Vision - Underwater (OSS)

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Humans, demihumans, and other humanoid species rely heavily on their vision to access information about their environment. For most of these surface dwellers, the teeming depths of an ocean, lake, or river present a visual challenge. Submerged adventurers will not only encounter a variety of exotic (and potentially dangerous) creatures, but will also experience a tremendous alteration in their ability to see.
==Absorption==
Perhaps the greatest factor that limits the underwater visual capabilities of humans and demihumans is a phenomenon known as absorption. When the eye perceives a specific color, such as the green exterior of a cloak, what it really sees is the color green reflected or radiating from the cloak. All of the other colors in the spectrum have been absorbed.
This process of absorption occurs quite dramatically when light passes into water. The colors with the longest wavelengths are quickly absorbed by water. Thus, red light begins to break down soon after it passes into water.
The deeper light travels in water, the greater is the absorption of its constituent colors. For example, red light is almost completely absorbed by water at a depth of about 25 feet, and all of the colors of the spectrum- except blue are absorbed at depths below 100 feet. Thus, even the clearest water appears a deep blue when perceived at this depth.
As one might expect, this process of absorption wreaks havoc with the infravisual capabilities of demihumans (see page 71 for details).
==Visual Distance Underwater==
Lakes, rivers, and seas contain much more than sterile, clear liquid. Rather, it would be more appropriate to categorize such places as thin soups. Large bodies of water serve as homes to millions of different creatures, from the smallest microorganisms to the largest whales. In addition, water carries around particles of silt, floating plankton, and other debris. Such a mixture is hardly conducive to visual clarity.
In fact, all of the particles suspended in water scatter light rays. Such rays bounce from particle to particle until their energy dissipates completely. This scattering process severely limits the available light and, thus, the range of a surface dweller's vision.
The following tables detail the limits of a surface dweller's underwater vision. The tables take into account the quality of water (clear, turbid, or murky) as well as prevailing surface conditions (clear day, overcast, or moonless night, for example).
 Table 39: Clear Water Disibility Ranges
SurfaceCondition	Movement	Spotted	Type	ID 	Detail
Clear Day 	100 	80 	60 	40 	5
Overcast 	50 	40 	30 	20 	5
Stormy 	40 	30 	20 	10 	5
Moonlit Night 	25 	20 	15 	10 	10 feet
Moonless Night 	15 	10 	5 	10 feet	5 feet
Clear Water refers to a body of water that is generally free of heavy silt, algae, or other visual impediments. This is the default classification for saltwater bodies, as their high salinity inhibits the growth of algae. However, DMs are free to rule that strong ocean currents, the presence of kelp or seaweed, or other factors create turbid or murky water conditions.
Depth Modifier: The deeper a character goes, the more the surrounding water scatters and absorbs light filtering down from the surface. For every 50 feet of depth, the DM should move the surface condition down one category. For example, Dagor has just entered the ocean on a Clear Day. Normally, he can spot movement up to 300 feet (100 yards) away. However, the brave wizard decides to investigate a shipwreck 60 feet below him. Because he has moved to a depth of 60 feet, the DM treats Dagor as if he were swimming in clear water on an Overcast day. Thus, the wizard can only spot movement up to 150 feet (50 yards) away.
Once a character moves beyond a depth of 250 feet or suffers a depth modifier that would move him beyond the Moonless Night category, he sees as if he were swimming on a Moonless Night in turbid water. If the character moves down another 50 feet, he is unable to see, as the available light does not penetrate the water in sufficient quantities for his eyes to make use of it.
 Table 40: Turbid Water Disibility Ranges
SurfaceCondition 	Movement	Spotted	Type	ID 	Detail
Clear Day 	80 	60 	40 	20 	5
Overcast 	40 	30 	20 	10 	5
Stormy 	20 	15 	10 	5 	5 feet
Moonlit Night 	15 	10 	5 	5 feet	—
Moonless Night 10	5 	10 feet	— 	— 	
Turbid Water refers to water that contains a moderate amount of silt, algae, swirling sands, seaweed, or other visual impediments. Because of the high growth of algae in freshwater, this is the default classification for all lakes and rivers. However, other factors may lead a DM to classify a freshwater body as murky water.
Depth Modifier: For every 35 feet of depth below the surface, the DM should move the surface condition down one category. Thus, a character adventuring at a depth of 40 feet on an Overcast day in turbid water would actually see as if he were adventuring in Turbid water on a Stormy day.
If a character moves down to a depth beyond 175 feet or suffers a depth modifier that moves him beyond the Moonless Night surface condition, he is unable to see. Light does not penetrate in sufficient quantities for his eyes to make use of it.
 Table 41: Murky Water Visibility Ranges
SurfaceCondition	Movement	Spotted	Type 	ID 	Detail
Clear Day 	20 	15 	10 	5 	5 feet
Overcast 	15 	10 	5 	5 feet	—
Stormy 	10 	5 	5 feet	— 	—
Moonlit Night 	5 	5 feet 	— 	— 	—
Moonless Night 	5 feet 	— 	— 	— 	—
Murky Water refers to water that contains excessive amounts of algae, seaweed, mud, or other visual impediments. This is the default classification of swamps, lagoons, and stagnant bodies of water.
Depth Modifier: For every 10 feet of depth below the surface, the DM should move the surface condition down one category. Thus, a character adventuring 15 feet below the dark surface of a swamp during a Stormy day would actually see as if he were traveling during a Moonlit Night.
If a character moves down beyond a depth of 50 feet or suffers a depth modifier that moves him beyond the Moonless Night surface condition, he is unable to see. Light does not penetrate in sufficient quantities for his eyes to make use of it.
