Infravision (DMG)

There are two definitions of infravision that can be used in the AD&D game. The first is simple but lacks detail. It is, however, a perfectly adequate definition for those who don't want to bother with the complexities of infravision. The second, optional, definition, adds another level of detail to the game. It allows the DM to create special situations in which the function of infravision becomes important, but requires the DM to keep track of more rules and more details.
==Standard Infravision==
The easiest definition of infravision is that it allows characters to see in the dark. Nothing more is said about how this works—it simply works. Characters do not see into the infared spectrum or "see heat or anything else. They just see in the dark as clearly as they do in normal light. However, since it is a somewhat magical power, the range is not that of normal vision—infravision ability extends only 60 feet. Beyond this only normal vision is allowed.
==Optional Infravision==
This definition is much more scientific and accurate to what we know of physical properties of the real world. To its advantage, this definition makes infravision very different from normal sight, with its own strengths and weaknesses. To its disadvantage, it introduces a certain amount of scientific accuracy (with all its complications) into a fantasy realm.
According to this definition, infravision is the ability to sense or "see heat. The best comparison is to thermal imaging equipment used by the armed forces of many different nations today. This special sense is limited to a 60-foot range. Within this range, characters can see the degrees of heat radiated by an object as a glowing blob translated into colors like a thermagram.
If this definition is used, there are several things that must be considered. First, large heat sources will temporarily blind characters with infravision just as looking at a bright light blinds those with normal vision. Thus, those attempting to use infravision must make the effort to avoid looking directly at fires or torches, either their own or the enemy's. (The light from magical items does not radiate significant heat.) Second, the DM must be ready to state how hot various things are. A literal interpretation of the rule means that characters won't be able to tell the floor from the walls in most dungeons. All of it is the same temperature, after all.
The DM must also be ready to decide if dungeon doors are a different temperature (or radiate heat differently) from stone walls. Does a different color or kind of stone radiate heat differently from those around it? Does the ink of a page radiate differently enough from the paper to be noticed? Probably not. Can a character tell an orc from a hobgoblin or a human? Most creatures have similar "thermal outlines"—somewhat fuzzy blobs. They do not radiate at different temperatures and even if they did, infravision is seldom so acute as to register differences of just a few degrees.
Be sure you understand the effects this optional definition of infravision can have—there are dangers in bringing scientific accuracy to a fantasy game. By creating a specific definition of how this power works, the DM is inviting his players to apply logic to the definition. The problem is, this is a fantasy game and logic isn't always sensible or even desired! So, be aware that the optional definition may result in very strange situations, all because logic and science are applied to something that isn't logical or scientific.