As previously mentioned, infravision is the ability to see into the infrared spectrum.
Thus heat radiation becomes visible and differences in temperature allow infrared sight.
Warm things are bright, cool things grey, very cold things are black.
Most infravision extends to 60' distance.
Dungeon-dwelling monsters have infravision to 120'.
All infravision is spoiled if a light source is shedding illumination upon the creature possessing the infrared sight capability.
Similarly, great heat will spoil the capability.

Thieves hiding in shadows are successful with respect to infravision only if there is a heat/light source nearby to mask their body heat, or a very cold object or radiation to provide similar cover.

DMG: Infravision +

Q. Are piercers, trappers and lurkers 
above detectable by infravision?
A. We think that they are not.  If they 
were, their prey (other than tasty 
adventurers) would spot them in time
to take action, as most creatures that 
live underground have infravision as 
an ability. 
    Furthermore, the descriptions of 
the monsters make it clear that they 
are indistinguishable from their surroundings. 
(Imagine #14)
 
 

Infravision, or “heat vision,” is the ability to see the shapes of
objects by sensing radiation in the infrared spectrum. All nonhuman
player characters have infravision, most of them out to a
range of 60 yards (outdoor scale). Hobbits other than pureblooded
Stouts have an infravisual range of 30 yards; drow,
duergar, and deep gnomes have infravision out to 120 yards. In-
fravision is useless in daylight or when the viewer or the target is
bathed in artificial illumination stronger than moonlight; in such
cases, the normally visible light overrides the heat radiation, and
the former is all that can be sensed.

A character using infravision sees heat as “light”: Very cold
objects are very dark, hot objects are very bright, and temperatures
between the two extremes are seen as varying shades of
gray. The “color” of objects being viewed with infravision is classified
on a scale from 1 to 10, with 1 being pitch black and 10 being excruciatingly bright:
 

"Color"	Temperature
1	Below -50
2	-50 to -11
3	-10 to 15
4	16 to 32
5	33 to 50
6	51 to 70
7	71 to 80
8	91 to 110
9	111 to 130
10	Above 130

The first thing to consider when determining the effectiveness <1st>
of infravision is the temperature of the air. This will dictate the
overall level of brightness that a character using infravision will
perceive. An object or creature with a surface temperature that is
warmer or colder than the air around it will appear as a brighter or
darker image against the backdrop of the atmosphere. Keeping
the phrase “surface temperature” in mind, use the folowing examples
as guidelines for what a character using infravision can or
cannot perceive:

    A human (body temperature approximately 98 degrees) wearing
    nothing thicker than normal clothing will appear brighter than
    his surroundings when the air temperature is 90 or lower. The
    same holds true for demi-humans and most size M humanoids,
    assuming that they have body temperatures roughly the same as
    a human’s.

    A human completely bundled in heavy clothing will have the
    same surface temperature as the surrounding air, and thus will
    not be detectable by infravision. However, if just a small part of
    the body (such as the face or hands) is uncovered or not fully protected,
    those areas will appear as small spots of brightness (or
    darkness, if the air temperature is very high).

In most cases, infravision will not detect the presence of an object
or creature when the line of sight is obstructed; a man
crouched behind a large rock is invisible to infravision, just as he
would be to normal vision. However, an object or creature that
gives off a great amount of heat or cold might be indirectly detectable
because its presence a    ctually changes the temperature of
the surrounding air. A campfire built behind a large rock would
warm the air around it, and the side of the rock facing the viewer
would appear as a dark shape outlined against a lighter background.

As indicated by the above “color table,” extreme heat or extreme
cold will render infravision useless, assuming that everything
in the range of vision is effectively at the same temperature.
In a very cold climate (temperature lower than -50 degrees),
everything will appear pitch black; in avery hot climate (temperature
higher than 130), infravision will detect nothing but a blanket
of almost blinding brightness. In fact, a character using infravision
can be afflicted by “heat blindness” if he suddenly finds himself
in close proximity to a large fire or a source of great heat. If a
wall of fire, for instance, springs up a short distance in front of a
character while he is employing infravision, the flash of brightness
he perceives will “burn out” his infravision and render the
power useless until cure blindness or similar healing magic is applied.
In a less extreme case, the viewer might be only temporarily
“blinded” to infrared rays, perhaps for a period of 5-10 turns
(ld6+4) while his eyes recover from the shock.

lnfravision will not detect the presence of cold-blooded creatures
(reptiles, insects, and most other nonmammalian creatures)
because the temperature of such a creature is, by
definition, the same as that of the surrounding air. However, if
magic is used to abruptly change the temperature of the surrounding
air, a cold- blooded creature will stand out against the
background for a short time thereafter because its body temperature
does not change that quickly.

L^: mu3	-	R#: T	-	D^: 12t + 6t*
C^: v.s.m	-	CT: 1r	-	S^: None
A^: Creature touched	-		-	-

Effect: By means of this spell the magic-user enables the recipient of infravision to see light in the infrared spectrum.

Thus, differences in heat wave radiations can be seen up to 6".
Note that strong sources of infrared radiation (fire, lanterns, torches, etc.)
tend to blind or cast "shadows" just as such light does with respect to normal vision,
so the infravision is affected and does not function efficiently in the presence of such heat sources.
(Invisible creatures are not usually detectable by infravision, as the infrared light waves are affected by invisibility,
just as those of the ultraviolet and normal spectrums are.)

MC: Either a pinch of dried carrot or an agate.