Most creatures rely on sight for even the most mundane, everyday tasks. Adventurers are no different, and the amount of light in a given area impacts their ability to perform both in and out of combat. Other environmental features (like fog or heavy rain) may obscure a creature’s vision and cause difficulty.
A lightly obscured area is one that creates a minimal, but not insignificant, amount of visual impairment. A moderately wooded area, lingering smoke from a sacked town, dim lighting, patchy fog, and the like can all cause an area to be lightly obscured. When within this area, creatures have disadvantage on Perception checks that rely on sight.
An area that is heavily obscured may be a shoreline cloaked in dense fog, a jungle full of thick foliage and hanging vines, or darkness (magical or otherwise), that obstructs vision completely. A creature in this area is considered blind.
The presence or absence of light in an environment creates three categories of illumination: bright light, dim light, and darkness.
Bright light lets most creatures see normally. Even gloomBright light is the most common. Even on a cloudy day the sun provides bright light, as do torches, bonfires, magical lights, and other similar sources (though often within a limited radius). Most creatures suffer no penalties from being in a brightly lit area, unless their stat block specifies otherwise.
Dim light creates a lightly obscured area. Dim light is typically found on the outer reaches of artificial light sources, such as the depths of a room lit only by candles or the outer ring of a bonfire. Dim light can also be found during dawn or twilight, or beneath a full moon.
Darkness creates a heavily obscured area. Creatures venturing out at night, delving underground, traveling into a windowless dungeon, or subjected to magical darkness without any alternate sources of light will find themselves at a distinct disadvantage.
A number of special forms of vision and non-vision based forms of sensory detection exist with the world of Dungeons & Dragons. The most common ones a character is likely to encounter are listed below.