Infravision

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.

Note: This information is taken from Chapter 13: Vision and Light of the "Dungeon Master Guide".