Jon Gedge dot com

Illuminating the Darkness

How to handle light.

RPG Illumination Level Chart

GURPS Bright
D&D Bright
D&D Dim
1 yds
5 ft
Chem Stick/Starlight
2 yds
6 yds
5 ft
20 ft
9 yds
27 yds
30 ft
80 ft
Torch Light
16 yds
50 yds
50 ft
150 ft
Bright as "Day"
This chart will make more sense after you read the article. And if you like this idea, it’s up front and ready for easy reference.

What can you actually see?

The subject of light in RPG's is not something that a lot of rule sets handle very well and this subject causes a lot of questions and concerns at the game table.

So I'll try to address this issue with the hopes of trying to set up some guidelines that will help in the future.

And I'm going to try to use GURPS as a baseline (because it's cogent) while also referencing D&D (because a lot of poor players are stuck playing with these rules).

Because I'm using GURPS, most of my distances that I'll be discussing are going to be in yards. And for those of you not used to freedom fractions, just read “yards” as “meters.” YES, I know there's a difference, but for the purposes of an RPG, the two are similar enough that the difference isn't appreciable.

Let's start with the practical question of how light actually works in real life and start by discussing your eyes.

As odd as it sounds, but your ability to see stuff is entirely dependent on what light that your eye's can discern. (Easy and obvious enough, I hope). However, the ability for your eyes to discern the world around you is entirely dependent on light bouncing off of the objects that are in your environment, and then that light being delivered to your eyes.

For that light to bounce off of the object, before it gets you your eyes, there needs to be a light source that causes that light to travel to the objects around you and then bounce into your ocular cavity.

(Please forgive me for being so elemental in this explanation, but it's come to my attention, that the basic understanding of these fundamental scientific principles isn't actually understood by everyone).

Here's the next principle to understand about light. Light travels in straight lines. Most importantly it travels in straight lines from the light source and then bounces off on an object and then travels in a straight line to your eyes.

It can get a bit more complicated, because light can bounce off of objects and then cause a secondary or tertiary “light source” for an object. However, your eyes capture an image based on the total light available at a given moment. So even if there isn't a direct light source, if light is sufficiently bounced, it's still possible to perceive some things that aren't directly illuminated.

So what exactly IS light? We're not sure, but there are two theories that seem to fit nicely but they fit best when you combine both theories. There's the wave theory and the particle theory. So light seems to be a stream of particles that convey different energy intensities that act like waves. Indeed, you can measure these wavelengths and discern the exact wavelengths of the particles being measured. However, because light travels in straight lines, they don't wrap around obstacles like other energy waves would. So because of that fact, light is considered a particle. We call these wave like particles photons. When photons enter your eyes, your retina measures the energy and then your brain combines all of these impulses to create an image of what you see.

And to become even more unnecessarily complex (and if you don't need this complexity, feel free to skip this paragraph altogether) through the study of what is call Quantum Mechanics, we've discovered that photons don't always travel in straight lines! They *only* travel in straight lines when something with intelligence is present to perceive the photons. Here's an example. If you have a light on, in a windowless room, the photons travel in random and erratic movements. If you put measuring equipment in the room, the equipment will record the photonic movement as being random and indiscernible. But the instant that you look at a camera of the room, those photons start traveling in straight lines (and the recording equipment will discern this change as well. How intelligent one might ask? Put a potted plant in that same room and the photons will travel straight). Is this fact relevant for your RPG? No! You can't perceive this change (because you are a sophont) so this oddity of physics isn't observable by you (or your characters). But it might help explain how a “darkness” spell might be possible . . .

Let's get back to more basic principles; during the day, the primary light source is the local star, a roiling ball of thermonuclear fire that puts out unbelievable amounts of light, that illuminates everything as far as the eye can see. So pretty much everything can be discerned, as long as you have line of sight and it's not too far away for you to discern. (Or it's not too small to notice).

Let me point out that in that kind of illumination, things are so bright, that the iris in your eyes close down to mere pin pricks, and your eyes restrict light exposure to the iris to a minimum. But as role players, we don't worry about those conditions. Those are easy for us to understand and model.

But when the sun goes down, that's when our perception becomes muddled and questionable.

There's also the issue of going underground into the ever present dungeons that make fantasy RPG's so appealing.

What happens then?

Without a light source, your eyes stop receiving photons and with nothing to discern, the eyes just stop working.

