Using Moiré Patterns To Guide Ships

Moiré screens for arrows
Moiré screens for arrows

[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.

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.

27 thoughts on “Using Moiré Patterns To Guide Ships

    1. Shadows are inherently 3D because we perceive and comprehend 3 dimensional space. Shadows appear 2D when observed from a 2D perspective or are projected onto a 2D space (i.e. a wall).

  1. So, it’s a horizontal analog of the aviation world’s (vertical) VASI https://en.wikipedia.org/wiki/Visual_approach_slope_indicator

    Though, where I grew up, the navigation markers were simple orange blazes, one behind the other by a few dozen meters. You’re in the lane when they are aligned.

    Presumably, this nautical version can be made passive, and not require electrical power. I can’t imagine what other advantage it might have over the much more compact aero version.

    1. Thanks for that. A friend of mine who owned a Cessna showed them to me back in the 1980s on my first plane ride but I’d long forgotten what they were called, etc. but I did remember that they were for indicating the slope of your descent.

    2. It’s a lot easier to buy a boat than to buy a plain these days so a system that literally tells you where to go to get back to the center of a channel with effectively no training is nice. Also it’s huge which is an advantage all its own.

        1. Just wanted to add that the reason why the definition is interesting is that I for one did not realize that the word ‘planet’ has the same root as ‘plane’, namely the Greek for ‘wanderer’.

  2. Not only does the arrow point ‘proportionally’, but the aligned half is more transparent than the unaligned half (see the GIF animation).

    Suppose the left half had a green backlight and the right half a red backlight (or 2 different IR wavelengths) then a receiver using 2 photodiodes with appropriate wavelength (or colour) filters should be able to determine the corresponding angular coordinate. To remove random room light one can use frequency filtering and flicker both background leds connected in series. multiple of these base stations horizontal and vertical each with their own frequency or code, and the receiver can locate itself? Cheap room GPS

    1. Hey, didn’t expect to see my own material here. I’d like to make one correction that a lenticular print isn’t a “real” hologram, it’s more of an stereogram. Also, I still haven’t been able to figure out how the 2D lenticular array image is able to store and reproduce depth of field.

  3. Way back in the day, I worked on developing a process that would use Moire’ patterns to test mechanical devices under stress. We would take a picture of a laser pattern on the device at rest and develop it into a film. We would then put the device under stress, project the pattern, and view it through the film and you could measure the distance between the Moire’ patterns to get micron accurate deformities of the device under low stress conditions. I think the first devices we tested were Fire Extinguisher tanks.

    1. This. Then you’re into holographic interferometry, polarized light analysis and all kinds of other fun things that don’t involve twitchy strain gauges. If you can find a copy of Gary Cloud’s “Optical Methods of Engineering Analysis” there’s a lot more information on all of it.

      (Disclaimer: I’m not him, but I did some work with him in grad school and really like the text).

  4. http://worldwidemarinetraining.com/blog/aids-to-navigation-range-markers/

    Generations of sailors have used visual ranges… they work night and day (if lighted at night)…but visual ranges are “clearly” a problem in fog! Not sure why this approach is better… existing method is pretty good and just needs paint and basic illumination to work!

    Me… more than a year at sea in coastal waters… best story about navigation marks (true or not)…

    https://www.snopes.com/fact-check/the-obstinate-lighthouse/

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