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A group of adventurers enters a dimly lit room. Suddenly monsters lurking in the semi-darkness attack. How do I find out whether the players are surprised?

Dim light in 5E creates a "lightly obscured" area. Lightly obscured areas give you disadvantage on Wisdom (Perception) checks. But if players are not actively searching, they don't do Wisdom (Perception) checks, but rely on their passive perception. If attacked it is Dexterity (Stealth) of the attacker vs. passive Perception of the defender that determines surprise.

So how does dim light play into this? Would you use the same passive Perception to determine surprise and the dim light has no effect? Or do exceptionally the players have to make a active Perception roll, with disadvantage? What about characters with darkvision, who see in dim light as if it was bright light?

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    \$\begingroup\$ You do not have to use the Passive score. You can just ask the players for WIS\Perception checks when their characters enter the room. The concept of a Passive Check is for when players are not active, not necessarily for when characters are not active. \$\endgroup\$
    – Greenstone Walker
    Commented Apr 12, 2017 at 22:06

3 Answers 3

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PHB, page 175:

Passive Checks

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Here's how to determine a character's total for a passive check:

10 + all modifiers that normally apply to the check.

If the character has advantage on the check, add 5. For disadvantage, subtract 5.

Since dim light gives you disadvantage on Wisdom (Perception) checks, a character without darkvision would subtract 5 from their normal passive wisdom perception score.

As for darkvision:

A creature can see in dim light within the radius as if it were bright light.

So they wouldn't have disadvantage and would use their normal passive score.

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I completely agree with @Purple_Monkey.

I just wanted to add two things:

  1. Dim light imposes disadvantage on Wisdom (Perception) checks relying on sight.

  2. Dim light does not affect your hearing. I would allow each character to pit their standard passive Perception score against the Stealth checks of the monsters. Any character that thus hears a monster cannot be surprised by it, but if the character can't see the monster - owing to disadvantage - the usual rules for unseen attackers and targets apply (PHB p. 194).

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  • \$\begingroup\$ Dim light does not affect hearing, true. But will monsters waiting in ambush, not moving, and trying to be stealthy make enough noise so that a party, opening doors and moving in armor, hears them? When Hiding, "you give away your position if you make noise, such as shouting a warning or knocking over a vase." \$\endgroup\$
    – Kirt
    Commented Feb 18 at 18:58
  • \$\begingroup\$ @Kirt, monsters waiting in ambush and trying to be stealthy are Hiding and so they must make a hide check. If the party can detect them with either a passive or active Perception check, then, despite their best efforts the monsters weren't good enough at hiding and the party knows where they are. Maybe the monsters twitched or breathed too heavily or kicked a pebble accidentally. \$\endgroup\$
    – Clearly Toughpick
    Commented May 16 at 18:51
  • \$\begingroup\$ Sure, if you want to have them make a Stealth check, you can. But if the monsters are not moving or attacking, that Stealth check is not made every round. Presumably the monsters who, several rounds ago, made a bad Stealth roll when they first hid and kicked a pebble are not repeatedly kicking the same pebble every round until the party enters the room. \$\endgroup\$
    – Kirt
    Commented May 17 at 6:56
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If characters aren't "actively" looking, then passive perception wins the day, modified by the environment (lightly obscured). It is remembered that hearing is part of passive perception, and sight and hearing are together part of active perception.

How these things work out in mixed parties (darkvision and no darkvision) is always fun.

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  • \$\begingroup\$ That's true, the accepted answer doesn't take that into account. How would you rule this situation then? A player with 15 passive perception enters a room, do you use the 15 or modify it to 10? How do you decide? \$\endgroup\$
    – user77842
    Commented Aug 4, 2022 at 0:29
  • \$\begingroup\$ Characters are assumed to be always actively looking by default, unless they are engaged in another activity that precludes this. See "other activities" in the PHB section on Noticing Threats \$\endgroup\$
    – Kirt
    Commented Feb 18 at 18:54

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