So it's time to break out a light source (or go to sleep). Either way, your retina is going to want more photons and your iris will open up, inviting more light to reach your retina.

Going back to GURPS, the GURPS Magic system lays out four different light sources and with that framework, let's start by looking at how much light is provided.

The four levels are, “Starlight,” “Moonlight/candle light,” “torch light,” and “bright as day.” I like these levels, because most RPG characters are going to be able to naturally produce these levels of light either with magic or tools.

I also like the concept that D&D 5E considers as “bright light” and “dim light” from a source. There's some very useful insights in the fact that light sources produce a variable amount of light. I don’t like the WotC assumption that if you simply double the bright light, range and call that “dim light,” that’s sufficient for a realistic approach to how fast photons loose their energy. Ergo my assumption that dim light reaches out double the distance.

Illumination Level 1

Let's start with the concept of starlight. This is, at best a “dim” level of light. Anything producing only this basic (RPG illumination level 1) of light is only going to produce dim light. You won't be able to read by this light (beyond and inch or so) and only vague outlines will be discernible. In GURPS Magic pg 110, this level of light gives a character a -7 to perception rolls. You won't learn much from this level of light and if you're holding such a light source, it will only illuminate things in your hex and the surrounding hexes, in what GURPS describes as a 1 hex radius. And this light will only be dim. While considering RPG level 1 illumination think of things like glow-in-the-dark stickers, glowing watches, chemical glow sticks, and other very dim light sources.

Illumination Level 2

At RPG Illumination level 2 that's the equivalent to decent Moonlight or Candlelight. This is the same as a match stick, a cigarette lighter, and candles. This kind of light will produce decent light out to two yards (20 feet for you D&D players). The photon's don't magically stop at two yards, but the energy that they carry diminishes quite a bit. But you will get to perceive things further out. D&D 5E has the assumption that this dim light is double the bright light distance. However, this dim light radius should be at least three times the bright light area. So one should be able to perceive objects out to 6 yards. So a candle can illuminate a room, but you won't be able to do any reading beyond 2 yards from the candle.

Illumination Level 3

The next RPG Illumination level 3 is described as torch light. I'd also categorize most camp fires in this category as well. Fire being fire, the type of photons generated are the same. If you get to bonfire, levels of fire, then you may want to increase the radius of illumination, but it's not going to be significantly bigger. But I digress. At this level of illumination, a torch, provides illumination out to 9 yards of in radius (beyond the hex occupied by the torch). And yeah, that's 30 feet for the D&D players. Realistically you should triple that radius to 27 yards for dim light. (You poor D&D 5E players will only get 60 feet of light by your hidebound DM's when you should be seeing out to 80 feet).

Illumination Level 4

RPG Illumination level 4 is never going to be true “daylight” but there are some technical light sources that will outshine your basic fire levels of illumination. Imagine for a moment that you're at a well illuminated sports stadium. You should be able to perceive everything at just about the same levels that you would expect to during day light. In reality, the illumination could be as little as half of what regular daylight is, but because your iris can open up and let in more light, it seems the same. Regardless, such a bright light would project bright light out to 16 yards (50 feet for you D&D freaks). Triple that to 48 to 50 yards for dim light.

But wait! There’s more to light!

So those are some basic guidelines for what you can see with the commonly available types of lights available to most adventurers.

But there's more to keep in mind.

Light from light sources is greatly diminished when in bounces off of an object, BUT direct light from that light source is at it's strongest. So, everyone should be able to see any light source if they have direct line of sight. Even that RPG Illumination level 1 light source will be brilliant in contrast to total darkness.

The other thing to keep in mind about these bright levels of light is that everything in that radius will be perceivable by everyone looking at them. People and objects in the dim area, will start to blend with the shadows. Check out GURPS Magic pg 110 for the negatives for using your stealth skill's in these different types of dim light.

Dark vision

Let's discuss Dark vision for a moment.

In some RPG's (including GURPS) some races have something called Dark vision where they can see others without the benefit of light.

Lets keep in mind, that your eyes (even with Dark vision) still operate by detecting photons.

However, we know that some species can perceive into the infrared spectrum (and humans can't perceive infrared). Infrared are low energy photos that emanate from body heat. And this fact can help us understand how dark vision works.

Characters with dark vision are just perceiving photons that humans can’t.

That sucks if you're a human, stumbling about total darkness, and your opponent has dark vision.