====Notes to Tables 39-41====
Unless otherwise noted, all ranges are in yards. Thus, an adventurer could see 20 yards in murky water on a Clear Day.
Clear Day is the equivalent of bright sunshine and a cloudless sky.
Overcast conditions have heavy cloud cover, light rain, light to medium fog, or any other meteorological phenomenon that blocks sunlight from penetrating the water's surface.
Stormy conditions are the equivalent of heavy rain, heavy fog, gale force winds, blizzards, and other heavy storms. Not only do these dangerous weather conditions block sunlight, but they also stir up sediment and other underwater debris that clouds vision.
Moonlit Night refers to clear nighttime surface conditions that allow moonlight to shine into the water.
Moonless Night refers to cloudy nighttime surface conditions that effectively block all moonlight and starlight from illuminating the water.
Movement is the maximum distance at which characters can spot motion underwater. Note that characters cannot obtain any other information (that is a creature's type or size) at this distance.
Spotted is the maximum distance at which characters can detect the presence of a stationary figure. Again, characters cannot obtain further information about the figure at this distance.
Type is the maximum distance at which characters can note the general classification (fish-like, humanoid, square) of the spotted figure or object.
ID is the maximum distance at which characters can clearly identify the spotted object or figure in question. Thus, at this distance, adventurers can identify the exact race or species of a humanoid figure. In addition, most actions can be clearly seen at this distance.
Detail is the maximum distance at which characters can see actions (including those as surreptitious as pick-pocketing), emotions, and identifiable physical features.
Artificial Light: Characters may use artificial light, including phosphorescent globes, magical weapons, or spells, to improve their vision while underwater. Any artificial light that illuminates with the same intensity as sunlight raises the prevailing surface condition to Clear Day within the light's radius or area of effect, regardless of depth. For example, Cordwain the Faithful, a cleric of Lathander, finds himself adventuring in turbid water on a Moonlit Night. Suddenly, he spots movement 45 feet away at the edge of his vision. Nervously, Cordwain casts a continual light spell. Because the spell gives off light equal to sunlight, the DM now treats Cordwain as if he were in turbid water on a Clear Day (but only within the spell's 60-foot radius). With the aid of his magical light, Cordwain can now identify any objects up to 60 feet away. The lucky priest discovers that the movement came from a small school of tuna.
Any light source that does not equal sunlight in intensity only raises the prevailing surface condition to Overcast within its radius. For example, the light from a phosphorescent globe (roughly equal to torchlight) would move the surface condition from Stormy to Overcast in a 15-foot radius.
Infravision: In campaigns that use the standard infravision rules found in Chapter 13 of the DUNGEON MASTER® Guide, the following rules apply:
Infravision functions underwater, though its range is only one quarter that of normal because of constant underwater currents carrying millions of tiny particles and creatures. The range remains constant regardless of depth. Exactly how much detail the character sees depends on the water conditions as noted on Tables 39-41. Use the Overcast line on each table. For example, if Cordwain had infravision with a 60-foot range, he could see 15 feet underwater. Within that 15-foot radius, Cordwain can see details as though he were swimming on the surface during an Overcast day.
===Optional Infravision===
Infravision suffers even more limitations in campaigns that use the optional infravision rules found in the DUNGEON MASTER® Guide. With this option, characters with infravision perceive objects through the heat (infrared) emitted by the object in question. Unfortunately, water (and any sediments suspended in it) quickly absorbs wavelengths of infrared light. Thus, the following rule applies:
* In clear water, a character's infravision is limited to one half of its normal range.
* In turbid or murky water, infravision is limited to one quarter of its normal range.
* At any depth, a character using infravision can see about as well as a character using normal sight just below the surface on an Overcast day. Infravisual sight, however, is by no means perfect. The scarcity of infrared light and the cold-blooded nature of much underwater life makes the exact identification of objects impossible.
Objects in a character's field of vision tend to blur and overlap. Characters with infravision cannot distinguish the exact number of approaching creatures especially if these creatures swim close together. For example, three sahuagin swimming within five feet of each other would register as one large heat signature to a character with infravision. Even single objects look indistinct. A single sahuagin, for example, looks like a vaguely human-shaped blur.
To reflect blurred vision, shift one column to the left when determining how much detail a character using infravision can see. For example, Cordwain is swimming at a depth of 100 feet in turbid water. Thanks to his infravision (which normally extends 60 feet), he can see 15 feet, but only indistinctly. A companion swimming five feet away appears only as a humanoid shape (Cordwain can identify what type of creature is there, but can see no details). A school of fish hovering 15 feet away is visible to Cordwain's eye, but he has no idea what it really is.
==Dungeon Mastery Tips: Vision==
Since vision is the main way that characters and players perceive their surroundings, smart DMs can use descriptive techniques to "set the mood" and keep players off-balance as they take their characters through a foreign environment.
One way to accomplish this is to emphasize the effects of absorption and refraction on vision. Players must feel that their characters are in an alien place, and what better way to accomplish this than by describing an environment where colors gradually blend into a monochromatic blue?
Some color variations do exist underwater, as various floating organic and mineral particles reflect light. In certain shallow areas-especially along coastlines the decomposition of organic matter tends to give a green tint to the water. Water may also appear brown in color as underwater currents move large amounts of sediment. This brownish coloration has the effect of limiting PC vision (turbid water), and therefore adding to the general unease of players.
Again, DMs should emphasize the strange variations in water color by using evocative phrases and descriptions to convey the experience of traveling in a completely alien environment. One of the best ways to accomplish this is to rent or watch National Geographic videos and other educational programs that focus on underwater life. Through immersion in such visual experiences, a DM can more easily convey the alien "feel" of underwater environments.