There are a number of discrepancies regarding non-standard vision, due either to ignorance or misinterpretation, in the D&D system. Infravision, Ultravision, and True-Seeing are prime examples. These three forms of non-standard sight are defined below, as well as several other, Gaeleth-specific forms of sight.
Visible light (the colors red, orange, yellow, green, blue, indigo, and violet) is just a small part of a much larger spectrum of light. If the human eye were just a bit more sensitive, it could perceive the infrared light from television remote controls, or the ultraviolet light from the sun. Ultraviolet light, like visible light, has more than one component. Visible light has reds and blues and other hues, while ultraviolet light's hues are referred to as letters (ultraviolet-A, and ultraviolet-B). Infrared has hues, as well. One of infrared's hues is what most people know as 'heat'. Heat is just another form of light.
While humans see such a small portion of the spectrum, our technology is allowing us to see more and more of the spectrum. X-rays are commonly used to see through the human body, and image broken bones, cancers, and other maladies. Microwaves allow us to use radar, cook meals, and transmit information. Infrared goggles allow hunters to see their prey at night, and military operatives to spot warm vehicles. In AD&D, certain creatures and spells can see more than just the visible spectrum, replacing technology with magic.
Infravision's Relative Spectrum
Elves, half-elves, dwarves, and other races can see infrared light, as well as visible light. This ability is referred to as 'infravision'. They can see a world completely alien to most humans, and so interpretations invariably err. Infravision among the inhuman races is limited in range, and therefore limited in resolution. At roughly sixty yards, the smallest heated object those with infravision can make out, is man-sized (five to six feet tall). At closer distances, infravision allows those races more detailed vision, up to the size of a rat, or there-abouts, at half of the sixty yard range. Even closer up, they can see the softer heat-trails of foot-prints, hand-prints, discarded clothing, used weapons, and the like.
For ease of comparison to human vision, it might be best to look at infravision from the point of human vision. A near-sighted individual can only make out objects that are closer, and a far-sight individual can only make out objects that are further away -- unless those objects are very large, or very bright. There's no reason an elf with 6" infravision (sixty yards outdoors, sixty feet indoors) could not see a giant lurking out at 12". On the other hand, the same elf might not see a dozing snake (cold-blooded, and therefore only slightly warmer than background) until it were too late.
Bright heat sources, of course, would overwhelm infravision, just as shining a Q-beam of one million candle power in a human's eyes would blind him. Infravision is thus useless with candles within 5’ of the observer, torches within 15’, small fires within 35’, and other sources as the DM determines.
Infravision is of so much less resolution than visible light, that most creatures' eyes will switch to visible light, no matter how dim, if any visible light is available. There are exceptions, of course, depending on the creature, and on the light source.
Ultravision's Relative Spectrum
Ultravision simply means that a creature can see ultraviolet light. Ordinarily, this would be of little value to an individual; however, it is thought that certain magical energies 'leak' ultraviolet light. It would be understandable, then, that evolution or spells that brought about ultravision would be useful.
Passive ultravision merely allows a creature to see ultraviolet light as it normally appears. Stars, and even the sun, give off ultraviolet light. Broad daylight may or may not be bright enough to overwhelm a creature's ultravision capabilities. Bees and spiders are not blinded in the daylight by their ultravision; however, at night, their ultravision is useless. A creature with very sensitive UV capabilities might be able to see by starlight, but it would be blinded in daylight. Passive ultravision would also allow a creature to see magical items and spells or enchantments, at certain ranges, under certain conditions -- daylight might overwhelm a magical item's aura. Also, not all enchantments or spells or magical items might give off ultraviolet light.
It is also conceivable, that just as humans see red, orange, yellow, and other colors in visible light, a creature might see other 'colors' in ultraviolet light, with just as varied and bright a contrast as visible colors. This might explain why some creatures can detect certain 'flavors', or spheres or schools of spells in effect, and avoid or prey upon them.
Active ultravision is a bit different than passive ultravision; it means that a creature's eyes are generating ultraviolet light. This allows the creature to see in the dark, much as a human with a flashlight would. Since ultraviolet light is not a spectrum that humans can see, active ultravision gives creatures uncanny vision capabilities, with humans or elves or dwarves none the wiser for it. A human would never know he is being 'painted' by active ultravision, unless he were paying very close attention to his clothes or articles. Ultraviolet light causes certain items, such as white cotton, to fluoresce a light violet color, in the visible spectrum. Active ultravision also means that the creature's eyes glow throughout the spectrum, from infrared to visible to ultraviolet; the glow is a byproduct of the energy required to generate ultraviolet light.
Supravision's Relative Spectrum
Supravision is the ability to see throughout a large range of the electromagnetic spectrum. It has all the advantages of infravision and passive ultravision, plus a few others. Beings with supravision could even see radar (which is often microwave radiation). Though they can almost see X-rays, they do not have 'X-ray vision'. The greatest advantage to supravision is that a creature with it cannot be blinded, except by nuclear detonations and similar powerful effects, because their eyes would switch over to a different part of the spectrum. Creatures with supravision can also see the different flavors of magic, as well as heat sources, and are completely at home in broad daylight, at night, or in total (relative) darkness.
One drawback to supravision is that it can cause 'information overload', supplying a creature's brain with too much information at once. The brain automatically filters out extraneous visual information -- therefore, creatures with supravision are not 'all-seeing', though they are far more difficult to surprise than most other creatures.
There are two different forms of true-sight; sorcerous, and deitific. Both give nearly the same information or results, though they come from different sources.
Sorcerous true-sight comes from supravision capabilities. Usually brought about by a spell, such true-sight allows the spell recipient to see a great deal, at once. The ultravision component of the true-sight allows the spell recipient to see magical effects, as well, cutting through a great deal of magical darkness, illusions, and the like. Range is usually sixty feet.
Deitific true-sight does not come from a spell's magical effects, per se. The caster's deity, or a secretary to the deity, such as a solar, archon, or planetar, divulges the information to the spell recipient. Just as with sorcerous true-sight, secret doors can be seen, misplaced or displaced objects become apparent, and invisible creatures can be seen. Unlike sorcerous true-sight, deitific true-sight lets the spell recipient know what the alignment of individuals is. Range is usually one-hundred twenty feet.
Creatures that are said to have 'true-sight' usually have supravision, although some such creatures may simply have a close contact with their deities.
Nightvision allows a creature to see at night, even with very low light levels. Such a creature is still blind if there is no light, as within most cave systems; however, if there is even the smallest bit of light, the creature can see very well. Nightvision does not confer the ability to see colors; creatures instead see very detailed shades of gray. Dim light, such as a candle, can temporarily blind the creature, and render the nightvision unusable (candles within 5’, torches within 15’, small fires within 35’) until the light is extinguished. Creatures with nightvision usually have photo-reflective eyes (i.e. their eyes usually reflect green light, when viewed from the proper angle). Most animals that hunt in the darkness have nightvision capabilities.