However, humans have something that most denizens who rely on dark vision rarely count on; artificial light sources!

Here's the problem with an adventuring party stumbling into a dark cavern and finding a bunch of dark vision dependent creatures. That torch will be blinding, to the dark vision critters.

To see in total darkness, that creatures iris is going to be open all the way up and straining to accept as much low energy infrared photons as possible. So when someone shows up with a torch, those retinas will be overwhelmed, causing temporary blindness (until the iris closes down) and quite possibly inflicting a stun condition. Just remember the last time someone woke you up from a deep sleep during nighttime by turning on the lights. It's not a pleasant experience.

The best way to avoid this calamity is for the other group to have their own light source (campfire, torches, lanterns, etc). And that means that they'll be illuminated by those light sources too.


So now that I've defined some pretty handy ranges for what could be perceived at various distances from the light source, there is the problem of highly reflective objects, like mirrors, shiny gold coins, and polished armor. The issue with surfaces like this is that they tend to return nearly 100% of the light that reaches it. While these kinds of surfaces are relatively rare, we should assume that objects like these should be perceivable at double the normal distance.

Here are some examples.

Example 1

The torch bearer (Illum 3), is going to be clearly visible to everyone in line of sight. While most opponents will be obvious out to 9 yards from the Torch Bearer, the opponents wearing polished metal armor will be obvious out to 18 yards. And with a successful perception roll, those same armored opponents would be detectable (perception skill check) out to 54 yards (instead of 27 yards for the darkly clad opponents).

The torch bearer's thief buddy, who is keeping 12 yards away, has a reasonable chance to remain undetected, until he makes a noise or the light from the torch glints off of his knife blade.

Example 2

The group of player characters have settled down for the evening and the party's wood elf (who needs limited rest and doesn't need to sleep) remains on watch, guarding a camp fire (Illum 3).

However, it's a moonlit night with partly cloudy weather, so the GM determines that the natural light is Illum 1.

During the day, the party's location was alerted to by a goblin scout and his group are sneaking up on the resting party (comprised of a dozen goblins)!

The wood elf, who is abhorrent of the camp fire, is sneaking about the camp site and staying 15 – 20 yards away from the fire and the tents set up by his human companions.

Though the campfire has settled down for the evening, the camp fire is clearly visible to everyone and everything within 9 yards is fairly visible.

The goblins have night vision (at level 5), but not dark vision, but with the low light levels, the goblins need to cluster together so that they don’t lose sight of each other.

To make this already long example shorter, the Wood Elf is going to detect the goblins before they “see” the Wood Elf (Dark Vision vs Night Vision at level 5).

The sleeping members of the adventuring party will be awakened by the universal alarm sound of a goblin screaming (as the Elf perforates it’s vitals with an arrow).

Now things will get interesting.

First everybody roll for initiative.

Since bows make noise, the goblins will be alerted to the general direction of the elf.

Since the wounded goblin is screaming, everyone should know the general direction of the invading force.

The player characters emerging from their tents, will be clearly illuminated. They will certainly make tempting targets for the goblin archers!

But what if they emerge from their tents away from the fire? Remember how light travels in straight lines? The back sides of those tents will only be illuminated by the starlight (Illum level 1). Even our Wood Elf companion could hide in those shadows, even though they are close to the light source.

Depending on how close the goblins are to the camp fire, they may or may not be visible, yet.

What if the group wizard casts a mage light at Illum level 4? That can move at a speed of 5 yards per turn, and if moved toward the goblins, they won’t be hidden for very long! And neither will the Elf! (For most of you D&D players, 5 yards per turn is the standard running speed for most people).

What if the fighters grab torches and spend a turn lighting those, then racing toward the sound of battle? While the fighters will be illuminated, anything within 9 yards will be clearly visible, and there’s a chance that they can detect enemies within 27 yards (with a successful perception skill check).

Conclusion; All done now.

As Game Masters and Players, knowing how low light levels actually work, can help everyone at the table understand their low light environment and to appropriately employ themselves more realistically.

And that also helps the group better appreciate the torch bearers or those mages with the light spells.

We also need to bear in mind that creatures adapted to darkness aren’t super human when it comes to perception. They may be able to navigate in total darkness, but when confronted by a bullseye lantern (Illum 4 in a cone), they’re going to be seriously messed up, even if only temporarily. But hopefully long enough to give the players a decisive advantage.