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Netscape 3 while in a comfortable chair, sipping a piña colada.
The human eye. It can see three colors and thier mixes (the primary colors, red, yellow, blue; or, speaking of light, red, green, blue). You have two eyes, enabling you to see three dimentions (mainly, depth). Staying on Earth, there are other ways to "see," which I will now explore (see Infravision & Your Fantasy Hero, an official TSR updated reprint of Sight in the Darkness, by Roger E. Moore, first published in Dragon Magazine #211).
Bugs have a few hundred lenses. It is still unclear to scientists exactly what they see, but it is believed that it improves their depth perception and decreases over-all sight; insects can see farther with less detail. A bug is attracted to you because it sees the light you reflect and mistakes you for a light source. To see in greater detail, an insect can use its highly sensitive antennae to feel details.
Bats are known for their blindness. It is most likely that most bats can see, just can "see" better using their sonar, therefore sometimes preforming activities with closed eyes. The sonar works by the bat's antennae emitting a specific signal in a direction to see (like right in front of itself) and the signal bounces around, returning information in the same way light would (but a little slower). Bats' vision enables them to see any bumps in an area that they are within range of "seeing."
Dogs, like most animals, are colorblind. They make up for this with an enhanced ability to smell things. For example, the bloodhound is used to track criminals by scent. Some dog breeds can identify specific objects and people by their scent.
Now, we delve into fantasy.
If a monster or character (like thri-kreen of Dark Sun) has insectoid eyes, then they can see movement very easily and can see great distances, but see most images as blurred and use antennae to "see" specific details of individuals. Such a creature also relies on the sense of smell to identify certain things.
Would vampires be able to use a bat's sonar? Only in bat form, and such sonar would not be as powerful and would not relay as detailed information as that of a true bat, since the vampire spends most of his/her time in other forms. (It is a good idea to state that vampires turn into mist instead of a bat -see the entry in the Monstrous Manual/Compendii). Sea elves have sonar. For details on sea elves, see the Complete Elves Handbook and/or the Monstrous Manual/Compendii.
Troglodytes use scents in all sorts of manners. It is a good defense when in combat. It is integrated into their language (see Ecology of the Troglodyte, of Dragon Magazine issue 235). It is used to aid vision. A troglodyte can detect other races, animals, and plants from great distances.
Infravision: There are two official versions of infravision. The first is the easiest; characters can see in the dark. The second is the scientific infravision, where heat patterns can be seen. A more scientific presentation can be found here, in an updated copy of a Dragon Magazine article from 1994. There are many ways in which house rules can mix these two forms.
Khopesh's twist on infravision is the addition of two colors (named whatever) to the standard three primary colors. A character doesn't "fade" into the infrared "spectrum," the infravision ability simply enlarges the "visible spectrum" to include some infrared light. Characters can see heat and its reflections, the new colors, to aid vision in darkened areas. Note that the sources (or reflectors) of this heat is not as clear as a source of light would be, and is therefore a difficult means of reading detail. This vision works as the most common mix of the above two variants used in the TSR novels and Player's Handbook Reference books.
Ultravision: Ah, ultravision. This is the other side of the visible light spectrum; there is infrared, then visible light, then ultraviolet light. Ultraviolet light reveals much more detail than normal vision, and can be considered the opposite of infravision. It allows certain beings to see in starry nights as if it were daylight. Ultravision is also called Starsight, and is used in place of infravision for Cerilian elves.
Khopesh's twist on infravision also works for ultravision. Four new colors (named whatever) enhance able characters in seeing in the dark. By a sky not completely covered by clouds, regardless of whether the moon and stars are out or not, a character with this version of ultravision can see as if it were a clear blue day (to the limit of the vision, usually 60 feet). In darkened areas, excluding the underdark but including the indoors, a character can see as if beneath a starry sky with a full moon. Even in the underdark, certain ultraviolet lights exist (although they are rare), and a character with this ultravision can see slightly better than one with only normal vision. A character with this version of ultravision may have problems seeing at noon on a summer day, especially near the equator (where the sun is strongest), and such conditions are equivalent to an average night for a character with only normal vision. During an eclipse (when the sun is INCREDIBLY strong -blinding to the human eye if stared at directly) or if staring at the sun during the summer, a character with ultravision becomes temporarily blinded (unless wearing sunglasses or simmilar apparel) at the DM's discretion. Characters with this form of ultravision also have better overall sight and can see more details at a larger range, in seven primary colors and their mixes.
