Random Roll: DMG, p. 59

The ability of several demihuman races to see in the dark is firmly established in Dungeons & Dragons. My first encounter with the game was through the Holmes-edited Basic Set and its rulebook calls this ability "infravision" without any explanation. The AD&D Dungeon Masters Guide, though, goes to some length explaining the nature of infravision.

As explained in PLAYERS HANDBOOK, infravision is the ability to see light waves in the infrared spectrum.

To say that I have disliked this definition for decades is an understatement. While I am on record as not being opposed in principle to the mixing of science fiction and fantasy, Gygax's explanation of infravision leans a little too heavily, in my opinion, on real world science, with infelicitous consequences, as we shall see.

Gygax elaborates with the following:

Characters and various creatures with infravisual capability out to 60' (standard) are basically picking up radiation from their surroundings. Therefore, they note differences in thermal radiation, hot or cold. They do not "see" things which are the same temperature as their surroundings. Thus, a room in a dungeon might look completely blank, as walls, floor, ceiling, and possibly even some wooden furniture are all of the same temperature. Openings in the walls should show up rather plainly, as space anywhere else will, and if you are generous, you can allow different substances to radiate differently even if at the same temperature, i.e. the wood in the example above would be discernible if care was used in scanning the room infravisually.

Leaving aside the not insignificant matter of what this does to the "magic" of D&D, the conception of infravision Gygax advances here seems intended to limit its utility. If an elf's ability to see in the dark is akin to 1970s era IR goggles, it's a rather narrow ability, almost to the point of uselessness. I imagine that's the point, though. He continues:

Except where very warm or very cold objects are concerned, vision of this sort is roughly equivalent to human norm on a dark or cloudy night at best. Note also that monsters of a very cold or very warm sort (such as a human) can be tracked infravisually by their footprints. Such tracking must occur within two rounds of their passing, or the temperature difference where they had trodden will dissipate. 

The ability to track via infravision is certainly handy, though, as one might expect, Gygax places limitations on it, which given his explanation of how the ability works, is not unrealistic. Of course, what he gives with one hand, he takes with the other.

Light sources which give off heat also absolutely prevent infravision from functioning within their sphere of illumination. (Explain this as the effect of trying to see into the dark when the observer is in a brightly lot area.) It requires not less than two segments to accustom the eyes to infravision after use of normal vision. 

Again, this makes sense, given his conception of infravision, but it's a potentially serious drawback when one notes that it takes two segments to shift between normal and infravision. A lot can happen during those 12 seconds of temporary blindness.

The section ends by noting that creatures with infravision with a range of 90' or more – the sort possessed by "most monsters inhabiting underground areas" – see much more clearly than those with standard infravision. 

Such creatures can easily distinguish floor, ceiling, wall, and other areas, as well as furnishings within the area.

Talk about stacking the deck in the monsters favor!

This whole section makes me unhappy, or at least disappointed. I much prefer granting certain creatures, like dwarves, elves, and many monsters, the magical ability to see in the dark without restriction. This is more or less what's implied in OD&D and the way I've always handled infravision (a term I now reject, owing to the scientific associations Gygax foists on it here). Chainmail, I believe, grants magic-users the power to see in the dark too and it's something I've long considered giving player characters of that class as a basic ability. 

My point, ultimately, is that I think this whole section reeks of an attempt by Gygax to rein in an ability he thought too useful. Since I neither share his likely concern nor like his reframing of infravision as thermal vision, there's not much here I would use.