What would it be like to see in the dark?
    My interest in this topic was sparked years ago when I tried
to figure out just what my half-orc AD&D(R) game characters could
see using infravision in a dungeon. I wanted every advantage there
was for those obnoxious little guys. Additionally, I wanted to
know just how well infravision-using monsters could see in the
dark, because I wanted my characters to avoid being seen and
promptly eaten, as a number of them were.
    This interest was sparked again recently by an article in a
science magazine on infrared vision. Some very intriguing points
came to light, and the results are offered here in the hopes that
AD&D and D&D(R) game players everywhere will find them useful.
(Certainly, my half-orcs would have gotten a longer leash on life
with this information.)

How infravision "works"

    We should really start with a look at real-world infrared
light and infravision. This makes certain game aspects of this
sensory power clearer, and also highlights inaccurate,
contradictory, and problematic aspects of infravision in game play
(which will be discussed in depth later).
    The science article that fired me up for this topic was
"Seeing the World Through Infrared Eyes," by Neil F. Comins
(Astronomy Magazine, June 1991, pages 50-55). This excellent
piece covers the basics of how infravision would work in realistic
terms. It's worth hunting for this article in your local library
and copying it for reference. That and a few encyclopedic entries
are the basis for the information that follows.
    Infrared radiation is normally invisible, lying just below red
on the electromagnetic spectrum. It is given off by hot objects;
the hotter the object, the more infrared light it gives off. Very
hot objects eventually give off visible light--red light at first,
then orange, yellow, and white as the heat increases. We can sense
heat radiation on our skin, the largest sensory organ we have, but
we cannot detect more than a general direction of the heat source
and an idea of how hot the source must be.
    Certain snakes called pit vipers are able to detect infrared
light more accurately than we can, though only within a short
range. Several sense organs called pit organs lie to either side
of a pit viper's head, between the eye and nostril. Changes in
heat radiation as little as 1 degree can be detected. The snake
senses the direction of the heat source by moving its head back
and forth, noting the direction and intensity of the heat it
senses.
    We've known about heat for eons, but infrared light itself was
discovered by an English astronomer, Sir William Herschel, in
1800. A very practical use for infrared light was found during
World War II, when electric sniperscopes were invented.
Sniperscopes were attached to rifles and gathered distant infrared
light coming from the bodies of soldiers, converting it to visible
light for the sharpshooter. This allowed sharpshooters to fire on
enemy positions at night. (As will become apparent, some versions
of infravision in the AD&D game were based on sniperscope
characteristics.)
    Infrared light has less energy than visible light, but it
behaves in much the same way. Some infrared radiation is absorbed
by molecules in the air. However, near-infrared light, which is
the part of the infrared spectrum closest to visible red light, is
reflected by most objects and thus can be used to detect them. We
see a chair by the light reflected from it; a pit viper can detect
nearby objects by the near-infrared heat reflected or emitted from
them.
    Our ability to actually see infrared heat in detail is blocked
by several major problems. Because infrared light is less
energetic than visible light, a human able to see near-infrared
light clearly would need eyes about 5-10 times larger than normal.
Worse, heat is emitted from many objects all around us; stoves,
furnaces, living beings, light bulbs, hot car engines, and
sun-warmed rocks, concrete, bricks, and asphalt for example.
Almost everything with any warmth would glow as if it were a light
bulb, though with an intensity proportionate to how hot it was.
Thus, an oven will be "brighter" than a warm rock.
    What this means, of course, is that anyone able to see
infrared light also will see his own body warmth. We have body
temperatures just below 100 degrees F., which is enough to blind
us with heat radiation. It's like trying to take a picture when
the camera itself emits light inside and out, ruining the film.
    To prevent such heat blindness, an infravision-using creature
would need some sort of insulation around its eyeballs to keep the
body's heat out of them, and some kind of refrigerant to keep the
eyeballs cool so they become sensitive to outside light. This
insulation and refrigeration would be done biologically. (Don't
ask me exactly how, but I'm sure Mother Nature would figure out a
way.) However, let's face it: We're dealing with magic, not
science, and magic can do anything. Our problems are solved at a
stroke, even if it doesn't please the scientists among us.
