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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 Apr 12 '17 at 22:06
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PHB, page 175:

Passive Checks

...

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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