Distance to see light in total blackness

[malachi]

1. How far away would a 30' radius of dim light need to be before you had trouble seeing it, assuming that otherwise you're in total darkness (such as in an underground cavern)?
2. How far away would a 30' radius of dim light need to be before you couldn't make it out at all, assuming that you're in total darkness?
3. Same as (1), but with 30' radius of bright light.
4. Same as (2), but with 30' radius of bright light.

I'm starting my own game, and the first session takes place in an underground cavern, where there are some patches of dim light for the PCs to find, but also a chance for them to be detected by their bright light (the Light cantrip and/or torches).

(Additionally, I houseruled Darkvision into Lowlight Vision, where they ignore the penalties of dim light on perception checks in X feet, rather than actually being able to see in the dark, for ALL races, including NPCs)

[Townopolis]

My understanding of how this situation would realistically work is:

Approximately 3 miles.

If any part of the light radius is outside total cover, you can see it. So, outside something getting in the way, you can see light until it is obscured by the curvature of the planet. On Earth, the horizon is ~3 miles from 6' person.

The thing that normally obscures lights from us is other light. So, if your otherwise in complete darkness, any light is going to be super visible for a super long distance. This is why you can see cities from space and also how lighthouses work.

If you're in total darkness, dim and bright lights will be (sort of) equally visible, with bright lights just being more attention-getting. Any of these lights could occlude other lights in their radius, or behind them, though.

[malachi]

There's a point at which things become so small that you couldn't make them out though, right? Otherwise, we'd be able to see every star in the sky at night without a telescope.

[Lord Vukodlak]

Miles, think about it. How far away are headlights visible on a car? Or the running lights on an airplane.

Without any obscuring objects or other distracting light sources you can safely say they can see it.


There's a point at which things become so small that you couldn't make them out though, right? Otherwise, we'd be able to see every star in the sky at night without a telescope.
Yes, but really think about how far away stars are. And one reason we can’t see many of them their light if drowned out by other stars.

Using stars as a reference point some scientists calculated you could see a candle flame 1.6 miles away.
Assuming no obscuring objects or distracting light sources.

[Foolwise]

There's a point at which things become so small that you couldn't make them out though, right? Otherwise, we'd be able to see every star in the sky at night without a telescope.

I think of it as having more to do with how our eyes are built wrt their cones and rods. Our eyes take a long time to adjust to night vision. An eagle can locate a mouse on the ground from a mile up or so.
It could probably see more stars than us even if it suffers from the same poor night vision (?). But a cat, for example, can see in the dark 6x greater than a human. It's thanks to their eyes being far more light receptive than ours. The night sky is likelier far more populated with stars to their eyes than it is to ours.

Taking that real world example into D&D, I would rule that darkvision races reflect light through their eyes the same way a cat does. Night watches would get interesting. Passive perception checks to catch the pair(s) of eyes staring into camp. Sentries facing away from the campfire to keep their own eyes dim. Seeing random eyes darting away as the party explores a dark dungeon.

[Contrast]

(Additionally, I houseruled Darkvision into Lowlight Vision, where they ignore the penalties of dim light on perception checks in X feet, rather than actually being able to see in the dark, for ALL races, including NPCs)

RAW - I don't think there's any guidance. Personally I'd have a discussion with your group and come up with an answer that feels right to your table - the important thing is that the PCs are adventurers who would know how light works in their world. Your PCs should be making a choice on how much light knowing it lights them up rather than it being surprising to them that someone could spot them from a mile away.

That said, there are quite a few nuances here.

A creature in total darkness, paying attention, will be able to see a light source from quite far away. How noticable it is is perhaps another question and would depend on the scale of the light source.

A creature in the bubble of a bright light source is probably going to have a very difficult time spotting that candle even a relative short distance away because the contrast will make the candle just vanish into the dark.

There's also a question about what is causing the light - a directional light source (hooded lantern, bullseye lantern) will be much more difficult to spot when the light is directed away from the observer even though in D&D terms there wouldn't be any difference there. Think a modern day torch pointed at your feet versus an illuminated lightbulb.