    Another option--useful for beholders, giant snails, and
crabs--is to put the eyeballs on stalks, separating them from the
rest of the body. The eyes are thus air cooled, so no other
refrigerant is needed. I don't think beholders and so forth have
infravision, though (as is noted later) if they've lived
underground for a long time, they've probably developed it.
    In some ways, the way that infravision is described in the
AD&D game rules implies that it works in the same way that our
night vision normally works. Rod-shaped cells in the retina of
your eyes can detect very dim light after a short period of
adjustment to darkness, which you should be familiar with each
time you go into a dark room. At first you can't see a thing, but
over a period of minutes you start to see more and more objects in
what little light there is. Eventually, faint light sources like
the full moon, digital clocks, and even pure starlight can seem
quite bright, even painfully so. However, because rod cells are
not color-sensitive like the eye's cone cells, night vision is
mostly black-and-white vision; maybe "shades of gray" vision is
more accurate. (Infravision was described as being like
black-and-white vision in the original AD&D game, too, as noted
later.)
    Night vision can be instantly spoiled by bright normal light,
which is why driving experts tell you to look away from oncoming
cars at night, to preserve your eyes' sensitivity. Infravision in
the AD&D game is spoiled by bright visible light, extremely hot
objects like fires, and magical light. Perhaps fantasy creatures
with infravision have magical cells in their eyes that work like
rods, but pick up heat instead of faint visible light. Who knows?
    So much for how infravision works. What can you see with it?

The infravisual world: Aboveground

    We'll assume that your campaign world resembles our own Earth
in that it has a normal day-night cycle with a sun like our own.
(If this is not the case, you can make adjustments as we go
along.) What would your heat/infrared picture of the world look
like, then? Let's use some logic as we look around.
    In the daytime in summer, everything bathed in sunlight is
warm. Things that retain heat well, like large rocks, will be
warmer and stay warmer longer than things that lose heat rapidly
in cool winds, like thin leaves or blades of grass. The greatest
normal heat source is the sun, which we can easily assume is too
bright to look at with any form of infravision. Sunlight in fact
ruins AD&D game infravision, so we would rely on normal vision
alone. Air is assumed to be invisible, whatever its temperature,
unless it is extremely hot (see below).
    So infravision is useless in broad daylight. Once darkness
falls, however, the landscape is still hot. Objects retain heat
from the sun and radiate it slowly away, which keeps the night
side of the world from freezing. (Even magical worlds need
thermodynamic physics!) With the sun gone, a creature with very
good infravision could see almost normally right after full
darkness falls, since the terrain will radiate light. We can
assume that a combination of rod-based night vision and
magic-based infravision would be a potent mix, allowing vision
about equal to normal sight in full daylight. Distant images would
be fuzzier and less distinct than usual, so a far-away orc might
look like an ogre or a halfling, but it beats seeing nothing at
all.
    Different parts of the landscape will cool off at different
rates, so things will look strange. Rocks would be "brighter" than
trees, for instance. Water is generally cooler than land, but
water also retains heat better than land; thus lakes and seas
might seem "brighter" than the shoreline, especially late at
night. Very hot air, such as that escaping from chimneys or fires,
will glow faintly like a luminescent cloud.
    Other warm things in the world include live animals,
especially the warm-blooded ones, and fire. A deer, a human, and a
chipmunk all radiate heat--more heat when they are ill or
physically exerting themselves, less heat when standing still or
asleep. Certain magical animals, such as salamanders and red
dragons, can be assumed to produce much more heat than other
creatures their size. I recall reading that drinking alcohol
causes the body to radiate more heat than usual, so a drunkard
could be detected by being "brighter" than other people.