X-ray Vision: Superman has X-ray vision. Oooh. Such vision makes little sense. The only way that this could be possible would be if Superman's eyes emitted alpha and gamma rays and he was immune to radiation poisoning. If he were immune to radiation poisoning, then he would be immune to kryptonite (which is radioactive and weakens him because he is SENSITIVE to it. This is the only way that it can harm him at a distance, and it has been stated that kryptonite's radioactivity can kill normal people if they are around it too long. It has also been stated this way in the comics). Superman is not immune to kryptonite, but lets assume that it is a unique form of radiation that penetrates his immunity to radiation(!). He can see with x-ray vision. He can see COLORS with x-ray vision. X-rays cannot detect light and therefore cannot see color. Okay, so lets say that he can't see color, that the comic books use color because it is assumed that one can figure out what is which color. Oh, wait. You can't do that; he frequently reads things through walls. Well, there goes the "natural" ability of x-ray vision! If a race has x-ray vision similar to Superman's, it is purely (or mostly) magical/psionic and should have a duration and very small range. The race might also be immune to the ill effects of radiation and be able to focus the rays used in the "vision" to a fine beam of fire, just like Superman. But why stop there? The race could also be able to fly, be really strong, have bullet-proof skin, be able to hide their identity by wearing glasses and suits, and be able to blow cold air at high pressures. As evidenced, Superman's abilities do have to do with each other, and are hard to separate (well, the flying, strength, constitution/health, and bulletproof skin fit together as a second catagory).
Spectravision (a Khopesh creation): Similar to sonar and Superman's vision, spectravision works by reading signs that are sent out by the character with this ability. Unlike the other visions (or the Khopesh interpetations of them), sonar and spectravision can be used without normal vision. Spectravision is sight by lifeforce; it works EXACTLY like light (colors are regognized and text is legable), except the only source of the spectravision "light" is the character using the ability. The character's lifeforce illuminates areas and the character's eyes see the lifeforce's reflections. Close relatives should be able to see each other's illuminated areas as slightly dimmer than their own, dimming as the relatives are farther from the character on the family tree. Obviously, using this form of sight in conjunction with another is extremely advantageous; it would be like always carrying a powerful lantern that burned a light that was invisible to anybody but you. Since your lifeforce is read by your eyes like light, it can be ignored by closing your eyes. The disadvantage to having this form of sight is that you can never see the sun, moon, stars, smoke signals, distant explosions or eruptions, or anything else outside your range. Fourtuneately, this light is really bright. The average character with spectravision should see 120 feet as if it were perfectly illuminated (with shadows appropriate for the light source, the character) and another 180 feet as shady area. The next hundred feet should be dark but not too dark to see "bright" objects (bright as in very reflective -white objects or mirrors, etc.). Note that fire does not give off light visible to those with spectravision; they see a strange concentration of dancing reflected light, which is recognizable as flame. A flame's color can be determined by approaching it (less than ten feet away is fine) and carefully studying it for 1d3 rounds. The light given off by a fire is not determinable by spectravision, and the only way a character could know (without asking) would be to test the range of the fire's heat.
If you refer to spectravision, please give me credit for being its creator. If you publish a web page reffering to spectravision, please link to this page and state that it is a concept concieved by Adam Katz for Khopesh, Ltd.
Omnivision: Okay, here's the thing you want. Bang! You see everything! Depending on the generosity of the DM, omnivision could mean that you *know* everything in the surrounding area as if seeing it in perfectly lit conditions. This includes figures hiding in shadows (note the perfect in perfectly lit conditions) and colors, text, and runes. Taking this farther, omnivision could allow one to see the six colors added to visible light by infrared and ultraviolet lights, and/or dust particles (therefore being a human barometer and air quality gague). Perhaps omnivision allows x-ray vision, too. Perhaps omnivision allows EVERYTHING to be seen, including the inside of closed boxes, through lead shields, and around corners (if within range). DM's be warned, this is very easy for players to abuse!
The other spectra: There must be a point at which to draw the line. It is a very good idea that the line be drawn here. One step in each direction, plus sonar, is far enough. It is probable that certain ranged magic trigger effects use radio waves, obvious that some spells or magical items use radio and microwave waves (to cook things internally, etc.), and probable that other aspects of these lines are used and therefore should NOT be used for sight. Other means of "seeing" things are explored in Infravision & Your Fantasy Hero, an official TSR updated reprint of Sight in the Darkness, by Roger E. Moore, first published in Dragon Magazine #211).