Alex Cooper is the lighting designer at Smithsonian’s National Portrait Gallery. (Andre Chung/for The Washington Post)

Museum lighting can make art shine.

But it also can make it suffer.

“Anytime you light something, you’re damaging it,” says Alex Cooper, resident lighting designer at the Smithsonian’s National Portrait Gallery.

With the empathy and calm of a family doctor, Cooper, 40, manages this fine line between enhancing and harming every day. He’s typically the last of the gallery’s specialists to contribute to an exhibition’s design, but his responsibility is great: to illuminate the art while minimizing fading. He must balance visitor experience against protecting the works.

The Portrait Gallery’s new Civil War exhibit, “Dark Fields of the Republic: Alexander Gardner Photographs 1859-1872,” crystallized the challenge. These highly light-sensitive paper photographs have immeasurable cultural value and require the lowest light possible. Cooper faced a tricky question: When does the visitor’s eye perceive enough information to “get” what’s in the picture? How much light do you need to understand all the detail?

Dramatic lighting on a portrait of Abe Lincoln in “Dark Fields of the Republic: Alexander Gardner Photographs, 1859-1872.” (Andre Chung/for The Washington Post)

The subjects of the photos — wary soldiers, dead soldiers, stoic families, Confederate spies, would-be assassins, Abraham Lincoln himself — are tiny, ghostly. In his small-scale works, Gardner captured the nameless, expressive nuances of American faces in that terrible time. If you spend a little time with them, you’re rewarded with rich detail and surprises. (There were so many different ways men wore their hair, for instance. Even in the House of Representatives, some sported the wild frontiersman look, others glossy curls.) So here, Cooper’s lighting design became an art of persuasion. His goal was to use just enough light to coax visitors to peer closely, creating an atmosphere where they’ll want to linger.

One of Gardner’s most consequential works is a cracked-plate image of Lincoln, which is rarely displayed. Taken during a photo session in 1865, it was the last formal portrait of the president before his assassination. The glass negative fractured, and Gardner pulled only one print before throwing away the plate. That print shows the crack ominously slicing across Lincoln’s head. Below the crack, his face is deeply creviced with the stress of the war.

Yet despite the somberness of these details, Lincoln seems warm and vibrant, with a slight smile. Cooper doesn’t want to take credit for that — it’s not his aim to “editorialize” the art, he says. But it’s unavoidable: The candlelight effect of Cooper’s lighting design in the room where the cracked-plate image hangs helps bring the slain president to life.

In another nearby portrait, Lincoln dotes on his school-age son, Tad. Tad was destined for an untimely death, like his older brothers and father. Yet here are the two of them, absorbed in the moment, eyes merry under a soft overhead glow. It’s as though they’re sharing a secret. The shadows around you feel like a shroud, and you’re pulled in closer, drawn into that father-and-son world — its brief happiness, its fate, its liveliness in Gardner’s hands, and in Cooper’s.

Luckily, Cooper says, subtle emotional tones such as these are best served by soft light.

“It’s a very fortunate intersection of the content suiting the technical conservation needs,” he says.

It is Cooper’s task to control the rooms emotionally as well as technically. Light-sensitive pieces that are brightly colored and feel energetic require a special touch. This was the case with a mid-century poster of “Singin’ in the Rain” displayed for the gallery’s 2013 “Dancing the Dream” exhibit. Although that show overall had a bright, celebratory feel, it wasn’t possible to bounce a lot of light off the vintage posters; that could hasten fading and deterioration. Cooper had to find other ways to inject light, consulting with the exhibition designer on wall colors and the strategic placement of the more delicate items in small rooms to enhance their “visual energy.”

Cooper got his start in theater lighting after earning a master’s degree in lighting design in 2003 from the University of Maryland — a relatively new degree at the time. The big difference between theater and museum work, he says, is that lighting visual art demands far more restraint.

“You have to rein things in,” he says. He stays away from colored lights and moving or patterned lights, tools a theater designer might use.

Lighting in both the performing and visual arts helps tell a story. And it’s the storytelling that continues to fascinate Cooper. This was especially true as he worked on the “Dark Fields” show.

“It was a huge revelation to me,” he says. “What got me was how in­cred­ibly vivid the images were. You just don’t think of historic photography as quite that vivid. It’s something you expect of modern photojournalism.”

So how did Cooper thread that needle of low light and lots of detail? Rich, dark wall colors — deep wine-red, grayish-purple, stormy blue — help make the photos pop. Your irises are adjusting to the darkness as you enter the rooms from the brighter hallway, and this also helps. The photos appear to glow.

Cooper also pondered how single and solitary he wanted the pictures to feel. Studying the walls a few weeks ago, as the show was being hung, he wondered aloud, “When you leave the room, do you imagine these pictures talking to each other?”

He wants observers to connect with the men and women captured so expertly within these small frames, to feel something of their lives. What he hopes, he says, is that we travel through an era of time in these small spaces. Whether he has achieved that, working with invisible waves and thin air, only the visitor can judge. But seeking to sculpt something as elusive as the art’s atmosphere, gently and with care, is what drives him.

“Light creates space and can create energy; the character of light will shift from room to room, if I’ve done my job correctly.

“This is what I love about every show,” Cooper continues, his gaze traveling from the photographs to the fixtures overhead, gauging the intangibles of small pools of light. “I’m learning as I go.”

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