[Xetheral]

There's a point at which things become so small that you couldn't make them out though, right? Otherwise, we'd be able to see every star in the sky at night without a telescope.

Yes, there is an angular size visibility limit, but you're not going to reach it in a cavern. The humidity level in the cavern is going to matter much more, with more-humid air making it harder to see small light sources. Also, the amount of particulate matter suspended in the air could make a large difference.

The same is true with stars, where the limitations on which ones can be seen from the surface of the earth depends more on Earth's atmosphere than on the distance to (and size of) each star. Many more stars can be seen from orbit (if shrouded from sunlight and reflected sunlight) despite being only trivially closer to those stars than one would be on Earth's surface.

On the plus side, the atmospheric conditions of your cavern are up to you and don't need to be explained in detail to your players. This means that you have a lot of flexibility in deciding how far away you want light sources to be visible while still making a plausible choice.

[Keravath]

1. How far away would a 30' radius of dim light need to be before you had trouble seeing it, assuming that otherwise you're in total darkness (such as in an underground cavern)?
2. How far away would a 30' radius of dim light need to be before you couldn't make it out at all, assuming that you're in total darkness?
3. Same as (1), but with 30' radius of bright light.
4. Same as (2), but with 30' radius of bright light.

I'm starting my own game, and the first session takes place in an underground cavern, where there are some patches of dim light for the PCs to find, but also a chance for them to be detected by their bright light (the Light cantrip and/or torches).

(Additionally, I houseruled Darkvision into Lowlight Vision, where they ignore the penalties of dim light on perception checks in X feet, rather than actually being able to see in the dark, for ALL races, including NPCs)

Two comments.

1) Apparently an average human should be able to see a candle at about 1.6 miles.

https://physicsworld.com/a/how-far-away-can-you-see-light-from-a-candle/

"Furthermore, the farthest from which an average unaided human could see a candle is about 1.6 miles."

In D&D terms:

"Candle. For 1 hour, a candle sheds bright light in a 5-foot radius and dim light for an additional 5 feet."

So whatever light source is causing your 30' areas of bright or dim light should be visible over much larger distances than a candle since the source has to be brighter to emit 30' of dim light compared to the 5' from a candle. Unless your cavern is utterly enormous - more than 10 miles - any 30' light source is likely to be noticeable in the pitch darkness.

2) If you have ruled darkvision to be low light vision in your games then there are a couple of other rules I am curious how you dealt with ...

a) Gloomstalker ranger - one of their key abilities is to be invisible in darkness where they can see creatures but the creatures can't see them. From what I can see, your house rule removes this ability entirely.

b) Have you changed how the Warlock invocation Devils Sight works? It allows characters to see perfectly in complete darkness. Did you remove it from the game? Otherwise, warlocks become superpowered in any form of darkness since they are the only ones who can see without a light source ... excellent scouts among other advantages.

Also, sneaking up on opponents through the dark becomes a strategy since the opponents will have to have a light source and so will be visible. If you can get into position without being noticed you can bombard them from the darkness and they won't be able to effectively fight back unless they can somehow light you up.

[Mellack]

The limiting factor will be the shape of the caverns. In total darkness, even a single candle is visible from more than a mile. You will need obstructions to block the light.

[Townopolis]

1) Apparently an average human should be able to see a candle at about 1.6 miles.

https://physicsworld.com/a/how-far-away-can-you-see-light-from-a-candle/

"Furthermore, the farthest from which an average unaided human could see a candle is about 1.6 miles."

In D&D terms:

"Candle. For 1 hour, a candle sheds bright light in a 5-foot radius and dim light for an additional 5 feet."


Nice. This is actually super useful if you want to extrapolate to more than just a 30' radius. Based on this, I might say:

5' radius bright: 1.5 miles.
10' radius bright: 2 miles
15' radius bright: 2.5 miles
20' radius bright: 3 miles, beyond which you're dealing with planetary curvature.