    Objects in close contact with living beings, like clothing,
weapons, tools, chairs, and beds, will radiate some heat after the
beings leave or discard them. In time, of course, those objects
will completely cool off. Standing on a spot or leaning against a
wall for a while also will leave residual heat behind, which could
be noticed. Scuffing or shuffling feet would leave infrared
"footprints" that could be tracked, though not for long. Friction
from dragged objects, like heavy sacks or combat victims, also
could be detected, as could places where surfaces have been rubbed
together for long periods of time (machine gears, gristmill
stones, axle joints, spinning wheels, etc.). Physical blows, like
being smacked with an open hand or a blacksmith's hammer, also
raise the temperature of solid surfaces for short periods of time.
    Fires produce vastly more heat than living beings. Seeing a
living being hiding next to a blast furnace in a dark room would
be almost impossible, like seeing a firefly's light next to the
sun's. Manmade and natural sources of fire include matches, pipes,
cigars, candles, torches, campfires, bonfires, hearths, furnaces,
forest fires, lava, and embers. All flame sources are assumed to
emit enough infrared and visible light to ruin infravision near
them. Note however, that a "dead" fire would radiate heat long
after the last ember has vanished, and likely would be detectable
at a great distance. A forest fire would "light up" the landscape
for many hours after the flames are gone.
    Remember, too, that infravision also detects the lack of heat,
just as normal vision detects the lack of light. Snow and ice will
look very dark in infravision if seen without visual light from
moons or stars. A cold-producing object like a refrigerator also
will look darker than objects around it. Cold-producing creatures
like brown mold or a lich (whose frosty touch causes frostbite
damage) will look very "black."
    It's worth a word on what sorts of creatures could not be seen
with infravision. Creatures that are normally able to turn
invisible, like pixies, should also be invisible to infravision
but not to other senses like smell. Any creature that is roughly
the same temperature as its surroundings, like a cold-blooded
insect, fish, amphibian, or reptile, would be harder to see at
night, though even cold-blooded creatures aren't always exactly
the same temperature as the environment around them. (Live things
move and so generate friction from moving, for one thing.) Magical
beings that radiate no heat at all, like undead skeletons and
zombies, would be almost invisible to infravision unless revealed
by reflected infrared light or else blocking a much hotter source,
revealing their outline.
    With so many heat sources at night, and so many things that
will reflect infrared light, there will be a multitude of
infrared-light shadows. The landscape will lack clarity and seem a
bit out of focus (even more so at greater distances), as well as
painted in shades of gray. It's a confusing, alien world, but any
creature born with infravision would be quite accustomed to it and
might instantly recognize any critical feature it sees.
    Neil Comins' article notes that the night sky itself would
change when seen through infravision, but modern-world astronomy
is considerably different from the AD&D game's SPELLJAMMER(R)
setting "physics." In essence, any heat source in wildspace will
glow fuzzily in infravision, but it's up to the Dungeon Master to
choose which things seen in the night sky are heat-emitters and
which are not.
    Stars, for instance, might radiate only visible light and no
heat at all, thus being invisible in infravision but not to normal
or night vision. Fire bodies, like suns, will certainly emit
enormous amounts of heat and be easily visible from a planet's
surface as enormous, brilliant, fuzzy balls in the sky. Air bodies
will emit or reflect very little heat (becoming invisible), and
earth and water bodies will only reflect heat (being barely
visible unless they are close to a heat source).
    Let's take three well-known crystal spheres and see what's to
be seen in the infravisual sky. Oerth, home of the GREYHAWK(R)
campaign, is set in Greyspace (described in the SPELLJAMMER
accessory SJR6 Greyspace). The local sun, Liga, is an enormous
fire body that will temporarily blind any infravision-using
creature dumb enough to look directly at it. Oerth's two
moons--Luna (a.k.a. Raenei) and Celene (a.k.a. Kule)--shine
clearly by reflected infrared light from Liga, passing through
normal lunar phases. Other planets are much farther away and thus
much fainter, though some have their own heat source, like the air
world, Edill (which has small fire bodies within it). Stars in
Greyspace are actually gigantic gemstones set in the crystal
sphere itself that emit light but no heat, thus being invisible to
infravision.
