Dimming the Light
Has your institution installed a light filtering system? The acetate base of solar films breaks down in 8-15 years. So even if the windows of your historic structures currently have protection, the filters will need to be replaced eventually. In order to guide your research into the best product for your institution, the National Park Service (NPS) has published a Conserve-O-Gram on the subject, charting its test results for a variety of types of solar films.
The NPS standard is to block all ultraviolet radiation exceeding 50 microwatts per lumen (roughly the amount generated by an incandescent light bulb). Advances in technology have allowed the production of new solar films that filter UV even more effectively.
Both visible light and ultra violet radiation must be controlled in order to protect your collections responsibly. Of course, in most exhibit circumstances, light should not be blocked entirely and is important for the authentic appreciation of historic interiors. Before climate control, heavy drapes or shutters worked to subdue direct light. These methods are still effective at reducing light’s damaging effects on the irreplaceable artifacts within our cultural heritage institutions. UV films do not eliminate the need for museum staff to adjust shades throughout the day, depending upon the sun’s path.
Many of the films NPS staff tested blocked significant amounts of visible light, while others were only effective at filtering UV. Use the NPS Conserve-O-Gram’s test chart to identify film varieties with lower than 50 in UV measurement and significantly lower than 1000 foot-candles. Remember that 1200 foot-candles was the amount of natural light in the test window, so films registering below that also offer some visible light control. Unfortunately, some of the most effective UV blocking films had little effect on natural light.
Twelve of the fourteen 3M products in the NPS test scored well (below 50) in the UV category and also functioned to reduce visible light. 3M film is no longer available at large hardware stores, but there are local businesses that both sell and install the products. One of these is Carolina Solar with branches in Raleigh and Wilmington.
An added feature of some of the films aids in disaster preparedness by impeding the glass from shattering. At the time of the NPS testing, however, these types did not block UV radiation effectively. As new types of film are developed, they may include all three advantages: shatter protection, UV blocking, and visible light reduction. Although there is some cost up front for this collections preservation solution, solar film may end up paying for itself in climate control savings during the warmer months.
Have you had good results with your light filtering systems? What products or services can you recommend to others in our North Carolina cultural heritage community?
Posted on December 6, 2011, in collections care, collections management, historic houses, historic sites and tagged Conserve-O-Gram, National Park Service, solar film, UV filters. Bookmark the permalink. Leave a comment.