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A lot of people whine about infravision in D&D and clamor for its return to the game. That's a really, really bad idea from a mechanical standpoint.
Infravision rules have a lot of holes in them, and cleaning up those holes isn't worth the time and effort to do so. Darkvision is an elegant solution (it works just like normal vision and doesn't rely on a science explanation that can be misinterpreted or exploited) and doesn't require all of this extra work.
Infravision is problematic because science tells us how heat should function, and infravision (which is supposed to see heat) doesn't work that way.
OK, first, infravision is effective at a distance, so the movement of air and such isn't enough to disrupt its functioning:
Second, warm-blooded creatures give off heat on objects that they touch. This means that (a) they leave warm footprints:
(b) they leave warm handprints:
(c) and in general objects they touch and the air around them become warmer:
Third, infravision is precise enough that you can notice fine heat-based details on a warm item:
or even recognize a person by their facial heat patterns:
So, we've established what the basic premises of infravision are and that they're not unreasonable from a physics standpoint, since we can duplicate them in real life.
Now let's take a moment to explain some of the science of infrared light and its relation to heat.
Temperature is defined as the average molecular activity of a given object's molecules. The more activity a set of molecules is, the more agitated those molecules are, and the more energy they have. That energy is generally perceived as heat. The molecules in a cup of hot water have more energy than the molecules in a cup of cold water, and the molecules in a cup of boiling water have more energy than those in a cup of hot water -- enough energy to push each other away violently and enter a gaseous state. So, a creature or object's temperature is a measure of how much energy it has, and is related to how much heat it radiates.
So, a warm-blooded creature such as a human gives off heat. This is because a byproduct of your metabolism (breaking down food and using it to make your body work) is heat, and if your body didn't give off this heat, your internal temperature would keep climbing and climbing and you'd eventually overheat and die.
Part of this heat is radiated as infrared light. A warm object gives off a broad spectrum of radiation in the infrared spectrum because those are low-energy wavelengths of light (objects that are even hotter start to glow red, and even hotter ones tend to glow white as red, orange, yellow, and other visible colors blend together into a muddled mess that we perceive as "white light"). It is this heat-as-light that a user of infrared vision sees.
In addition to infrared radiation, a warm body is in contact with the air around it and heats it by direct conduction of heat without using infrared at all. Just like sticking a thermometer on a light bulb causes its temperature to increase, a warm body causes the air around it to warm as well. That's why a crowded room tends to heat up. Simple, eh? This means that in addition to the IR a body directly gives off, the air directly around a body gets warmer and radiates heat -- at least, more heat than the unwarmed air near it gives off. This means that a creature with infravision should see a cloud of warm air around or above a warm body, just like you can see the air shimmer above a hot fire (see the couch photo above for an example). This warm cloud trails off quickly because the air around a creature is always in motion and that air (1) gets shuffled away from the warm body by the action of other air, and (2) that warmer air transfers some of its energy to other adjacent air molecules. This is the same effect as a blacksmith sticking a hot piece of iron into a bucket of water, only less effective (water sucks up heat even better than air does) -- the warm object slowly bleeds heat into the surrounding air. If it weren't a living creature, eventually the object and the air would reach the same temperature and it would be hard to tell them apart with infravision. A living warm creature would continue to produce heat, keeping it warmer than the environment.
A hot body not only gives off heat into the air, but into whatever solid objects it touches -- say, the floor. So, a warm creature is going to leave warm footprints, warm handprints, and so on, on anything it touches (as we've seen in the pictures above). Sure, they fade relatively quickly, and there are fine gradiations in temperature between the range of human skin, human clothing, handprints, footprints, the air, and the cold ground (from hottest to least hot), but if infravision is supposed to allow you to recognize individuals (as it must, else the dwarves and the drow have to turn on the light every time they answer the door), and then its sensitivity must be fine enough to allow for these subtle differences, since the human face radiates heat in different places (as the face picture above shows). Heck, you can even see veins under the skin.
So, we have established (1) that a warm creature radiates heat via infrared and by direct conduction, (2) air and touched surfaces radiate heat temporarily after being touched, and (3) AD&D infravision has to be sensitive enough to detect these things.
Infravision has a fixed range (usually 60 ft.). This meats that a red dragon standing in the dark 70 ft. away from a dwarf cannot be seen by the dwarf, even though it should be putting out plenty of heat to reach into the dwarf's 60 ft. infravision range. To account for these effects, infravision would need rules to determine how far out a creature's heat signature extends beyond its body (suggestion: I'd add it's smallest face statistic in each direction as a rough guideline). Of course, that means that infravision is not longer a simple flat value -- more work for the DM.
If infravision is sensitive enough to be able to identify facial features and such, then infravision must be sensitive enough to see tracks on the ground from a warm creature (as we established above). AD&D doesn't have any rules for how long it takes for these tracks to fade. It also doesn't have any rules for the difficulty of tracking in this manner.
