[kragen]

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.

[hamburglar]

> 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.

[kragen]

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.

[hamburglar]

> 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.

[kragen]

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.