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Old 10-06-2018, 03:36 PM   #1
Eukie
 
Join Date: Dec 2013
Default Darkness Penalties Under Water [Enhanced Senses-related]

The human eye requires light to see, and in water light quickly attenuates with depth. The visible distance light attenuates in water is typically taken to be 74 metres in blue water oceans and a half to a third that in green coastal water. In blue oceans, blue light attenuates at a much slower rate, with visibility distances of about 275 m. The visibility distances used in this measure corresponds to a 98% decrease in brightness, or approximately -3 in GURPS terms.

With this in mind, and some convenient rounding:

Darkness Penalties Underwater
When underwater and the sky is the strongest light source, increase the penalty for Darkness by -1 for every 25 yards you're from the surface in blue water, or -1 for every 10 yards in green water. Apply an additional -1 for every multiple of the respective distance to the target.

In turbid regions with lots of particulate matter in the water, this penalty can easily double, and in brown water it can often go as high as -1 per yard.

Seeing Blue
In blue water blue light can penetrate far. After the first -4 increase in darkness penalties, blue light becomes dominant: instead of the usual penalty, you may increase the penalty by only -1 for each additional 100 yards. If you do this you're only seeing in blue and are colourblind! (p.B127)

In turbid blue water, increase the penalty to -1 for each 50 yards after the first -5 in penalties. In green or brown water blue light doesn't help at all.

Seeing Ultraviolet
Ultraviolet light penetrates deep in clear water, but there's less of it. You can use ultraviolet vision underwater at -1 per 10 yards in blue water, or -1 per 5 yards in turbid blue water. In green water you can use it at -1 per yard at best!

Seeing Infrared
If you have Near Infrared Vision, you can use it in blue water at -1 per yard, -2 in turbid blue water, -3 per yard in green water, and in brown water you're limited to 1 yard of vision.

Thermal Infrared vision and Terahertz vision are almost completely useless underwater: you basically have to touch what you want to look at with your sensor.

Seeing Clearly
A regular human eye is Farsighted (p. B123) underwater because it can't focus clearly when in contact with water. You can ignore this if you wear diving goggles.

Last edited by Eukie; 10-06-2018 at 05:48 PM.
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Old 10-11-2018, 03:05 AM   #2
Pursuivant
 
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Default Re: Darkness Penalties Under Water [Enhanced Senses-related]

Useful and seems to be accurate.

I'd only add that turbidity in water - even in "blue ocean" if there is enough organic matter - can significantly degrade visibility beyond the limits you suggest.

High levels of turbidity, like those found in muddy rivers or estuaries, can reduce visibility to inches/centimeters, even in good light, and can create near blackout conditions just a few feet/meters below the surface.

In such cases, it's not unreasonable for the GM to set a penalty to vision per X yards/meters of distance, doubling this penalty when attempting to look downward rather than horizontally, and halving it when attempting to look upward (assuming that there is daylight on the surface so objects closer to the surface are silhouetted against the sky).

Darkness on the surface, or material floating on the surface which blocks light (e.g., oil slicks, large ships), gives the usual Darkness penalties in addition to any penalties for depth.
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Old 10-11-2018, 12:41 PM   #3
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Default Re: Darkness Penalties Under Water [Enhanced Senses-related]

Turbidity is very similar to fog and dust in air. I'm not sure it needs special rules beyond noting what lets you mitigate it (not a whole heck of a lot except switching sensory modes, IIRC).
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Old 10-11-2018, 01:43 PM   #4
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Default Re: Darkness Penalties Under Water [Enhanced Senses-related]

As a side point, the standard for visibility is an e^3 (approximately 95%) reduction in contrast, not brightness.
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Old 10-12-2018, 06:38 AM   #5
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Default Re: Darkness Penalties Under Water [Enhanced Senses-related]

Quote:
Originally Posted by Bruno View Post
Turbidity is very similar to fog and dust in air. I'm not sure it needs special rules beyond noting what lets you mitigate it (not a whole heck of a lot except switching sensory modes, IIRC).
Space 162 to the rescue!
Quote:
Originally Posted by Space 162
Some organisms on Earth have
polarized vision, which helps cut glare
and allows them to see better through
clouds and haze. Such creatures typi-
cally have two to four levels of Acute
Vision (Accessibility, Only to compen-
sate for glare and haze, -50%) [1/level].
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Old 10-12-2018, 08:48 AM   #6
ericthered
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Default Re: Darkness Penalties Under Water [Enhanced Senses-related]

Quote:
Originally Posted by Eukie View Post
The human eye requires light to see, and in water light quickly attenuates with depth. The visible distance light attenuates in water is typically taken to be 74 metres in blue water oceans and a half to a third that in green coastal water. In blue oceans, blue light attenuates at a much slower rate, with visibility distances of about 275 m. The visibility distances used in this measure corresponds to a 98% decrease in brightness, or approximately -3 in GURPS terms.

With this in mind, and some convenient rounding:

Darkness Penalties Underwater
When underwater and the sky is the strongest light source, increase the penalty for Darkness by -1 for every 25 yards you're from the surface in blue water, or -1 for every 10 yards in green water. Apply an additional -1 for every multiple of the respective distance to the target.

