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.
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