Now let's take a look at some spells and how infravision would affect them (or, if infravision exists, how these spells require answers on other aspects of the game rules).
Blur: Wouldn't infravision negate this spell? A person using infravision is used to looking at a creature with a blurring, shifting outline (since the creature's body is heating the air around it, which the infra-user can see), so they should have no miss chance, or at least a reduced miss chance.
Burning Hands: Does this (and other fire spells) negate infravision temporarily because it's so hot? Likewise, is an ice creature (like a gelugon, or ice devil) invisible to infravision because it doesn't give off heat?
Cone of Cold: If this spell drains heat, do creatures that have been hit by it become harder to see with infravision for a while?
Darkness: If darkness prevents infravision, it's keeping heat from flowing through the air. Shouldn't that give some benefit against fire attacks?
Displacement: Shouldn't infravision negate this, too? Even if the target's radiating heat is displaced, the target is also heating the air and ground, which shouldn't be displaced.
Faerie Fire: Not so much of a 3E D&D problem as an AD&D problem. AD&D faerie fire outlined a subject in a glow, making them easier to hit. A creature being viewed with infravision is glowing and should therefore be easier to hit than one viewed with regular light.
Gaseous Form: Can you see the gaseous person with infravision? A wandering hot zone of air?
Heat Metal: Even if it's not enough to blind infravision, shouldn't this spell make the target easier to hit, since they'd be outlined in extra heat?
Invisibility: Infravision breaks this spell. If a creature is
invisible to infravision, then something must be either (a) preventing
the heat from escaping your body or (b) rapidly cooling the body heat leaving
your body so it's indistinguishable from the surrounding air.
If it's the first option, then the invisible creature will heat up over time and eventually succumb to heat exhaustion and die; it also should reduce the damage of spells like cone of cold which "drains heat," and keep you warm in arctic areas (since you're not losing heat to the cold air).
If it's the second option, shouldn't it be a transmutation spell instead of an illusion? And wouldn't the spell have to be very "smart" to recognize the different patterns of heat and cold around you? Basically the spell would have to being doing double duty, muting two entirely different types of radiation to compensate for the senses that detect those types. It's like asking a French horn player (a fairly complex instrument) to play the keyboard at the same time -- quite a lot of work for a 2nd-level spell.
Furthermore, it doesn't address the secondary effects of a warm body -- the heated air and solid objects it touches. Does invisibility affect the cloud of warm air? If doesn't, an infravision user should be able to see the warm outline of an invisible creature and therefor be able to locate it in some way. If invisibility does affect the cloud of warm air, does it affect the parts of the ground they touch? What about the weirdness of an invisible user holding a torch? The light is visible … is the heat visible? You're asking a lot out of a 2nd-level spell.
Iron Body: If you're "living iron," do you radiate heat? Or are you room temperature like an iron golem? Heck, does an iron golem generate heat? Its parts probably grind together at some extent, and friction creates heat (that's why rubbing your hands together in cold weather helps). While we're at it, if infravision is used, you'd need to note for each creature whether or not it gives off heat, and if it's a low, medium, or high level of heat (relating to the gelugon/ice devil question earlier).
Mirror Image: Since this spell is a figment, it cannot produce real effects. It can't make light, and therefore it can't make infrared light, which means that the mirror images don't show up in infravision -- the spell would never be used by drow or other dark-dwelling creatures that don't use light.
Otiluke's Spheres: Does infravision penetrate these spells? If so, at what threshhold does the spell stop allowing heat? Could you do a "low heat" spell that could still damage people through these force effects?
Statue: Same issues as iron body.
Wall of Force: Same question as Otiluke's sphere spells: Is a creature on the other side invisible to infravision? It allows visible light but not damaging sorts of energy, but enough heat will damage someone.
All right, enough of spells. Get my point? Compared to infravision, darkvision is simple, elegant, and it works
without all of these questions and extra work. If you really want it, go
ahead and put infravision back into your home game, but understand the
problems and extra work because of it. You won't see it in mine, and I
doubt you'll ever see it show up in official D&D again.
10/26/03: Scott a.k.a. "Ironsword" also pointed out something.
A warm-bodied creature using infravision would blind itself with its
own body heat; it's like standing with your face right nekt to a lamp
and trying to see in an otherwise dark room. Real-world creatures with
heat-sensing abilities (mainly snakes) are able to do so because they
aren't warm-blooded, (their body temperature is generally the same as
the environmental temperature); their ability is also less sensitive
than D&D infravision because they can only detect relative changes
in heat (things that are significantly warmer than the enviroment), not
absolutes, so they wouldn't be able to recognize faces, follow a
creature's tracks, etc.