    Krynnspace, home of the DRAGONLANCE(R) setting, has two fire
bodies: the sun and Sirion, the small innermost planet. The sun,
of course, is staggering bright (just like Liga, above, or our own
Sun). Infravision makes Sirion look like the second-brightest body
in the sky, however, as it gives off so much more heat than any
other planet. Reorx, an earth body that is the second planet out
from the sun, is third brightest; it reflects heat from both the
sun and Sirion, which often appear widely separated when seen from
Krynn. Krynn's three moons also reflect heat from both sources,
becoming the fourth, fifth, and sixth brightest bodies. Outer
planets reflect less light and are thus dimmer in Krynn
"infra-sky." The stars here, as in Greyspace, are invisible to
infravision, emitting only light from the quasi-elemental plane of
Radiance. DMs should read SJR7 Krynnspace for more.
    Finally, from the justly famed surface of Toril (home of the
FORGOTTEN REALMS(R), Kara-Tur, Horde, Maztica, AL-QADIM(R), and
MALATRA(TM) settings), the sun is the dominant object in the
"infra-sky." It is interesting to note that Coliar, the second
planet out from the sun, is almost invisible because it is an air
body; the sun's heat goes right through it. However, because of
the small "islands" of rock circling within Coliar, infravision
might show the planet to actually be a spherical collection of
pinpoints of light, reflecting heat from the sun. Selune (Toril's
great moon) and the Tears of Selune (a cluster of asteroidal
bodies circling Selune's trailing Lagrange point) shine clearly by
reflected heat from the sun, when seen from Toril. Other worlds
appear very faint, with the exception of Anadia, an earth body and
the closest world to the sun, and Garden, which is illuminated by
one of its moons, Sunson, a miniature fire body. Garden's other
moons might also show up clearly in infravision through a
telescope, reflecting Sunson's heat and light. Again, the stars
are infra-invisible. See SJR2 Realmspace for more.
   Given the above settings, you can see that even when the sun
goes down at night, infravision might still reveal a great deal
about the surface world if a heat source is still in the sky.
Garden's Sunson might be bright enough infravisually to cast
shadows at night on Toril, like a full Moon on our Earth. This
would be especially true when Garden is in conjunction with Toril
(i.e., when it is closest to Toril). Sirion would appear from
Krynn to be an extremely bright "morning star" or "evening star"
(in the manner of Mercury or Venus, as seen from Earth), but it
would also reveal the local landscape quite clearly.
   Collisions between celestial bodies will produce brief bursts
of infrared light (heat). As asteroids in the Grinder of Greyspace
or earth bodies within Krynnspace's air-body Zivilyn collide, an
infravision user elsewhere in local wildspace would see a brief
burst of bright light that fades away rapidly.
   If a DM is designing an original campaign world using a
completely different astronomical set-up, he can use logic and the
above examples in designing the "infra-sky" of his world. What
about an air body full of nonluminous but very hot gases? A cold
air world that eclipses the sun, not reducing visible light but
cutting down heat radiation briefly?
   You might also consider creating a large selection of
infravision-using monsters on your world, with behavioral patterns
that vary according to the heat-brightness of the sky. Carnivores,
especially, will be more active when they can see clearly.

The infravisual world: Underground

    Infravision is remarkable enough to surface-dwelling
creatures. Let's look at what it's like for subterranean beings,
and what advantages and disadvantages they gain from it.
    In our real world, deep caverns tend to have a uniformly cool
temperature; Mammoth Cave, in central Kentucky, has an average
temperature of 54 degrees F. that varies only slightly year-round.
