From our earliest adventures in AD&D, we've dealt with characters who can see in the

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dark; but rather than pretend that these characters and creatures could see where there is no light, the game suggested that they could see light--electromagnetic radiation--in frequency ranges below or occasionally above that perceived by humans.  Thus we were told that many creatures and most non-human races could see infrared light--radiated heat.  There were even a few creatures who could see ultraviolet light--a less common, less detailed, and less useful skill.  However, the details which were given for either were scanty at best.

  We were told that if a character was standing in a well-lit area, he could not see into the darkness beyond, much as a character trying to look down a dimly-lit hall while standing in a bright ballroom cannot see.  We were told that with infravision, the hotter an object was, the bright it would appear; ultravision was not so well described, other than to suggest that those with ultravision could see as well at night as the rest of us could see by day.

  This was expanded in the Wilderness Survival Guide(tm) to include descriptions of color to temperature relationships for infravision, and weather condition modifiers for ultravision.  A good effort was made to flesh these out.

  I was dissatisfied with these concepts.  It wasn't exactly that I had fleshed out the details of my own concept of these abilities, but rather that some of what was implied was contrary to what I had inferred from the earlier material and my limited understanding of the physiology of vision.  I have since given it some consideration, and would like to offer a few points to, er, illuminate these ideas.

  I would start by examining the way vision works in the real world.  The way we see--and the way animals in our world see--involves aspects of the properties of light, the structure of the eye, and the function of the brain.

  An intriguing book, Voyage to Arcturus, suggested that because the planet on which the action was set had two suns of different colors, there were six primary colors.  This is a mistake.  Primary colors are not a function of the light itself, but of our perception of it.  Light itself is comprised of waves of photons moving at many different frequencies--that is, vibrating rapidly but at different speeds, much like different notes if they were sound.  (For some of you, this is a very basic concept; however, the readers of this page have very differing backgrounds, and I need to lay this foundation for those who do not have it.)  Although to us, these frequencies appear to blend together in a continuum, light produced is invariably at very specific frequencies.  Each element produces bands of light at particular frequencies when heated or otherwise excited to certain temperatures.  We identify the elemental contents of stars by passing the light from the star through a prism (a spectroscope) and matching the color bands to those of known elements.  Although when we view a rainbow it appears to be continuous color, it is actually close bands of different colors.  And that brings us to the structure of our eyes.

  When I was young, I wondered how it was that biologists were so certain that some animals could see colors and others could not.  The answer lies in the structure of the eyes:  some eyes are equipped to perceive different frequencies, and others are structured for light intensity only.  Those which can perceive color are also structured for intensity as a separate function.  The human eye, and other eyes which are capable of perceiving "color", contain two different types of nerve endings, named for their shapes, rods and cones.  Other eyes contain rods only.  The rods are sensitive to light across the entire spectrum of all that we can see:  whenever a photon of any frequency from the darkest red to the faintest violet hits a rod, it sends a signal to the brain, registering it as "bright".  Rods are most sensitive in the middle of the range to which they respond, which is why yellow is the brightest color we perceive, with orange and green less so, and red and blue darker yet.  The cones are actually less sensitive.  Each cone is sensitive to a narrow range of light; they divide into those which are sensitive to blue, those sensitive to green, and those sensitive to red.  These cones will send a signal to the brain only when struck by a photon whose frequency is reasonably close to that which we would regard as the center of each color.  They will respond less well to frequencies near that color, and not at all to other frequencies.  But since the cones respond to a narrower range of light than rods, at low light levels they don't react enough to send a sufficient level of signal to the brain to register color.  Thus in low light we tend to see "black and white"--or at least we fail to perceive color as distinctly as we do in bright light.  And when the light levels drop further, the rods are no longer stimulated, and no signals reach the brain at all.

  Animals who from our perspective see in the "dark" don't do so.  They are capable of seeing at lower light levels.  Generally they accomplish this by admitting more light into the eye.  I am unaware of any animal which can see "heat", light in the infrared frequencies (vipers have a sense of temperature which is directional, but not imaging).  There are animals--insects most notable among them--which can see ultraviolet light; they generally cannot see that which we perceive as red, but they are sensitive to different frequencies of light in the same way as we perceive color.

  But that perception of color is a separate aspect of vision.  It has nothing to do with the nature of light itself, and very little to do with the structure of the eye.  Color is a construct of the brain; it is our mind's way interpreting frequency variation, in the same way the pitch is our mind's way of interpreting the frequency variation of physical vibrations in the air--interpreted by us as "sound".  When red-frequency photons strike red-sensitive cones, our mind creates the color red in the constructed image; and so for green- and blue-frequency photons.  When photons of two different colors come from the same point in the image, our brain creates an intermediate color; when photons between these frequencies hit the eye, they partially stimulate cones of different sensitivities, having the same effect.  Thus our brains invent color as a way of interpreting frequency data.  At the same time, the data gathered by photons across the entire spectrum striking rods in our eyes is mixed into the image and interpreted as brightness in the image.

  We use our computers in much the same way in imaging today.  CT scans, geographic information, infrared interpretation, planetary scans, weather maps, satelite images--repeatedly we convert information into brilliantly diverse color coding to assist in understanding it.  This is nothing new.  Our brains have been doing that with frequencies of light for as long as we can tell.

  But I promised to apply this to infravision and ultravision.  I see two ways in which this can be done.  The first is simpler; but the second is far more interesting.

  The simple way to do this is to widen the frequency response of the rods, but to keep the same cones.  With this conception, color as we understand it would be the same, but the presence of light in the infrared and/or ultraviolet ranges would increase the perception of "brightness" in the image.  To a degree, a hotter object would be brighter than a cooler one at all times; but unless the character is familiar with that object he would not know whether the brightness was from a greater level of reflected visible light or from the presence of heat.  However, as light levels fell, those objects which were radiating heat would continue to do so, and although they would fade as they ceased to reflect light in the "visible" range, they would gradually be perceived as brighter than the surrounding objects due to the heat which stimulated the rods in the eyes.  Eventually, if all reflected light vanished, objects would be brighter or dimmer according to the amount of heat radiated by them.