This article shows how to protect art from sunlight using smart glass (also known as electro-optic glass or switchable glass). This involves not only reducing light exposure on museum quality works of art but also allowing exhibition of the object in human photopic vision.
This is achieved by tuning the amount of light entering the vitrine and the time that the object is exposed to light, according to the sensitivity of the materials on display.
This article has the following implications for any museum, private or corporate collection of rare, fragile works on paper, textiles or objects with light-sensitive dyes or pigments:-
the pros and cons of displaying the object
the importance and popularity of the piece
the most cost-effective way to preserve the collection
Lets now break all this down by first looking at the types of materials and their sensitivities to light.
For this we turn to the CIE Technical Report “Control of Damage to Museum Objects By Optical Radiation, CIE 157:2004”, which we will call ‘CIE 157’ and is summarised in Table-1.
We see that the most sensitive types of materials (CIE category 4) include silk, fugitive colourants and newspapers, which should not receive more than 50 Lux illuminance and not more than 15,000 Lux-Hours light exposure per year.
Even under these strict display conditions, and even with UV-filtered light, you would see a noticeable deterioration on CIE-4 objects within 2 – 21 years.
The light exposure corresponds to the time-integral under a graph of incident light. i.e. we are multiplying the incident lux levels by the time that the object is exposed, which gives us the Lux-Hours exposure, or total energy received by the object.
Lets now break this down further, using a conceptual model, where we analyse:-
the light impacting the display case from windows, skylights and artificial lighting
the light received by the object through the glass of the display case
the light received at the observer after being reflected by the object
Fig. 1 shows a conceptual model of a display case, housing a light-sensitive object and illuminated by a light source. The light source is assumed to have a Correlated Colour Temperature (CCT) of between 2900K and 4200 K and a Colour Rendering Index, CRI >= 85.
An observer perceives light reflected from the object as a field-of-view dependent Luminance, L. The display case glass has a Transmittance (i.e. how transparent the glass is), modelled as T.
The Illuminance incident on the display case is marked A, of which T% is transmitted by the glass into the display case.
This results in B Lux entering the display case and illuminating the object (assuming no other internal artificial light sources are present).
The object reflects a fraction R of the light as C Lux, of which T% is transmitted out by the glass and leaves the display case as the Exitance D in lumens / m2.
Assumption from Ref 2 :- as long as the exitance D is above 10 lumens / m2, the observer will receive a luminance L of 3 cd / m2 in the field of view, sufficient to allow photopic (i.e. colour) vision, which allows better perception of fine details and colours present on art.
Table-2 now compares a normal (non-switchable) glass versus a smart glass vitrine where the objects are on display under an illuminance of A=100 Lux, for J=3000 hours per year and the glass transmits light during Y% of that time (called the duty cycle) at a specific transmittance, T% . A representative object reflectance R of 85% is assumed but this will of course be different for each work of art.
It can be seen that in both cases, observers would receive enough light, L (> 3 cd / m2) to be in photopic vision but that objects of type CIE category 4 displayed in a normal display case would be exposed to light levels of X=285,000 Lux hours / year, which is almost 20 times the CIE recommended figure, P.
It is evident that the object would be liable to severe damage from light, which can result in embrittlement, surface cracking, structural breakdown and colour fading.
See our Blog article Antique Art and the Light Spectrum for more details on how light can affect fragile works.
The smart glass display case, on the other hand, can alter its transmittance when an electrical signal is applied and the Duty Cycle Y can be limited by ensuring activation only when someone is present, e.g. by proximity sensor or push button.
If the activation of the smart glass is triggered by visitors, the Duty Cycle Y effectively captures the visitor popularity of this object, which we have modelled as 10% in this case, corresponding to 300 hours per year.
This fine-tuning allows the object to be displayed in human photopic vision, whilst also controlling the overall light exposure to within CIE 157 limits.
This article shows how smart (switchable / electro-optic) glass can be used to protect art from sunlight.
Photopic luminance at the observer can be achieved and minimal light damage suffered by the work of art, depending on various factors such as the transmittance of the switchable glass, object reflectance and exposure limits per object category.
If you would like us to provide further analyses based on concrete values for your specific display requirements, please contact us.
1. “Control of Damage to Museum Objects By Optical Radiation”, CIE 157:2004, ISBN: 978 3 901906 275
2. “Light for Art’s Sake: Lighting for Artworks and Museum Displays”, Christopher Cuttle, ISBN: 978-0-7506-6430-1