This coolness makes everything look the same, bland shade of gray
to an infravision user, but there is plenty of hope here for
diversity. For one thing, large openings radiate only faint heat
(from objects beyond them), so such openings will look dark. More
distant objects radiate less visible heat than closer ones, so
distant objects are dimmer and darker. You could thus pick out the
shape and direction of an unused tunnel with little trouble.
    Running water underground is often extremely cold, so cave
water will seem very black, as will the rocks surrounding it. If a
cavern complex is near a geothermal heat source, like a geyser or
(heavens forbid) volcanic magma, the entire cavern will grow
warmer and "brighter" as an infravision-user gets closer to the
heat source.
    Caves often have a variety of life in them, especially in
fantasy worlds, and living beings will radiate enough heat to
"infra-illuminate" their surroundings. The more beings, the
brighter their living space; a thousand goblins should be able to
see their underground lair quite clearly with no other "light"
than the heat from their own crowded bodies.
    Heat-producing magical creatures, like red dragons, will of
course radiate vast amounts of infrared light. A red dragon would
have an advantage, too, in that one short puff of flame will ruin
the infravision of any approaching creature, with fatal results
for the blinded ambushers. Some cold-blooded creatures like
slithering trackers would be invisible to infravision, again with
fatal results for cocky adventurers. The special dangers of
skeletons, clay golems, and other "heatless" monsters becomes
highly apparent.
    Some undead, however, radiate cold. Liches, for instance,
cause damage from their chilly touch; they and their hands should
"glow black" in infravision, standing out against warmer
backgrounds, even cave walls. Read the descriptions of monsters
carefully if you want to produce a more detailed and intriguing
picture of underworld life to adventuring dwarves, gnomes, and
elves.
    Speaking of fantasy races, a short history of infravision, as
it appears in TSR's fantasy games, is in order.

Infravision and the AD&D game

    References to infravision are scattered throughout the AD&D
and D&D game rules, but it becomes obvious that the concept
underwent much expansion and refinement over the years since
either game first appeared. It would help to start out with a look
at what infravision used to do in fantasy games, and what it does
now--as well as collect the rules on infravision together in one
spot for ease of reference. A few areas of omission and
contradictions that have confused the playing of infravision will
become apparent.
    Certain races in the Chainmail rules (the war-gaming rules
from which role-playing sprang) were able to "see well in dimness
or dark." Dwarves, gnomes, goblins, kobolds, and orcs, as
subterranean races, needed the ability to get around in caves and
mines when candles and oil lanterns weren't available. If you
dumped the infravision concept entirely, this sort of vision could
be either light-intensifying vision, making the most of every
visible-light photon in the area, or a form of magical radar,
allowing for an accurate map of local surroundings without
recognition of color or "flat" things like paintings, handwriting,
etc. It could even be magical vision that makes dark areas seem to
be lit by sourceless light, so there are no shadows (color is
optional). Take your pick.
    Hard on the Chainmail game's heels in 1974 came the D&D
Original Set, those three tan booklets in the white box. There,
the infravision spell first appeared. The original version of
the spell allowed the user to "see infra-red light waves, thus
enabling him to see in total darkness." (Of course, you still
might not see in total darkness if there were no heat sources
around.) The spell lasted for one day and had a range of 40-60
feet. Interestingly, it wasn't until a later D&D supplement
appeared (the Greyhawk book) that dwarves, gnomes, and elves
were noted as having infravision allowing them to see monsters up
to 60 feet away in the dark. (Elves were probably allowed this so
they could see at night, though light-intensifying vision would
have been more logical.)
    The original AD&D game's Player's Handbook and assorted
monster descriptions gave infravision to many creatures, including
every demihuman PC race except certain halflings. Different types
of infravision began to appear, too, defined by range. Poor
infravision was effective only out to 30 feet, and was found in
certain halflings and derro, an evil dwarflike race. Normal or
standard infravision, good out to 60 feet, was the most common
variety. Superior infravision extended out to 90 feet, as was the
case with trolls and troglodytes, or 120 feet, for drow and
duergar (evil dwarves). In one place (page 102), the Player's
Handbook says that monsters living in dungeons have infravision
out to 120 feet; why then do some have shorter ranges? Hmmm.
