Thursday, November 19, 2009

Moire Patterns Around Us

Across the street from work, they have been building a new addition to the Student Recreation Center. Construction has been ongoing for over a year or so, and soon to conclude. Almost every day, many staffers at the Mirror Lab cross the street to a convenience store, Circle K, to fill up on snacks, get a newspaper, or just get out of the lab for a few minutes to see the sun. About a month ago, I noticed something new - evidently they've incorporated a moire pattern as an architectural accent in the addition!

Now I've been a fan of "found" Moire patterns in everyday life for decades. Farms in Iowa have them all over if you look for them. In the simplest forms, if a repeating pattern meets another pattern, light and dark fringes are generated as the patterns work against each other to block or pass light. For instance, the two patterns of horizontal lines, slightly tilted, can generate vertical bands. This image is slightly modified from the Wikipedia entry on Moire patterns. On a farm like where I grew up, corncribs, fencelines, even power lines can form Moire Patterns.

In an urban area, they are harder to spot, but can still be seen - in protective screens over pedestrian highway overpasses, even in the reflection of a screen in a window. Here is a shot of a pattern formed by a window screen with it's reflection. The effect is often seen in some high-tech applications - sometimes when photographing a repeating pattern, say a brick wall, when examining the image on the camera's viewing screen, a Moire pattern can form with the limited sampling of the screen. Similarly, in the work we do measuring mirror surfaces, frequently the interference fringes we use to measure the mirror form patterns with the sensor's pixels.

But the Moire on the Rec Center was the first I've ever seen that were built on purpose. As soon as I had a chance, I took a short walk to see how it was formed. The pattern is caused by two separated screens - what looks like steel sheets with hundreds or thousands of regularly spaced holes a couple mm in diameter. The sheets are about 30cm (1 foot) apart, and when backlit with the morning sun falling on the east older wall of the Center, patterns form as the holes alternately let light through or block it. What is also interesting is that when driving along 6th Street westbound past the screens, the pattern has a dizzying motion that surly has a hypnotic effect on drivers. It would be interesting to come back and look at accident statistics in 10 years, comparing it to before it was installed!

While not exactly common, the use of Moire patterns in architecture is not unknown. I found an incredible blog while doing a Google search and found "Moire and Shadow Art". The blog hasn't posted anything new in over a year, but what he's posted is captivating. I recommend marching down all of his posts listed on the right hand side of the blog. Among my favorites besides the full building Moire at the above link is the photo of Jason Alexander shown at left. The pattern is caused by a repeating pattern in his suit interfering with the original printer's screen at some point in reproduction - another common place to find Moire patterns. He also has some videos and a link to a Moire-driven digital sundial - Neat stuff!

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