Infra-red vision vs. dark vision...

[Bruno]

Originally Posted by tanksoldier

An IR sensor where the "third eye" supposedly use to be?

On humans that's on the top of our craniums, so not terribly helpful for upright humanoids unless it's purely to watch out for falling rocks and looming giants/dragons/whatever. Unless you're talking about the "mystical" third eye, instead of the pineal gland and pineal eye (which is a different thing with a totally cool evolutionary history and with actual-third-eye parallel structures in other species).

[Johnny1A.2]

Thanks all!

This more-or-less confirms that I already suspected, that 'night vision' is a more plausible idea for near-human hominids than infra-vision. The SFnal 'Dwarves' I have in mind will have extremely sensitive retinas and eyes adapted to use whatever visible-frequency light is about very efficiently. They can't see in _total_ darkness any more than anybody else, but very small light sources for us will light up the cavern for them. A single candle for one of them would be just as good as a very bright flash-light for a human.

This sensitivity would be a huge drawback in sunlight, but they are not nocturnal. Instead, their pupils can narrow down very quickly, reducing light input, faster than a human eye can adjust to sudden light (which does mean that they will have an advantage over a human going from dark to sudden light in battle).

Also, I assume that they have evolved a reaction whereby bright sunlight (defined in part by the presence of a substantial percentage of UV) triggers a biochemical reaction that also cranks down the light-sensitivity of their eyes, in effect muting their night vision somewhat. This reverses in the absence of UV-rich light.

The upshot is that they are not nocturnal, they can operate freely in bright daylight, but with a slight discomfort in really bright sunlight. It's not penalties, just a Quick-level discomfort in bright, noon sunlight. They're actually more comfortable in gentler light, and often prefer levels of light that humans would find inconveniently dark.

"Dark for dark business."

Note that for someone who understands how their eyes work, intense visible light unaccompanied by UV can be used to cause them considerable pain and discomfort, because the 'step down' reaction doesn't happen.

Additionally, they have very sharp hearing, and are better at determining direction and sorting through echoes in enclosed environments than humans (about a +1 to IQ rolls for that purpose.)

[Bruno]

If you're interested in realism, after "bleaching" their eyes with bright light they will need around 20 minutes to recover their night vision.

Slit pupils can be very useful for getting those tiny pupils quickly, as you don't have to draw it down to a pinprick, you can use those vertically-closing eyelids to help restrict the vertical cross section. The downside to vertically-slit pupils is that they tend to restrict clear vision to a much narrower horizontal plane. That's why you don't see them outside small animals.

An alternative to slit pupils is a semitransparent third eyelid (nictitating membrane, the real type not the GURPS advantage), acting as natural tinted sunglasses with built in UV blocking. Most real third eyelids are translucent at best, but since snakes as an order have an entirely transparent eyelid [1], they don't necessarily have to come with vision penalties.

[1] It's a myth that snakes don't have eyelids, but the eyelid they have is clear, and fused shut over their eyes. It's true that they can't "close" their eyes, but it's because they're already closed all the time.

[Flyndaran]

It's a bit misleading to call the transparent scale over their eyes eyelids. No one believes their eyes lack skin.

And why only small close to the ground usually crepuscular animals have slit pupils is a bit debatable last I heard. Then again, I'm often behind the times in terms of scientific knowledge.
I've heard it helps to reduce chromatic aberrations in color seeing animals that are about in both bright and dim light like house cats, but I don't know the reliability of that "fact".

[Bruno]

Originally Posted by Flyndaran

It's a bit misleading to call the transparent scale over their eyes eyelids. No one believes their eyes lack skin.

The brille is not a scale, it is a fusion of the upper and lower eyelid.

[Anthony]

Originally Posted by Flyndaran

It's a bit misleading to call the transparent scale over their eyes eyelids. No one believes their eyes lack skin.

And why only small close to the ground usually crepuscular animals have slit pupils is a bit debatable last I heard. Then again, I'm often behind the times in terms of scientific knowledge.

There's no real question that a slit lens has better resolution perpendicular to the slit than parallel, that's basic optics.

[Flyndaran]

Originally Posted by Bruno

The brille is not a scale, it is a fusion of the upper and lower eyelid.

Take it up with Wikipedia then. It defines brille as skin or scale in animals that lack eyelids using snakes as an example.

[Flyndaran]

Originally Posted by Anthony

There's no real question that a slit lens has better resolution perpendicular to the slit than parallel, that's basic optics.

Yes, but seeing as how not every animal has them, they can't be exceptionally good, right? Small cats have them, but big cats don't.

[Anthony]

Originally Posted by Flyndaran

Yes, but seeing as how not every animal has them, they can't be exceptionally good, right? Small cats have them, but big cats don't.

They have overall inferior optics. Night vision adaptations aren't free.

[Bruno]

Originally Posted by Flyndaran

Yes, but seeing as how not every animal has them, they can't be exceptionally good, right? Small cats have them, but big cats don't.

That's the point, they aren't "exceptionally good", they're merely "acceptable" in the horizontal plane, and they're substandard in the vertical plane. Cats can't focus clearly on objects too close to them (I think within a foot), have a maximum focal range of about 20 feet, and on top they see everything slightly blurry due to the tapetum lucidum.

Oh, and there's a spectacular blind spot from about their cheakbones down, the pretty much exact region covered by whiskers and sense of smell (no coincidence).

Their vision isn't great; it's optimized for motion sensitivity, and low light sensitivity, not clarity or significant distance.