The light transmission through the water, it's cloudiness, is just way to variable for something like this to work constantly, reliably.

Wouldn't the cloudiness itself be a signal?

Presumably it's essentially clear of suspended particles, most of the time.

The issue is that the cloudiness of water, even open water, varies greatly. So there would be either lots and lots of false positives, or the system would be too insensitive.

Think driving in fog

A signal of what?

Something solid ahead

To clarify: I mean that, generally speaking, the presence of 'cloudy' water would presumably signify that the seafloor was close. In the open ocean, maybe a sea mount is surrounded by essentially a cloud of particles.

Edit: I think your reply made the thread reach maximum depth, so I'll end it here.. I should do more reading on the topic.

The problem I'm highlighting is it also gets cloudy randomly in open ocean, for lots of other reasons.

Have you ever dived? Max visibility is like 30-50m unless you’re in a filtered environment

Your two requirements contradict each other. You can choose one or the other, but not both.

The world's militaries have invested a great deal of time and money into researching underwater sensors. The solution is not going to be discovered by random people on the Internet.

> The solution is not going to be discovered by random people on the Internet.

I'm fairly certain someone knows a number of things that have been discovered by random people on the internet.

Water absorbs photons at most frequencies outside the visible spectrum. What we deems as visible light is actually where water is most transparent. Since eyes evolved underwater, it makes sense they would use this section of the spectrum.

https://commons.wikimedia.org/wiki/File:Absorption_spectrum_...

That makes so much sense and actually unravels a few other mysteries for me. Thanks!

> Any people with knowledge in the field care to comment on whether it's feasible for a light (em?)-type, active, near-range detection system to work?

I’m not in the field, but I follow it enough to say: in theory, yes, (blue-green lidar), and there is a lot of work on moving this from theory to practice.

But approximately everyone is working on it (for bathymetry and for detecting subs, and similar blue-green laser tech for underwater comms), so its likely once its deployed so that a sub could in principle use it as a “headlight”, it’ll raise similar concerns to sonar, because if it is used actively, passive sensors looking for active use will also be common.

the issue is to detect these while maintaining stealth.