Roleplaying Games

Illuminating Dungeons: Light and Darkness in RPGs

Last week I started pondering what the inside of a dungeon looks like with only artificial lighting. The way D&D and other F20 games clearly delineate between light and dark, with an area of shadowy light between. I know that Dungeon Fantasy is built on GURPS, so it is able to use finer resolution. In practice it too splits the illumination into light, dark, and semi-dark. I don’t know how the statistics of light sources in the AD&D first edition were derived, but I think they were mostly winged (and set for game balance). From there these statistics have been copied to later editions and F20 games. They don’t really answer the question. If a location is not in complete darkness, the light from light sources will be visible far and allow vision outside of its “visibility radius”.

Data on Human Vision

Human vision can adapt to a very large range of illumination. The full range from darkness to sunlight is 12 magnitudes, that is 1,000,000,000,000. The light energy (photon flux) coming from a light source diminishes according to the square of the distance to the light source, i.e. a light source that is 100 times brighter illuminates 10 farther. The human eye is a pretty sensitive instrument. This means that a candle light is visible on a dark night up to 2.5 km (1.6 miles) away. Of course this doesn’t mean your character is able to use this candlelight for anything else at this distance. In total darkness, an adventuring party approaching with a light source will be spotted long before they will see the other party.

To complicate matters, the light receptors in the human eye are split into two types: one (cones) for color vision in bright light and another (rods) for night vision. The range of the first is called “photopic vision” and the second “scotopic vision”. In between is a range where both cells are active, known as “mesopic vision”. The visual acuity (resolution) of the human eye is much (25 times) worse with scotopic vision.

Unfortunately the light in the photopic range is too bright for the night vision receptors. This means that in the bright light the night vision is overloaded and the eye is not able to adapt to dark conditions quickly. Moving from bright light, it takes 5 minutes for minimal dark adaptation and 45 minutes for full adaptation with the most light sensitivity. Therefore an adventurer standing within the bright light radius of a light source is not able to see as far with their light source as someone outside of it. So even without any “darkvision” or “infrared vision” creatures outside your circle of light will be able to see you better and farther away than you can see them.

How far does the useful range of light extend?

Digging out some experimental data on light sources and human vision I created this table. It shows how far from a light source you have enough light for certain tasks. It also shows how the light compares to natural sources and the limits of human vision ranges. From this data we can learn something new.

The Dark Ages

The first thing we can learn is how dim all of the (portable) light sources available to adventurers in a pre-industrial society are. The light intensity of the brightest example (a torch) is lower than on an “overcast day” within it’s bright light radius (NB! An improperly made torch will burn brighter for a few minutes, but well made torches are dimmer). It will fulfill modern illumination standards for homes and workplaces close to it (within 1 m/3') and dim working areas within double that. It is bright enough to read by (A text written with Times New Roman 16 pts at a distance of 60 cm/2') for 40 m/120'. That distance is close to the limits of mesopic vision (and better resolution).

The Long Shadow

The second notable finding is how much farther from the bright light radius reading is possible. For example with a “standard candle” of light intensity of 1 candela, the bright light radius is 10 m/30'. For the same light source the maximum radius for scotopic vision (where the adventurer will be able to see objects like walls or chasms) is 1000 m/3000'. And as mentioned in the beginning, the same candle is detectable to a radius of 2.5 km (1.6 miles).

How to Use This Data

Some rules of thumb can be derived from the data and the formulae behind it. The bright light radius for a standard candle is about 0.5 m/2'. The limit of dim light is 20 times farther, the maximum night vision range is more than 2000 times the bright light and the spotting distance is more than 4000 times the bright light radius. To compare a light source’s illumination range to the standard candle take the square root of the brightness of the light source in candela and multiply the candle’s range by the result.

A 1 m/3' object is resolvable (discerned from its background or other objects) at over 3000 m/10 000' (3 km/2 miles) in bright light, at 1000 m/3000' (1 km/0.6 miles) in dim light and at 100 m/300' in night vision conditions.

Similarly a 1 cm/0.4" object is resolvable at 30 m/100' in bright light, 10 m/30' in dim light and 1 m/3' in night vision conditions.

Note that full sunlight (real or with a spell) is really bright. Reading is possible at a distance of 1000 m/300' from it, night vision limit is up to the horizon (ca. 5 km/3 miles). In dungeons and other confined spaces a light source this bright will illuminate basically anything with a line of sight to it (and ruin everyone’s night vision).

Non-human Vision

Of course we should remember that not every creature has vision similar to a human. For example geckos can see in color in lighting conditions that would be scotopic for humans. And although humans have a three color vision, most mammals are color blind.

Cat’s Eyes

Cat’s are known for their excellent night vision, and their vision in dim light and darkness (scotopic conditions) is 20 times as sensitive as with humans. This means that the distances above can be multiplied by 4.5. It has to be noted that the acuity (resolution) of cat eyes is only 1/10 of humans. Therefore all of the resolution distances have to be divided by 10.

Best Possible Night Vision

The human eye is able to detect light as dim as 50 photons, so a perfectly sensitive eye would have a sensitivity times 50 the human sensitivity. This perfect night vision can be used as a limit to non-magical night vision in creatures, multiplying the vision ranges above by 7. A more realistic “best” night vision would probably have a multiplier of 5–6.

Dark Vision or Night vision?

For some campaigns the DM can prefer to have light as a resource for all player characters (and most other creatures). In those campaigns, dark vision can be replaced with night vision. Almost everyone living underground would carry at least some sort of light source and dungeons would be at least dimly illuminated. If these data are used for the vision ranges, the radius of dim light and night vision would be a lot longer than in regular campaigns. This leads to new strategies and tactics, as the characters and their opponents have to think about the benefits and drawbacks of their light sources. Relying on someone else’s light allows for stealth and some visual perception. On the other hand, to be able to perform any tasks requiring sharp vision or resolving small objects (reading runes or removing traps) requires bright light. Only creatures with blindsight would ignore lighting completely.

A Hybrid Example

Here’s a small example of real and fantastic light sources based on the data, but using visual ranges similar to D&D (bright light close to mesopic range and dim light double it). It also has some adjusted values on duration based on real world data. Even the fantastic light sources are inspired by real world light sources of later eras (e.g.carbide light) or from the nature (e.g. bioluminescence).