreases and the paper becomes less white. When framing, however, the question arises as to whether OBAs are still effective if glass with high UV protection is used. Does the paper remain bright white or does the effect disappear?
We asked photo paper expert Jochen Kohl, Marketing Manager Europe at Ilford, what effects UV-protective glass has on fine art papers in this process and how the ageing process can be stopped.
The performed tests
The following series of tests show that the white of the paper is slightly less bright under UV protection glass, but is still clearly bright white. Using the example of ILFORD GALERIE Gold Fibre Pearl paper, which contains OBAs, a clear fluorescence is still observed in UV light, even under UV protection glass, although somewhat paler in comparison to exposed paper. The fact that UV-protective glass can extend the life of OBAs is of great importance to artists, collectors, and museums.
Matt vs. anti-reflective. Matt white glass above, anti-reflective Artglass AR 92 below
Interference-optical coatings are absolutely color-neutral and, by eliminating reflections, even provide better color rendering, especially of dark colors. Thus, Artglass offers a color rendering index of Ra=100. Normal glass, in comparison, only has an index of Ra=90.
UV Protection
Even though many museums are free of UV radiation due to modern LED lighting, high UV protection is still a sought-after feature. Outside of museum exhibition rooms, UV protection is mandatory for the presentation of sensitive works. Glasses without special UV protection only protect against approx. 45% of the rays, so they offer hardly any UV protection. UV protection means blocking light up to 380nm wavelengths. For UV protection beyond this, the glasses are further refined.
Transmission curves of different glasses
With mineral glasses such as Artglass AR 92, a UV protection of 92% is achieved by an additional coating based on silicon oxide. Here, too, the effect of interference is used so that UV waves cancel each other out.
In double-pane safety glass such as Artglass AR 99, the laminated PVB film between the panes provides over 99% protection against UV radiation. Here, the plastic film contains the UV filter. The situation is similar with Optium Museum Acrylic®. Here the UV filter is incorporated into the acrylic glass. In this way the glass also achieves a protection of over 99%.
Blocking UV rays affects color rendering because the filter extends slightly into the visible range. Glasses with high UV protection therefore have a minimal yellow tint, but this is only noticeable in direct comparison with white glass. When developing the UV filter, Tru Vue worked with restorers and curators, for example, to find the ideal balance between maximum UV protection and color neutrality. In this way, the aesthetics and intention of a work of art are not compromised.
Test af friktionshjul
DIN ISO 3537 2018
Transmission curves of different glasses
Test 02
On the left ILFORD GALERIE Gold Fibre Rag with OBAs, right ILFORD GALERIE Gold Fibre Pearl without OBAs under UV light behind Optium Museum Acrylic with over 99 % UV protection. Fibre Rag shows a clear fluorescence in comparison to Fibre Pearl, which is slightly attenuated by the UV protection in the glass.
Test 03
Left ILFORD GALERIE Gold Fibre Rag with OBAs, right ILFORD GALERIE Gold Fibre Pearl without OBAs in daylight with natural UV content behind Optium Museum Acrylic with over 99 % UV protection. Fibre Rag is significantly whiter than Fibre Pearl, the effect is hardly noticeably influenced by the glass.
Test 04
On the left ILFORD GALERIE Gold Fibre Rag with OBAs, right ILFORD GALERIE Gold Fibre Pearl without OBAs in artificial light without UV behind Optium Museum Acrylic with over 99 % UV protection. Fibre Rag and Fibre Pearl do not differ noticeably. An effect due to the UV protective glass is not recognizable in this test and can therefore be neglected .
Conclusion
The UV protection of the glass has no negative influence on the brilliant white of OBA-treated papers in daylight. This means that the paper still retains its brilliant white, but is protected from the harmful effects of UV rays, which presumably extends the life of the OBAs . When selecting paper, it should be borne in mind that the effect of OBAs disappears under UV-free artificial light and the white tone of the paper does not fully come into its own. Even modern bright white papers really come into their own under museum glass with high UV protection .
Test 01
Color fan under black light behind Optium Museum Acrylic with over 99 % UV protection. The papers with OBAs literally glow.
Test 02
On the left ILFORD GALERIE Gold Fibre Rag with OBAs, right ILFORD GALERIE Gold Fibre Pearl without OBAs under UV light behind Optium Museum Acrylic with over 99 % UV protection. Fibre Rag shows a clear fluorescence in comparison to Fibre Pearl, which is slightly attenuated by the UV protection in the glass.
Test 03
On the left ILFORD GALERIE Gold Fibre Rag with OBAs, right ILFORD GALERIE Gold Fibre Pearl without OBAs in daylight with natural UV content behind Optium Museum Acrylic with over 99 % UV protection. Fibre Rag is significantly whiter than Fibre Pearl, the effect is hardly noticeably influenced by the glass.
Test 04
On the left ILFORD GALERIE Gold Fibre Rag with OBAs, right ILFORD GALERIE Gold Fibre Pearl without OBAs in artificial light without UV behind Optium Museum Acrylic with over 99 % UV protection. Fibre Rag and Fibre Pearl do not differ noticeably. An effect due to the UV protection glass is not recognizable in this test and can therefore be neglected .
Conclusion
The UV protection of the glass has no negative influence on the brilliant white of OBA-treated papers in daylight. This means that the paper retains its brilliant white brilliant whitebut protects from the from the harmful effects of UV rays which presumably extends the life of the OBAs . When selecting the paper it should be borne in mind that under UV-free artificial light the Effect of OBAs disappears and the white tone of the paper does not fully come into its own . Even modern bright white papers really come into their own under museum glass with high UV protection .