Photo of the Month: June 2009

June 14, 2009 / no comments


Pantheon, Rome, Italy

The Emperor Hadrian, who commissioned Rome’s Pantheon (completed around 126 AD), was an architectural enthusiast. The building incorporated amazingly innovative technologies, so it’s not surprising that it also used the best available lighting technology of the day: daylight.

With the exception of a little light coming through the doorway, all of the daylight in the space enters through a single oculus at the peak of the dome. It’s about 30 feet in diameter, which sounds big, but if we take that as a fraction of the floor area below (142 feet in diameter), the oculus works out to be about 4.5% of the floor area. And that provides very comfortable levels of interior daylighting, in a space that’s also 142 feet high.

There’s no glass in the aperture – it’s open to the sky – so if we corrected for modern low-E insulating glass at 70% light transmission we’d get the same amount of daylight from an aperture of about 6.5% of the floor area. To also account for absorption by lightshelves, baffles, lightwells, etc., we might double it, to around 15%. That’s not a bad rough starting point for thinking about a modern daylighted building. And by the way, Rome’s not in “the south”: it’s at virtually the same latitude as Boston, so the sun-angles there would be the same also.

If Hadrian could have built an all-glass building, would he have chosen that instead? Would that have been better? We’ll never know, but I know my vote…

Photo Credit: stanrandom