In a puzzling episode of Things You Might Not Know, host Tom Scott made an honest attempt to respond to a viewer’s question about Moiré leading marks, a subject completely unknown to him. So he did whatever research he could do. He eventually learned that this idea was developed and patented by the Inogon Corporation in 1983. The beacons were intended to mark underwater hazards with invisible Moiré arrows to guide ships safely into port. However Scott had some questions, specifically about the Southhampton UK location where he was filming.
Once you know that this is an Inogon light, all the rest of the research becomes easy. All except one thing. This Inogon light isn’t being used to guide people in. The light follows the path of an underwater cable where you shouldn’t drop anchor. So the signal here is all wrong. You don’t want to follow these arrows, you’re required to stay away from them unless you’re passing straight through. …So why is it here? I don’t know. But if anyone out there does know, for sure, with evidence…do get in touch.
Here’s a demonstration of a Moiré Pattern in use.