Dark Days in Sion
|Low Light Vision|
Characters with low-light vision have eyes that are so sensitive to light that they can see twice as far as normal in dim light. Thus, if a group of adventurers passes down a dark passage with a torch illuminating a 20-foot radius, an elf with low-light vision can see everything within 40 feet of the torch. Low-light vision is color vision. A spellcaster with low-light vision can read a scroll as long as even the tiniest candle flame is next to her as a source of light. Characters with low-light vision can see outdoors on a moonlit night as well as they can during the day.
|-Dungeon Masters Guide|
Darkvision is the extraordinary ability to see with no light source at all, out to a range specified for the creature. Darkvision is black and white only (colors cannot be discerned). It does not allow characters to see anything that they could not see otherwise—invisible objects are still invisible, and illusions are still visible as what they seem to be. Likewise, darkvision subjects a creature to gaze attacks normally. The presence of light does not spoil darkvision. If a character has darkvision with a 60-foot range, and he stands within a 20-foot radius of light, the character can see normally in the light, and 40 feet beyond the light because of his darkvision.
|-Dungeon Masters Guide|
That’s the standard definition from the DMG, but it leaves a lot to interpretation. How can eyes, see anything if there is no light, especially if all you can see is black and white, and not some kind of infrared or thermal vision?
My idea about darkvision is that its magical, and it works because a creature with darkvision project an invisible magical spectrum of light, out of its eyes, out to whatever range that it can see with its darkvision. Within that range it can see with perfect clarity in shades of black and white only. It can read, it can recognize faces, etc.
Darkvision even works in the presence of light. A being with darkvision can hold a candle and seamlessly see color within the bubble of light from the candle and then black-and-white beyond the range of the candle up to its range of darkvision. Beyond that range… if there was say another candle 100ft away, the creature with darkvision would pick that tiny glow up with its usual ability to see normal light beyond the range of its darkvision.
Darkvision is not something that can be turned off at will, consciously or not, it is constantly on as long as that creature or being has their eyes open.
Infravision is in between low light and darkvision and something else entirely. It is a sort of hyperspectral thermal infrared, to use the parlance of our times… infravision allows a creature to receive the full spectrum of light in through its eyes. In conditions of no light, it can see the heat radiated by something hiding behind a rock for example… and in conditions of normal light, it can still pick out the undead vampire in a crowd of people because it emits no body heat. If a mage cast Mirror Image, a being with infravision would have no trouble determining which image was the real one.
A being with infravision who also has the craft skill can use it to their advantage as a metalworker, gaining a +2 to craft checks whenever they use heat to forge and/or shape metal.
That’s not to say there aren’t drawbacks to infravision. A creature with infravision is no less able to stare directly at the sun for example, and it would even have difficulty seeing someone standing in front of the sun. In conditions of mixed light infravision is not as bad as some interpretations made it seem back in the 2nd edition days of yore…
A creature with infravision is a highly calibrated visual instrument. It can see a wide range of temperatures simultaneously. So, for example a being with infravision sitting before a campfire can still easily spot a warm blooded creature like a rat scurrying through camp, as long as the campfire wasn’t between its line of sight with the rat. An infravision user wouldn’t have any bonuses spotting a cold blooded snake slithering through camp though.
In a cave underground, or on the frozen tundra without the moon or stars above, it might seem like everything is the same temperature so where is the frame of reference for infravision? Everything must look like a blank blue screen to a creature with infravision. Well, that is simply not true. They can also emit light with their eyes, similar to the magical light someone with darkvision does, except theirs is purely in the infrared spectrum.
In that spectrum they can see as clearly as if they were looking through an infrared camera, (a sort of dulish washed out color, yet still high in contrast and sharpness) up to a certain range, and cannot be seen doing so except by other creatures with infravision. This is known as ‘Projecting Infravision’, and it can be turned on or off at will.
Creatures with infravision however are capable of seeing the magical light creatures with darkvision use to see. That light is very faint though, so if for example a person with darkvision was holding a candle, the creature with infravision might not notice the magical projected darkvision because the normal light of the candle was so bright by comparison.
Creatures with infravision cannot use their ‘projected infravision’ to see clearly beyond the brighter visual light of a torch or even a candle, but they can still see things less clearly with their receptive infravision past the range of light offered by the torch or candle. And if they are using their ‘projected infravision’ to see things up close more clearly they can still use their ‘receptive infravision’ to see things past the range of their projected infravision.