Photo of the Month: August 2009

August 3, 2009 / no comments

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One of my favorite local beaches is Coffin’s Beach on Boston’s North Shore. It’s a spectacular flat arc of sand about 1-1/3 miles long and, at mid-tide, about 120 yards wide. Here’s a thought experiment to illustrate the incredible adaptability of human vision. On a nice, sunny July day around noon, the daylight on the beach measures 10,000 footcandles. It’s a little uncomfortable without sunglasses, but I can read a newspaper with some squinting.

My beach towel is 30 inches wide and 5 feet long. Let’s imagine that I spread it out on the beach and collect the daylight falling on the towel at that moment. I come back at midnight, and spread that same “towelful” of daylight illumination evenly over the entire area of the beach, about 2.5 million square feet, more than 57 acres. That will give me enough light to still read the paper. Yup. The headlines are easy, the article text is a little slow, but I can make it out. Wanna bet? That towelful of light will give us about 0.05 footcandles over the entire beach, which is the ASHRAE/IES recommended minimum for a walk in the park.

So I can read the newspaper over an illuminance ratio of about 200,000 to 1. We can actually see over a still greater range. For just walking around, my eyes aren’t even trying hard at 0.05 footcandles. In fact, I still have some sense of color vision, and my colorless night vision is just beginning to kick in. In order to get my night vision to take over completely, I’d have to spread my towelful of light over a hundred Coffin’s beaches – almost 60,000 acres.

Photo Credit: Google