    Superior infravision, however, involved more than simply
receiving heat radiation. Creatures with long-distance infravision
were noted in the 1st Edition DUNGEON MASTER(R) Guide (page 59)
as emitting infrared light from their eyes (magically, of course),
then seeing the reflected radiation. (This would not be possible
in normal science, as noted earlier, but this is a magical
universe we're talking about.) The eyes of any creature with
infravision out to 90 feet or more are noted as glowing red quite
brightly when seen by any other creature with standard
infravision. Most monsters in underground areas were said to have
superior infravision.
    This brings up a curious point: How far away can an adventurer
with standard infravision detect one with superior infravision?
Can the adventurer see danger coming before the dangerous creature
sees him? Well, if you get picky about it, you can say that the
standard range of 60 feet is fixed; you can't see farther than
that, no matter what heat source is out there. On the other hand,
it is clear that the original intent of the rules was to have the
60 feet range be that at which the body-heat radiation from
monsters (and normal people) could be seen. The implication is
that stronger sources of infrared light could be seen if they were
farther away.
    A liberal DM should note that a creature with 90 feet or 120
feet infravision is actually emitting infrared beams out to 180
feet or 240 feet, respectively. All infrared light going out from
its eyes must be reflected back to its eyes to be seen, so in
theory those eye beams should be detectable by infravision out to
those doubled ranges (assuming those eye beams don't first
encounter a surface that causes them to be reflected).
Furthermore, near-infrared light reflects from most normal
surfaces just like normal light. A monster with superior
infravision "paints" everything it sees with powerful heat rays,
just as if it were carrying a double-beam flashlight. (Perhaps
dwarves and gnomes have appropriate expressions like, "That troll
was so close that its eyes could've burned the skin off my arm!")
    Thus, a gnome wandering an abandoned mine tunnel might see the
corridor ahead of her "light up" with faint infrared light if
there was a duergar 240 feet ahead of her. The duergar has the
advantage in having a much broader range of accurate vision, but
the gnome has the advantage of early detection. The gnome can
immediately flee or hide, unseen by the approaching duergar.
    This argument is buttressed (and contradicted) by the note in
the original DMG (page 59) that, outdoors, infravision allows
for detection of warm or cold figures at a range of 100-300 feet.
Vision is said to otherwise be equal to "a bright, starry night,
with full moonlight." Cannot the duergar then see the gnome at 240
feet? What heat sources are present that allow for this greater
range of vision? And if you can see up to 300 feet outdoors, why
can't you see that far indoors? Game logic breaks down at this
point.
    To the rescue, perhaps, comes the earlier notes about a
sun-warmed landscape and rod-based night vision. As a rule of
thumb, let's say that a creature with infravision can see three
times as far outdoors at night as it can in a deep cavern, because
the landscape is warmer and radiates more infrared light. A
halfling with poor infravision thus can see most outdoor objects
out to 90 feet, and a duergar (with infrared eye beams) can see
out to 360 feet. The gnome in the earlier example should obviously
avoid meeting duergar at night in open fields; the duergar will
see the gnome first.
    In the original PHB (page 102), things seen with infravision
are described as appearing in a colorless way to an observer. Warm
things look bright, as if they were emitting light. Cooler things
look progressively more gray, and cold things appear black. This
fits with the black-and-white view of infravision developed
earlier. Recent versions of the D&D game have instead substituted
certain colors for different heat temperatures (D&D Cyclopedia,
pages 24-25), and there is that nagging PHB note about the
red-glowing eyes of a creature with superior infravision. The
optional rules for infravision in the AD&D 2nd Edition game DMG
(page 119) also allow for "pseudo-color" infravision, as typically
appears in a thermogram. I'll still opt for the simpler no-color
view, which makes it just like the view you get from a
sniperscope.
