Luminism was a term created to describe some attributes of the Hudson River school, that group of painters who idealized upstate New York landscapes in the mid-19th century. As the word implies, these artists were preoccupied by light-filled skies — an obsession that the photographer Alfred Stieglitz would push to abstraction some 70 years later. In "Equivalents," San Francisco artist Elliot Anderson combines the idealism of the Hudson River school with Stieglitz' abstraction by creating digital photocomposites of skies. To get his images, Anderson collects snapshots of clouds taken from the Internet and runs them through a kind of averaging software he's concocted. The resulting photographs, some of which are backlit, glow with eerie superdrama. Storm clouds, more bulbous than life, ooze a pus-colored radiance. A baby-blue vault of clouds coyly reveals a silvery celestial backdrop. You could call this luminism, too, if it weren't quite so vertiginous.