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February 2012

Protecting Artworks from Light: A Few Rules of Thumb

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By Scott Rosenfeld, LC, IESNA
Smithsonian American Art Museum

A precious watercolor, a child's drawing, a photograph of newlyweds kissing--these are three examples of artworks that may be in your home and slowly fading because of excess lighting. As a lighting designer at the Smithsonian American Art Museum I am faced with the daily paradox of knowing that the light which allows visitors to see is also causing permanent and irreversible damage to the artworks I am charged with preserving. The way museums deal with this conundrum is to limit the quantity of light, limit how long artworks are exposed to that light and adjust the spectrum of light to minimize energies we can't see.

Light by definition is energy we can see. In addition to "light," some lighting fixtures also create energies that are invisible but extremely destructive such as ultra-violet energy (UV) and heat. The main source of UV in our homes is daylight and fluorescent lighting. The application of filters and window films does a great job eliminating destructive UV light. Regular light bulbs (incandescent lamps) typically produce very little UV, but the bulbs need to be placed far enough away to not heat up the artwork. The rule of thumb is that if you can feel the heat from a lamp on the back of your hand, it's too hot for artwork (just like testing baby formula). Many of the new LED spotlights on the market are fabulous because they produce very little heat, no UV and the best of them provide an excellent quality of light for illuminating artwork. For very light-sensitive materials (works on paper and fabric), it's a good idea to choose a warm white color of light (under 3500 Kelvin) to reduce harmful near ultra-violet blue rays. It’s worth noting that Kelvin temperature is the metric used to describe color and it ranges from warm/yellow-white light (2700K-3500K) to cool/blue-white light (5000-10,000K). The rule of thumb for art conservation is that sunlight at 6000 Kelvin will fade artworks twice as quickly as an equal amount of incandescent light at 2850 Kelvin (CIE report on the Control of Optical Radiation on Museum Objects, 2004). 

Sunlight is gorgeous, but it must be carefully controlled because it's both super bright and has a high blue content. While the invisible energies of heat and UV will destroy the structure of materials, plain "light" is primarily responsible for fading sensitive colors. Keeping artworks out of the path of direct sunlight is an essential step if the goal is preservation. Daylight shifts throughout the year; in the northern hemisphere winter light is often the most damaging because it's low on the horizon and floods into windows that face south, east and west. Depending on the geometry of your home, the space between windows is often the darkest and safest place to hang light-sensitive artworks. It's highly recommended to filter daylight with shades, draperies and by applying window film that eliminates UV and reduces the total quantity of light.

Lastly, it's important to control both the brightness of the light on your artworks and the amount of time they are exposed to light. Museums carefully rotate their most light-sensitive artworks, only exhibiting them for a total of 12 months per decade. Museums measure the illuminance in lux (metric) or foot-candles (imperial) and adjust the level to the recommended values. Very light-sensitive artwork, such as works on paper and fabric, receive the smallest amount of light needed to appreciate the artwork; the agreed upon value is typically between 50 and 75 lux (5-7 foot-candles). For artworks that are more durable, such as oil paintings, we light the artworks with a more optimal quantity of light (200 lux or 20 foot-candles). For comparison's sake, a sunny day can easily be 100,000  lux while a fluorescent lit office is typically 500 lux. Artworks such as glass, ceramic and metal are not light sensitive so the quantity of light will not fade the color or destroy the structure of these materials (the rule of thumb is: if the artwork was made in an oven, it's not light sensitive). The most light sensitive artworks (ISO 1 & 2) will noticeably change in as little as a year with minimal quantities of light (50 lux), while artworks with more durable pigments (ISO 7 & 8) will survive our lifetimes unchanged if lit at 200 lux for 10 hours a day. Museums measure these values with an illuminance meter, but the intention behind these values is to provide sufficient light so artworks can be seen while controlling the rate of damage. When in doubt, I consult a conservator for expert advice on the light sensitivity of a particular artwork and to receive guidance on exposure limits.    

That brings us back to the paradox--if you can see a light-sensitive artwork it's getting damaged. So make sure you are spending the life of your object well! Reduce the amount of light with shades, choose lower wattage lamps and add dimmers. But equally important is to light artworks so they can be enjoyed, otherwise they are getting damaged without any benefit. I love the idea of special table lamps or track lights that are only switched on to showcase a particular work of art or add some drama with an old-fashioned veil that both protects artworks and makes seeing artwork a special occasion. Most importantly, when no one is in the room with the artworks simply turning off the lights and closing window shades will save energy and save your artworks. 
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