And dim light may be visible at half that distance.

Keep in mind, though, that I am not a physicist, mathematician, optometrist, or anyone else who has any sort of expertise in how vision works. Except for the distances pulled from sources, these are all just some amateur's "good enough for a game" reckonings.

[JackPhoenix]

But a cat, for example, can see in the dark 6x greater than a human. It's thanks to their eyes being far more light receptive than ours. The night sky is likelier far more populated with stars to their eyes than it is to ours.

Kinda off-topic, but no. Cats see well in the dark, but they have very poor long-distance vision. To a cat, the night sky is propably just a blur.

[stoutstien]

1. How far away would a 30' radius of dim light need to be before you had trouble seeing it, assuming that otherwise you're in total darkness (such as in an underground cavern)?
2. How far away would a 30' radius of dim light need to be before you couldn't make it out at all, assuming that you're in total darkness?
3. Same as (1), but with 30' radius of bright light.
4. Same as (2), but with 30' radius of bright light.

I'm starting my own game, and the first session takes place in an underground cavern, where there are some patches of dim light for the PCs to find, but also a chance for them to be detected by their bright light (the Light cantrip and/or torches).

(Additionally, I houseruled Darkvision into Lowlight Vision, where they ignore the penalties of dim light on perception checks in X feet, rather than actually being able to see in the dark, for ALL races, including NPCs)

All n' all the rules are silent on stuff like this so it's up to the DM to decide how the logic works in a given setting. At face value there isn't any max vision limit to see light in 5e. the closest they give you is in pg ~240 of the DMG talking about visibility over large distances and environmental factors. They give a rough starting point of 2 miles on a clear day or until view is blocked.

For your specific scenario it actually comes down to how you're going to decide the material the caverns are made up of and the general layout. Anyone spending anytime spelunking in caves will tell you some caverns eat light while others seem to allow a simple pen light to be seen for days. Water/moisture is probably the biggest factor but any highly reflective surface would be something to consider. A lot of twists and turns combined with lateral movement can also be a large factor. Distance should be drawn in lines from surface to surface so you could have something putting off light relatively close but nearly invisible just due to a hairpin tunnel and a small drop.

Sound on the other hand would be much harder to control even of pin pointing the original location is difficult. Still dense cold in a naturally acoustic enhancing environment is perfect for sound to be carried far. Though it could be deceiving, one could be clever enough to map out parts of the cave with sound.

[malachi]

Two comments.

1) Apparently an average human should be able to see a candle at about 1.6 miles.

https://physicsworld.com/a/how-far-away-can-you-see-light-from-a-candle/

"Furthermore, the farthest from which an average unaided human could see a candle is about 1.6 miles."

In D&D terms:

"Candle. For 1 hour, a candle sheds bright light in a 5-foot radius and dim light for an additional 5 feet."

So whatever light source is causing your 30' areas of bright or dim light should be visible over much larger distances than a candle since the source has to be brighter to emit 30' of dim light compared to the 5' from a candle. Unless your cavern is utterly enormous - more than 10 miles - any 30' light source is likely to be noticeable in the pitch darkness.


Ok, from this and other notes, looks like I just need to create more terrain blockers and ambient patches of bioluminescent material in order to keep the civilized portion from being spottable right from the start of the cavern. Thanks guys!



2) If you have ruled darkvision to be low light vision in your games then there are a couple of other rules I am curious how you dealt with ...

a) Gloomstalker ranger - one of their key abilities is to be invisible in darkness where they can see creatures but the creatures can't see them. From what I can see, your house rule removes this ability entirely.

That would be an issue for me, and I don't know how I would rule it. Thankfully, my players are all new, and it was simple to limit things to just the PHB.



b) Have you changed how the Warlock invocation Devils Sight works? It allows characters to see perfectly in complete darkness. Did you remove it from the game? Otherwise, warlocks become superpowered in any form of darkness since they are the only ones who can see without a light source ... excellent scouts among other advantages.