In turbid regions with lots of particulate matter in the water, this penalty can easily double, and in brown water it can often go as high as -1 per yard.

Seeing Blue
In blue water blue light can penetrate far. After the first -4 increase in darkness penalties, blue light becomes dominant: instead of the usual penalty, you may increase the penalty by only -1 for each additional 100 yards. If you do this you're only seeing in blue and are colourblind! (p.B127)

In turbid blue water, increase the penalty to -1 for each 50 yards after the first -5 in penalties. In green or brown water blue light doesn't help at all.

Seeing Ultraviolet
Ultraviolet light penetrates deep in clear water, but there's less of it. You can use ultraviolet vision underwater at -1 per 10 yards in blue water, or -1 per 5 yards in turbid blue water. In green water you can use it at -1 per yard at best!

Seeing Infrared
If you have Near Infrared Vision, you can use it in blue water at -1 per yard, -2 in turbid blue water, -3 per yard in green water, and in brown water you're limited to 1 yard of vision.

Thermal Infrared vision and Terahertz vision are almost completely useless underwater: you basically have to touch what you want to look at with your sensor.

Seeing Clearly
A regular human eye is Farsighted (p. B123) underwater because it can't focus clearly when in contact with water. You can ignore this if you wear diving goggles.

Ok, this is awesome, and it needs to be archived somewhere more permanent than a forum post.
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Old 10-12-2018, 11:40 AM   #7
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Default Re: Darkness Penalties Under Water [Enhanced Senses-related]

Quote:
Originally Posted by vicky_molokh View Post
Space 162 to the rescue!
Though 2-4 levels is rather generous (I've used polarized lenses plenty; they help but it's more like 1 level), and is not quite the right benefit, as it's mostly related to looking sideways -- it doesn't change the actual opacity of the liquid, it reduces the amount of scattered light that you pick up.
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Old 10-16-2018, 10:29 AM   #8
Eukie
 
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Default Re: Darkness Penalties Under Water [Enhanced Senses-related]

Quote:
Originally Posted by Pursuivant View Post
I'd only add that turbidity in water - even in "blue ocean" if there is enough organic matter - can significantly degrade visibility beyond the limits you suggest.

High levels of turbidity, like those found in muddy rivers or estuaries, can reduce visibility to inches/centimeters, even in good light, and can create near blackout conditions just a few feet/meters below the surface.

In such cases, it's not unreasonable for the GM to set a penalty to vision per X yards/meters of distance, doubling this penalty when attempting to look downward rather than horizontally, and halving it when attempting to look upward (assuming that there is daylight on the surface so objects closer to the surface are silhouetted against the sky).
True. My unstated assumption is that if you have so much turbidity in blue water that the penalties get much higher than the ones suggested, the water is in fact green. For brown water, I didn't intend -1/yard as the maximum: it was simply a guideline I came upon when looking at an average across a variety of lakes.

Quote:
Originally Posted by Anthony View Post
Though 2-4 levels is rather generous (I've used polarized lenses plenty; they help but it's more like 1 level), and is not quite the right benefit, as it's mostly related to looking sideways -- it doesn't change the actual opacity of the liquid, it reduces the amount of scattered light that you pick up.
My naive assumption would be that you filter out half the light, which according to the Size/Range table is worth +2, but darkness penalties are half that, so it's just +1. Thinking a bit more, it's really closer to a 22% reduction in intensity when you filter out one dimension of unpolarized light. Which is at most +1 before halving, and hence not terribly useful.

Where polarization really shines is looking through reflective surfaces and filtering away specular glare, which is a very different topic altogether.

*begins gathering data on specular reflection on different surfaces for a GURPS: Light And Darkness fan-supplement.*
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Old 10-16-2018, 11:23 AM   #9
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Default Re: Darkness Penalties Under Water [Enhanced Senses-related]

Quote:
Originally Posted by Eukie View Post
My naive assumption would be that you filter out half the light
The percentage that gets filtered out depends on how highly polarized the light you're filtering is. It's fairly easy to experiment with this, decent polarized sunglasses are maybe $20.
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Old 10-17-2018, 06:41 AM   #10
Eukie
 
Join Date: Dec 2013
Default Re: Darkness Penalties Under Water [Enhanced Senses-related]

Quote:
Originally Posted by Anthony View Post
The percentage that gets filtered out depends on how highly polarized the light you're filtering is. It's fairly easy to experiment with this, decent polarized sunglasses are maybe $20.
Looking into this a bit more, there's two different sources of polarization you can filter away underwater:
  • The polarization of the incident sunlight
  • The polarization of the scattered sunlight

Filtering away the incident sunlight provides maybe a 30% increase in contrast. Filtering away the polarization of the scattered sunlight is a lot more effective, but works best when you're perpendicular to the sun rays. For instance, if the sun is shining straight down at the water, the light has peak polarization when you're looking horizontally, and no polarization (other than the inherent polarization of the incident sunlight) if you look up or down.

This article suggests up to 80% increased range in detection. This is roughly equivalent to halving the penalty (possibly also granting a +2 offset) in the horizontal plane.

Coincidentally very similar to the bonus Enhanced Senses suggests for using Ultraviolet Vision underwater!
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