Vision and Light
The most fundamental tasks of adventuring–noticing danger, finding hidden objects, hitting an enemy in combat, and targeting a spell, to name just a few–rely heavily on a character’s ability to see. Darkness and other effects that obscure vision can prove a significant hindrance.
A given area might be lightly or heavily obscured. In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight.
A heavily obscured area–such as darkness, opaque fog, or dense foliage–blocks vision entirely. A creature effectively suffers from the blinded condition when trying to see something in that area.
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 gloomy days provide bright light, as do torches, lanterns, fires, and other sources of illumination within a specific radius.
Dim light, also called shadows, creates a lightly obscured area. An area of dim light is usually a boundary between a source of bright light, such as a torch, and surrounding darkness. The soft light of twilight and dawn also counts as dim light. A particularly brilliant full moon might bathe the land in dim light.
Darkness creates a heavily obscured area. Characters face darkness outdoors at night (even most moonlit nights), within the confines of an unlit dungeon or a subterranean vault, or in an area of magical darkness.
Blindsight
A creature with blindsight can perceive its surroundings without relying on sight, within a specific radius. Creatures without eyes, such as oozes, and creatures with echolocation or heightened senses, such as bats and true dragons, have this sense.
Darkvision
Many creatures in fantasy gaming worlds, especially those that dwell underground, have darkvision. Within a specified range, a creature with darkvision can see in darkness as if the darkness were dim light, so areas of darkness are only lightly obscured as far as that creature is concerned. However, the creature can’t discern color in darkness, only shades of gray.
Truesight
A creature with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceives the original form of a shapechanger or a creature that is transformed by magic.
Furthermore, the creature can see into the Ethereal Plane.