Two art collectors with important works on paper left their living–room blinds open during an extended summer vacation. It was a decision they lived to regret.What happened reads like a third–grade science experiment. First, water seeped into the wall behind a valuable Impressionist gouache. Because there was no foam–core barrier inserted behind the picture, moisture got trapped inside the frame. When sunlight hit the gouache, creating heat, mold began to grow and eventually spread across the whole image.“It was a field of moss,” recalls Renée Vara, a fine–art specialist in collection management at the Chubb Group of Insurance Companies. “The work was a total loss.”Collectors understand the damage that fire and water can do to their art and usually take the proper steps to protect the work, but they tend to be less wary of the harm that can be caused by light. That’s probably because light is needed to see art in the first place. But collectors need to strike a balance between its benefits and dangers. “Works on paper, photography, and textiles are particularly vulnerable to light,” Vara explains. “And the damage is pretty irreversible.”In Japan people understand that woodblock prints are so light sensitive that they shouldn’t be framed, says Margaret Holben Ellis, chairman of the Conservation Center at New York University’s Institute of Fine Arts. “They are kept in books or folios,” she says. “People look at them and put them away. Americans think the prints were made to look faded, but in fact they were garish.” As far as textiles, she notes, “the Unicorn Tapestries were fabulously brilliant. You should see the back of them.”But not all works suffer equally from light exposure. “A Richard Serra installation is not going to be as sensitive as a Day–Glo Andy Warhol painting,” says Ellis. “Day–Glo is very sensitive to light and will lose its ability to fluoresce.” As a general rule, organic materials—paper, certain watercolor pigments, vegetable inks—change rapidly in light, while nonorganic materials—stone, metal, mineral pigments—hold up. The latter, however, can be sensitive to light’s drying and heating effects.Energy wavelengths that are not visible to the human eye are the most damaging. These include ultraviolet rays, the same ones that cause sunburn. So what you can’t see hurts the most. “Bright doesn’t mean bad, and dim doesn’t mean good,” adds Vara. Ellis concurs. “It’s the type, intensity, and duration of exposure to light that counts,” she says.Protecting art from the damaging effects of light begins at the framer’s. The glazing—the glass or acrylic sheet over the image—should always screen out 80 to 98 percent of UV rays. Collectors who want to enjoy their art in daylight, the best time for viewing but also the most hazardous, should opt for the highest percentage possible. Because of its static charge, acrylic is not recommended for work that incorporates pastel, gouache, charcoal, or other flaky materials.But while installing UV–screening glass over your art is crucial, it’s not the only protective measure you should take. Art should not be installed in direct sunlight or on the same wall as windows. Windows themselves can be fitted with UV film or UV–filtering scrims. Experts recommend turning off lights and closing drapes when you leave a room. If you live in a light–drenched space, consider covering your art with fabric or taking it off the wall and putting it in a closet when you go on vacation. And you may want to have the intensity of illumination—known as “footcandles” in the United States and “lux” in Europe—measured in your home. “Art in a private home is usually up for far longer than art in a museum,” says Ellis. “A museum show goes up and down in 90 days. This does not lessen the collector’s responsibility as the work’s temporary caretaker.”Artificial illumination can also be harmful. Experts suggest avoiding halogen lighting because it emits high levels of UV rays. Even when fitted with UV filters, halogens still pose a fire risk. Tungsten lights—regular light bulbs—are much better, but should not be placed close to the work. Picture lights mounted at the top of a frame are verboten. “Lots of important collectors love them,” says Vara, “but they give off uneven light and heat near the top of the canvas and can cause discoloration. And if the light falls off the frame, it can scrape the picture.”Sometimes, it is impossible to make important changes. People who live in historic properties with landmark status, for instance, may be limited in what they can do. Museums may receive gifts that come with specific instructions on where the works should be displayed. “Not everyone understands the ramifications of that kind of mandate,” Ellis laments.At some museums, the art is deemed too valuable to be exposed to light, except briefly. The Albertina in Vienna hangs reproductions in its galleries. Ellis recalls signing a document at one museum that said, in effect, that she had seen a drawing and would never see it again. “It’s one look per visitor per lifetime,” she says.The challenge for museums and collectors, notes Wendell Colson, the head of research and development at Hunter Douglas, the world’s premier window–covering manufacturer, is that they “need to strike a balance between enjoyment of the work and conservation issues.” Ellis agrees—to a point. “What are you going to say to the Andy Warhol Museum?” she asks. ” ‘You can’t put up the Warhols’?”Michelle Falkenstein last wrote for ARTnews on art dealer Marian Goodman.