Speaking of membrante potential changes, I wonder if somehow produces some kind of extra electricity -- like if he is somehow able to harvest the electric potentials created through his neuronal signalling, and convert this electric energy into heat energy.
Hmmmmm...interesting thought, remember, he's supposedly a walking, talking solar collector/reactor. In the newer comics the explanation of heat vision is that it is simply the venting of solar energy. Maybe we could theorize that he can channel that existing energy along his optic nerve or perhaps another type of nerve that he may have in his alien biology (we're not stuck wondering how it could work with our physiology because as an alien he could have all sorts of little "extras" that have evolved in Kryptonians
)
I think the solar energy thing could even explain x-ray vision because I seem to remember reading somewhere (I may need to be corrected on this) that the sun gives off gamma rays, x-rays, etc. because of the massive amount of heat the nuclear reaction of the sun produces (the same thing being true with nuclear bombs)
The "weird optic nerve" explanation may also explain his other vision powers (telescopic, microscopic, infrared, etc.) because maybe Kryptonians could always see better than humans thanks to evolving in a red sunlit atmosphere where visibility would be a lot poorer than a yellow sunlit atmosphere (maybe explaining the evolution of "extra stuff" in his optic systems) and now that he is super-energized his vision "extras" translate into "super-extras".
(It's how early on a Sunday morning??!!)
As for the original question about visibility of heat vision, it has been established in the comics that in normal usage it is invisible to the naked eye, but that this all depends on it's intensity. The more powerful the burst the more visible it is. I'm not sure how scientifically this would be explained, but perhaps it would have to do with why stars are different colors depending on how hot they burn (that is why they're different colors, right?)
-Michael