    Does infravision work underwater? Yes, but badly. Water is a
very poor conductor of heat, despite what any game rules say.
Though the original DMG allowed infravision to work underwater
to a limited extent, but it would be more accurate to cut it off
completely. Cool water will dampen out nearly all heat radiation,
and warm water will obscure it. I'm no scientist, but I'd give
infravision an underwater range of about 1 feet, no more. Very hot
sources, like a volcanic vent, will boil all the water near them
and make an infravisual view of them merely bright, fuzzy blobs
that fill your field of vision. If you are liberal, you can keep
the limits set by the original or AD&D 2nd Edition rules (i.e.,
normal underground ranges).

Getting clever with infravision

    What new tricks can infravision bring to a typical AD&D game?
Here are some possibilities:
    Given that infravision is not as precise and focused as normal
vision, the chances for mistaken identity increase when only
infravision is used. An orc at a distance looks like a human or a
hobgoblin; long experience and closer inspection (at great risk)
will tell the difference. DMs should play up on this at every
opportunity.
    As a rule of thumb, a DM could say that accurate
identification of a creature can be made using infravision only
when the target being is one-third the distance of the spotter's
infravision range. Thus, a dwarf can accurately identify a comrade
at a range of 20 feet (one-third of 60 feet), and a duergar can
identify a fellow monster at a distance of 40 feet.
    Can you read by reflected infrared light, if no normal light
is present? For the record, we will assume not, unless the heat
source is very strong and the writing is only inches from one's
eyes.
    Thieves with infravision can learn to hide themselves from
other creatures with the same power. A very powerful, blinding
source of heat or the presence of many separate, man-sized sources
of heat (like a group of bodies immediately after a battle) can
conceal the thief's presence quite well. However, simply hiding
behind a rock is no help at all, as the thief's own heat radiation
will be seen around the rock's edges and "painted" over background
objects. Wrapping up in a blanket might help at first, but the
blanket will slowly grow warmer (and brighter). Hiding against a
cold object will make the warmer thief stand out as if he were in
a spotlight. If you are playing a thief (as a player or DM),
imagine that character as a permanent, glowing light source. How
can you hide that light? Magical invisibility might be the only
foolproof recourse--but even that can be challenged by creatures
with superb senses of hearing or smell.
    The descriptions of monsters should be carefully examined to
determine if any being might radiate more or less than the "usual"
amount of heat. Considerable leeway is given for the DM here. A
dragon turtle, which breathes steam, and a remorhaz, which is
incredibly hot, are likely to put out enormous amounts of infrared
light. What about a flametongue long sword or a necklace of
missiles? Though it is tempting to rule otherwise, magical items
might not put out any heat at all, no matter what their powers,
unless the description of them in the DMG says they do.
    Consider the infravisual effects of certain spells. Fireball
will produce a burst of infrared light that will temporarily blind
any creature totally dependent on infravision. Incendiary cloud
is easily distinguished from all other cloud-type spells because
the cloud radiates so much heat that it glows brightly in
infravision. Burning hands could briefly illuminate a large area
like a flash bulb, if the infravision users had their backs to the
spell effect so they weren't blinded by it. Dancing lights,
which radiates no heat, could be instantly told apart from real
torchlight by infravision users. Chill touch makes the user's
hand seem black (cold) in infravision. Use your imagination with a
careful touch of logic and reason for other spells.
    Everything that a dwarf knows about infravision is likely
known by a goblin, and vice versa. Creatures who have no
infravision are more likely to fall for certain traps set by those
who can see heat. For instance, a goblin stonework trap that was
recently used or tested will be visible to a dwarf, who can detect
the heat from the friction of stones sliding across each other. An
ambush site will radiate enormous heat from the bodies of the
gathered ambushers, tipping off other experienced dark-dwellers. A
tank of cold water, set over a thin, wooden ceiling, will made the
area around it very dark. A corridor recently hit by a fireball
spell will radiate much heat (and probably smell burnt as well);
the same corridor recently hit by an ice storm will seem very
dark. Fresh blood and body wastes will retain high temperatures
for a short time. You get the idea. Dwarf-kin and goblin-kin love
battling the ignorant armies of surface dwellers who enter their
realms, but hate battling each other, since they already know all
the best tricks.
