Here’s my slightly tweaked take on lighting conditions in 5th edition. This houserule reduces the effects of less-than-optimal lighting, having darkness penalties apply only in pitch black conditions.
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 in a heavily obscured area effectively suffers from the blinded condition (see appendix A).
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. The soft light of twilight and dawn also counts as bright light. A particularly brilliant full moon might bathe the land in bright light.
- Dim light, also called deep shadow, 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. Characters face dim light outdoors on most moonlit nights or indoors when embers in a fireplace or moonlight through a window provide some light to see by.
- Darkness, also called complete darkness or pitch-black, creates a heavily obscured area. Characters face darkness outdoors on a moonless night, within the confines of an unlit dungeon or a subterranean vault, or in an area of magical darkness.
From the Sorcerer's Scroll 