[Tom Scott] ran across an interesting visual effect created with Moiré patterns and used for guiding ships but we’re sure it can be adapted for hacks somewhere. Without the aid of any motors or LED animation, the image changes as the user views it from different angles. When viewed straight on, the user sees vertical lines, but from the left they see a right-pointing arrow and from the right, they see a left-pointing arrow. It’s used with shipping to guide ships. For example, one use would be to guide them to the center point of a bridge. When the pilots see straight, vertical lines then they know where to steer the ship.

US patent 4,629,325, Leading mark indicator, explains how it works and how to make one. Two screens are separated from each other. The one in front is vertical but the one behind is split in two and angled. It’s this angle which creates the slants of the arrows when viewed from the left or right. We had to convince ourselves that we understood it correctly and a quick test with two combs showed that we did. See below for the test in action as well as for [Tom’s] video of the real-world shipping one.

Creating slanted lines with combs The two combs

The test was done by holding up two hair combs against a light background, in our case a computer monitor displaying mostly white. The teeth of the two combs were held touching at the bottom. The front comb was held vertically and the back comb was leaned backward. Instead of moving the camera to look at the combs from the left and right, the combs were swiveled as one left and right. As you can see in the animated GIF, looking at the boxed area, the slant of the Moiré pattern changes, just as the upper or lower part of the arrow in the shipping light does.

Not convinced that Moiré patterns can be useful to hackers? Check out these DIY precision calipers which use the Moiré effect.

Our thanks to [Lindsay Wilson] for sending in this tip.