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Comment by flak48 | Hacker News Reader

> Does it get warm?

> Brian S (Creator): Nope not at all. LED's do not emit much heat and the Titanium body of the eye offers great heat shielding from any heat emitted.

[https://www.tiktok.com/@bsmachinist/video/713267837541158430...]

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LEDs of a couple of watts do emit a noticeable amount of heat.

Being titanium doesn’t mean anything - here the heat has to go somewhere eventually. Either out of the front of the eye where it’s “uninsulated” or it’ll get hot enough internally that the body will get warm.

Being titanium means it acts as a heat spreader and a buffer that can absorb and dissipate the small bursts of heat you might expect from brief flashlight use. Rather than the LED touching one spot in your eye socket (which would probably be a noticeable amount of heat even if not uncomfortable), the heat is spread over the entire titanium body.

Given the size of the battery I’d assume you could only pump out “max power” for a short time anyway. If that titanium does act as a buffer, things might be good.

Honestly the heat thing was the first I thought of too. It would nice if the dude goes into more detail about it because I’m sure it is one of the top questions asked.

> Being titanium doesn’t mean anything

This is a pretty confident answer, but I think you miss the point of the titanium. It can act as a heat battery and slowly let it dissipate all around vs a focused point of heat.

Titanium has much less thermal mass than the same volume of water. Water is 4.2 J/g/K and 1 g/cc, thus 4.2 J/cc/K. https://en.wikipedia.org/wiki/Titanium is 25.060 J/mol/K, 4.506 g/cc, and 47.867 g/mol, so 0.5235 J/g/K and 2.359 J/cc/K.

I think water is also a better thermal insulator if you can keep it from convecting, and a better heat spreader (to eliminate "a focused point of heat", which I agree is a problem) if you do let it convect. But that's the opposite of titanium being a good heat shield.

Thanks for commenting. I'm a layman here so providing hard numbers helps me wrap my head around the comparison. I think you're saying that water is roughly double a better thermal insulator? I wonder if the man could devise a water layer around what generates the heat in order to reduce the chance for burning his body?

He probably has to weigh the chance for water to mix with the components vs a simple titanium hunk of metal. I guess that's what materials engineers are for?

I'm a layman too.

No, I didn't provide numbers on thermal conductivity, just specific heat, though it is now unfashionable to call it that.

Yes, putting some water inside the titanium would increase the thermal mass of the overall assemblage, and I don't know of anything that would work better than water for this. This would reduce the temperature to which the eye-socket tissue is heated by intermittent but high-intensity LED usage, reducing the risk of burns, because it smooths it out over time. If the LED is not at the top of the eye, or if the eye rotates, liquid water would also help to smooth it out over space.

However, to minimize the temperature to which the eye-socket tissue is heated, it would also be beneficial to interpose thermal insulation between the tissue and the heat source, such as a thick layer of teflon around the outside of the eyeball (or saline-solution-soaked teflon foam). This will slow heat transfer from the hot eyeball to the eye-socket tissue, giving blood flow more time to convect away the waste heat and thus producing a lower temperature in the sensitive tissue.

I think the more important issue here is that he's using low enough power with a low enough duty cycle that it isn't much of a problem.

> I think the more important issue here is that he's using low enough power with a low enough duty cycle that it isn't much of a problem.

Yeah, I think debating whether water or titanium is a more effective heat battery ignores the fact that titanium is obviously good enough, and there are plenty of reasons he might have preferred to construct the thing out of titanium, such as the fact that it looks cool.

Because titanium is opaque, solid titanium looks the same as a thin shell of titanium on top of another material. Because water is liquid at body temperature, it isn't a suitable material to replace titanium for the outer shell; it would just splurt out of your eye socket.

> Because titanium is opaque, solid titanium looks the same as a thin shell of titanium on top of another material.

I obviously haven’t done the math and don’t have access to the data that would allow me to, but I suspect that for this application, you’d have to have an extremely compelling reason to complicate design and manufacturing with such an approach.

Most people would consider not burning their eye socket and possibly brain an extremely compelling reason. Heating an entire 7-cc titanium ball from 37° to the 44° necessary to start causing burn damage would take about 30 joules: 500 mW for a minute, for example. If the heat source is close to the surface of the eyeball (on the inside) you would need less energy because only a part of the eyeball is reaching burning temperatures, maybe as little as a joule. If the ball is almost entirely full of water and properly insulated, you could probably handle hundreds of joules before it started to burn you through the teflon or whatever. A hundred joules is a 3.7 volt Li-ion battery with 7.5 milliamp hours of capacity, so this is a practical amount of energy to put inside your eye socket.

Well sure it's a confident answer - that's just how physics works. You generate heat in an enclosed area, it's going to leak out. The hotter it gets inside, the faster it's going to dissipate through the material containing it.

The question is how hot will it get for some amount of LED on-time.

But it kind of misses the point. The titanium does do something. It slows down the heat dissipation to an unnoticeable level according to the creator.

The comment was confident and correct, but not accounting for the whole picture. Couple that with a level of snarkiness, and it's a classic internet comment that makes the internet worse IMHO.

I'm always reminded of that comment on hackernews that shits on dropbox when it was launched. It was correct and of course dropbox was "trivial" to copy using a couple of commands, but it misses the point.

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