Artificial Skies

Some years ago, Paolo Di Trapani, a professor of optics at the University of Insubria in Italy, discovered something startling about daylight: he didn’t fully understand it. He had been asked to give a lecture on Light and Color in the Outdoors, a book by a Dutch astronomer called Marcel Minnaert, first published in 1937. Minnaert catalogues oddities and strange phenomena that can be produced by sunlight in certain conditions—mutant rainbows, freak reflections, stray iridescence and fluorescence, colors out of space. It’s a fascinating book, but also quite a tease. Di Trapani found himself vexed. “I realized that I hadn’t seen any of these phenomena,” he says, “even though I was trying desperately to see them.”^[1]

To give a typically frustrating example, Minnaert says that where the shadows of two equally thick branches of a tree overlap, they produce a bright line in the midst of the darker shadow.^[2] “How is this possible?” Di Trapani complains. “A shadow is dark, and if you superimpose two shadows it is even darker. How can you get light?”

And there were hundreds of these effects. Rippled water reflects vertical, but not horizontal, forms. When the sun is low in the sky, only the vertical wires of a wire-mesh fence cast shadows, not the horizontal wires.^[3] “It’s really a symphony of effects. But I was unable to see any. I believed, finally, that Minnaert was cheating, because I am a professor of optics and I could not see any of these.”

So Di Trapani went to the laboratory to find Minnaert’s errors. He set up experiments to recreate the conditions and effects described by Minnaert and confirm with his own eyes what the astronomer, who died in 1970, had claimed. “And of course everything worked perfectly. It took a while, but I managed to reproduce several of these phenomena. And afterward, I opened the window and I saw.” The experiments had trained him to see what Minnaert described.

Armed with this new understanding, Di Trapani decided to exploit it more thoroughly. He planned to build an artificial skylight window, through which artificial sunlight would pour and from which an artificial sky could be seen. Such an invention had the potential to revolutionize interior design, bringing “daylight” into windowless rooms underground or deep inside large buildings. It could change architecture itself, breaking dependence on “natural” windows. But this wasn’t a new dream.