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Dec 12, 2016

Antique Art and the Light Spectrum

By Manoj Phatak, Founder and CEO, ArtRatio

It is a common misconception that only ultra-violet light can damage art and antiques.

As stated in Technical Report 157 from the CIE (International Commission on Illumination, based in Austria), there are two principal processes by which museum art can degrade:

1. Photochemical action

2. Radiant heating effects

Photochemical action

This can result in irreversible molecular change to the structure of the material itself due to the incident light, not only ultra-violet (UV) and infrared (IR) but also visible light, and the final result depends on the 'activation energy' of the materials in question. The most noticeable change can be colour fading of textiles, manuscripts and photographs but can also include embrittlement and surface cracking of many other types of museum artefact.

Radiant heating

Radiant heating is an increase in the surface temperature of the object above the ambient temperature due to the absorption of infrared radiation. This depends on the spectral power distribution of the incident light and the spectral absorptance of the material. The increase in surface temperature can cause unwanted chemical activity as well as dimensional changes, resulting in mechanical stresses when the thermal coefficients of expansion of adjacent materials in the object are not matched. Localised surface heating can also cause migration of moisture with a resultant dehydration of the artefact, resulting in surface hardening, cracking and discolouration. This will be especially visible in hygroscopic materials (i.e. most organic substances).

Characteristics of Electro-Optic Glass

It is due to these effects of broadband light that we chose some years ago to build museum vitrines from electro-optic glass to mitigate the effects of light damage. Electro-optic glass can change its transparency when stimulated by an electrical signal.

See Graph Below - Figure 1 shows the characteristics of the SPD electro-optic glass we use in all our vitrines.

- The blue line indicates the light transmitted by the glass when in its default inactive state (opaque).

- The red line shows the light transmitted by the glass when it is activated (i.e. transparent).

Here is how to read the graph:-

a. The horizontal axis of Fig.1 shows the wavelength of electromagnetic radiation, measured in nanometres (nm), from 300nm which is in the ultra-violet part of the spectrum to 1000nm (well into the infrared). The visible spectrum extends from 380nm to 740nm. The vertical axis shows the amount of light transmitted by the glass, as a percentage.

b. The blue line shows minimal transmission of light across the visible spectrum with a 12% transmittance peak at 420nm, giving the glass a slight blue tint in the off state when it is opaque. The glass limits the IR to 50% when opaque, which means that half of the radiant heating effects are eliminated when the glass is inactive (which from our experience is most of the time). That is why our vitrines measure the popularity of the work of art (in total seconds viewed), so we can factor in how much time we can expect to be illuminating the object at different times of the year.

c. The red line shows a fairly uniform (flat) distribution of light transmitted when the glass is activated, proving that the glass - when transparent - apportions no extra colouration to the artefact. The transmittance in this state exceeds 60% across the entire visible spectrum but please note that the glass is also transmitting IR when activated. This is why we also monitor the temperature inside the vitrine at all times and flag any sudden increases in the conservation data.

d. At all times, UV light (any wavelengths below 380nm) is completely eliminated, minimising photochemical deterioration, so whether the vitrine is transparent or opaque, the work of art is protected 24/7 from harmful UV radiation, as required by CIE 157.

Fig.1 shows the optical characteristics of the 'Light' version of Suspended Particle Devices electro-optic glass (trademarked as 'SPD-SmartGlass' by our partners Research Frontiers).

The version we commonly use is the 'Dark' version, which extends down to very close to 0% transmittance.

The 9mm thick sample used in these tests is a laminated glazing, fabricated with clear float glass having an interlayer which blocks UV up to 380 nm. The overall visible light transmission is 2.6% (when dark) to 62.8% (when clear).

Customised fabrications are available including UV blocking to 400 nm, IR-rejection, anti-reflective coatings, and low iron (i.e. higher clarity) glass substrates as well as impact- and blast- resistant interlayers. 


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For more information about ArtRatio, designer of art conservation furniture for museums, private collectors and luxury retailers to protect rare, fragile items from light damage, check out their listing on The Clarion List or visit their website: http://www.artratio.co.uk/