LOW LIGHT AND DARK VISION: My $0.02 worth...
...and as always, you may just be able to get some change back from it, is this:
Dungeons and Dragons, when the 3rd Edition was created, their was a concious choice to avoid the pitfalls of infra- and ultra-vision. And in my opinion, it was a good choice -- from a game design standpoint. This way, using the mechanics presented, low-light and darkvision are simply game mechanics that can, in what-ever world you are playing in, be defined via what-ever means the game-world designer wishes it to operate.
If you use infra-vision and ultra-vision, as terms, this would tend to indicate that the people of that world understand what the infra-red and ultra-violet light spectrums are. In the real world, the science of light was pretty much invented by Newton (if I recall my high school history classes correctly), and the discovery of infrared spectrum did not take place until 1800 when (again, if I recall) Frederick Hershell (sp?) with either his wife or his sister (that I cannot recall) did astronomical work and stumbled onto it. A year later John Ritter discovered ultraviolet light. Thus, but not calling low light and dark vision by these terms, you avoid a rather odd terminology for a medieval fantasy.
Second, if you avoid calling them by these rather scientific names, you avoid problems like white-out (being blinded by relatively high heat sources -- at what range? at what temperatures?), differentiation (how many degrees variant does it take to distinguish an object from the ambient temperature?), depth perception, resolution (human vision, for example, is broken into rods and cones; rods are highly light sensitive, but low resolution while cones are not nearly as sensitive, but of a much higher resolution and can see color), and so on.
By calling this by a game-functional name: low light vision and dark vision, you get the effect and allow the game master to define the how and why. And in my estimation, this is a good thing (tm).