Also, sneaking up on opponents through the dark becomes a strategy since the opponents will have to have a light source and so will be visible. If you can get into position without being noticed you can bombard them from the darkness and they won't be able to effectively fight back unless they can somehow light you up.

I was definitely leaning toward having the invocation work as written, given that it already penetrates magical darkness. If I have an warlock NPCs (probably won't, since I'm a fan of non-symmetric things) then that would be a huge advantage if they took it. It'll make devils (and things with blindsight / etc) a scary thing, since they can actually see in total darkness.
My PCs didn't make a warlock, finding them a bit too complicated to build (even though I was trying to make it simple, so I could compare how SR / LR casters work).


Kinda off-topic, but no. Cats see well in the dark, but they have very poor long-distance vision. To a cat, the night sky is propably just a blur.

Huh, that's interesting. They also have very poor vision close to their face, despite needing to bite things up close. They just have weird eyes.

[Dork_Forge]

Huh, that's interesting. They also have very poor vision close to their face, despite needing to bite things up close. They just have weird eyes.

This is because they don't have the muscle structure to change the shape of their lenses to focus up close like a human can.

They are midsighted, being able to see things very sharply at about 20 feet, whereas human vision is sharpest at 100-200 feet.

What you have to remember about cats (and a lot of other animals) is that they interact with the world far differently then we do, they don't need to see up close to bite: They acquire a target, pounce and then adjust as needed up close with their whiskers. Their smell and hearing is also far better than our own.

A good case in point about this kind of thing: We have an African Pygmy Hedgehog and one of our greatest fears is that she'll just be walking around one day with less eyes. They're spikey creatures and, well pretty careless. However every time on a forum I've read about it happening the hog was basically unhindered, they rely on their nose, ears, and whiskers more than their eyes so it's barely a loss for them.

More in line with the OP:

I'd just use that candle at 1.6 miles guidance, but be very explicit that you're just seeing a pinprick of light, the PCs don't get to see anything else.

[malachi]

This is because they don't have the muscle structure to change the shape of their lenses to focus up close like a human can.

They are midsighted, being able to see things very sharply at about 20 feet, whereas human vision is sharpest at 100-200 feet.

What you have to remember about cats (and a lot of other animals) is that they interact with the world far differently then we do, they don't need to see up close to bite: They acquire a target, pounce and then adjust as needed up close with their whiskers. Their smell and hearing is also far better than our own.

A good case in point about this kind of thing: We have an African Pygmy Hedgehog and one of our greatest fears is that she'll just be walking around one day with less eyes. They're spikey creatures and, well pretty careless. However every time on a forum I've read about it happening the hog was basically unhindered, they rely on their nose, ears, and whiskers more than their eyes so it's barely a loss for them.


The more you know!



More in line with the OP:

I'd just use that candle at 1.6 miles guidance, but be very explicit that you're just seeing a pinprick of light, the PCs don't get to see anything else.

Yeah, that's my plan now, where the PCs will eventually have a chance to see some sources of light that are occasionally being blocked by people moving around.

[Carlobrand]

1. How far away would a 30' radius of dim light need to be before you had trouble seeing it, assuming that otherwise you're in total darkness (such as in an underground cavern)? ...


My understanding of how this situation would realistically work is:

Approximately 3 miles. ...

That sounds about right to see the light source itself. That doesn't mean you see what's illuminated by the light. Human eyes are really very good at detecting light, which is why the night sky is so lovely on a moonless night, but we do need something to have a big enough visual angle and enough light on it in order to be able to discriminate it as anything more than some distant tiny light blob. Put another way, if you take a ruler and hold it at arm's length away from your body, the thickness of those little up-and-down lines denoting the fractions of an inch - not the distance between them, but the thickness of the little lines themselves - are very roughly equivalent to about a ten-foot distance at three miles; you can extrapolate for yourself what you can see at that distance from that.