    Certain "clean-up crew" monsters, like gelatinous cubes, take
on special significance for infravision-users. A 'cube is assumed
here to radiate no heat, and it likely blocks heat transmission as
well. It might become "visible" to a dwarf or goblin because it
cuts off the normally expected scenery down a corridor, as if the
corridor ended abruptly in a cold wall. Humans wouldn't figure it
out, but a clever dark-dweller would stop, probe, then go another
direction.
    Newly discarded items like clothing, armor, and weapons would
reveal much to infravision, like how long they had been abandoned
(depending on how cool the items were) and whether the items had
been used--any warm blood on the blade? A newly set underground
trap, placed by a human who was unaware of his own heat effects,
would be avoided with laughable ease by a hobgoblin or gnome.
    A few new magical spells suggest themselves for dark-dwellers
and wizards. If there can be light and continual light, why
not infrared light and continual infrared light, at the same
levels of ability and with the same restrictions? A pebble with
continual infrared light would make a dandy lantern that no
human could see, though it would immediately give away itself and
its user to any other infravision-using being within range.
    A "light bomb" can be created by enchanting a pebble with
continual light, then coating it with mud. Once dried, the
pebble can be carried in a pouch, emitting no heat at all, until a
group of infravision-using foes is met. The pebble can then be
thrown against a wall as the "bomb"-carrier retreats; the burst of
light will temporarily blind the foes and allow for escape.
Optionally, an adventurer with the blind-fighting proficiency
could close his eyes, throw the pebble (probably by the bunch),
then attack, unaffected by the burst of light.
    A pebble enchanted with continual infrared light could be
used as a signalling device invisible to normal sight. Placed
inside a lantern with a shutter, the pebble's radiance can be
blocked or revealed by opening and closing the shutter. Given a
form of Morse code, underground creatures could signal to each
other, silently and unseen, if surface dwellers approach them.
    (A scary thought: In total darkness, a drow can communicate in
Morse code with another drow 240 feet away merely by blinking her
eyes. Think about it!)
    A pouch full of cold dust would be useful for detecting
approaching foes. When scattered on the ground, the perpetually
low-temperature cold dust would quickly reveal the exact
location of any being walking over it, even if the being were
cold-blooded. (The cold dust would be much colder than the
surrounding environment, providing great contrast.)
    Finally, a game rules variant: sighting ranges for different
sizes of target creatures. This will complicate the game a bit,
but I've tried to keep the basics simple.
    First, find the infravision range of the spotter (30 feet, 60
feet, etc.). Next, find the size category of the target (Tiny,
Small, Man-sized, Large, etc.). Multiple the infravision range by
the sighting range modifier, and that's how far the target must be
before it is normally seen. It's thus harder to spot a rat with
infravision than it is to spot an ogre, and you can see the ogre
coming from farther away.

Target's size   Spotter's sighting
category        range modifier
Tiny            1/3
Small           normal
Man-sized       normal
Large           normal
Huge            4
Gargantuan      10
 

    Using this table, a dwarf can see a hill giant (Huge) coming
from 240 feet away, since the giant is so big and puts out so much
heat. A goblin won't be able to see a rat (Tiny), however, until
the rat is 20 feet away.

Last thoughts

    Infravision is not the only special sense that real-world and
fantasy creatures have. Minotaurs and hell hounds have superb
senses of smell (as do normal canines), bats use ultrasonic sonar,
certain fish sense pressure changes in the water, and electric
eels sense nearby electric fields, such as those from other fish.
A little research and some imagination could bring these other
peculiar senses to life just as this article has hopefully done
for infravision. It's a strange world, and fantasy makes it all
the stranger (and more fun).