I actually wanted to talk to a biologist about what changes would be plausible for Nostraman eyes. Because I was curious. Thanks to @kyroptera-more-like-chiroptera , whose specialty is evolutionary biology, we ended up with quite a lot of interesting and very plausible brainstorming. I won't reproduce every little detail, since it would get terribly long, but if people want, I can post all the biology details separately.
Below the cut are biological and cultural headcanons for Nostraman vision. Enjoy!
First off, Nostraman eyes being "mostly" pupil with not much iris is actually decently plausible given the environment. It's also likely their eyes are a bit bigger than human standards, due to the need for good vision in the dark. Which would mean calling Jago Sevatarion cute isn't out of the realm of possibility, if you will excuse the self indulgent aside.
Also, vision depends on what are called rods and cones. For better vision in the dark, you need more rods. But that is only the beginning when it comes to interesting deviations from the standard human configuration. UV sensitive photoreceptors are a thing. And they would be very valuable on Nostramo. So it's likely that Nostraman vision has developed along a very different spectrum than the human norm. According to @helloitsmadamehyde , it's actually mentioned in the Night Lords trilogy that Nostramans need to modify their HUDs, since Imperial standard is pretty much useless. Which only makes these biology headcanons more plausible. Everything suggested for how vision works is actually something that occurs somewhere in real life.
This next part is verbatim from @kyroptera-more-like-chiroptera :
Different animals also have different adaptations depending on their environment. One of the important ones is called Critical flicker fusion frequency (CFF) (don't be intimidated by the fancy terminology, it's just the rate at which a flickering light source appears to be a steady, continuous light). so that determines whether you see something as 'flickering' vs it emitting a more steady light. This is affected by a number of physical factors such as stimulus intensity, color, size, contrast and eccentricity, as well as light adaptation conditions and the subject’s age, and when looking at it from an evolutionary perspective you usually have to choose, either be better at say seeing in the dark vs seeing lots of colors, or seeing really good across distances vs depth perception and visual range/acuity.
For example flies and insects seem to be able to 'avoid' us when we swat at them, and that's related to their temporal acuity, meaning their CFF is diffrerent than ours and they see our movements almost like stop-motion frames, making it very easy for them to avoid us.
That might also be relevant to Nostramo. Indeed, this does fit very well with how Sevatar sees others' movements in combat. It is supposedly a result of his repressed psyker powers. But it's equally likely he has an extra mutation (not the Chaos kind, but the kind that naturally occurs in a population. Or even a combination. It fits too well.
I would also like to note that Nostramans likely have what’s known as Tapetum Lucidum, which makes the eyes reflect light, like cat’s eyes. It’s a modification that aids in night vision. Not to mention that it would make running into a squad of Nostraman Night Lords very creepy, since all you would really see is several pairs of glowing eyes. Fits very well with the whole predator vibe they have.
@ladyalisette also raised the possibility of Nostraman vision being at least partly in the infrared spectrum. Which is another interesting angle, though personally I would go with UV being the main spectrum, it offers too many interesting social and cultural possibilities. Besides not everything you would need to see in the dark radiates heat. But arguments are welcome.
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Because of the difference in the wavelengths perceived by Nostramans, it is also possible that if you sent a Terran to Nostramo, they wouldn't be able to see the gang tags and graffiti, because the pigments are tailored to very different eyes.
Cue a brisk black market trade among Night Lords for appropriate pigments that would work as substitutes for Nostraman colors. While people naturally don't have the UV patterns that birds' feathers do, which are invisible to us, but very bright to birds themselves, it is possible that over time, they started occurring in the population of Nostramo, becoming a sort of identity marker. Through selection for such patterns, a Nostraman might be able to tell where you're from, which section of the city, which family you're from (for instance Sevatarions could have specific patterns or pattern combinations), maybe even which gang you're with, assuming gang membership is passed down through families.
Sevatar definitely has patterns common to City's Edge, for instance, in addition to typical Sevatarion markings, not that Sevatarion is a high status family.
But he would look amazing if you could see UV light. Even more than he already does.
To come back to the Nostraman dyes, I think those flayed skins Night Lords so love to wear only look blank to baseline human sight. Using Nostraman dyes, they paint them with oaths of the moment, Nostraman battle chants, traditional patterns and so on.
And this comment from @kyroptera-more-like-chiroptera needs to be quoted in its entirety, too juicy not to.
Also there has to be one NL who's barely competent enough for combat, like every other Legion would have tried to put him on kitchen duty ages ago, but like, secretly he's the best with all those invisible paints and pigments and shit and he *really* has an eye for detail so everyone just shares the burden of pitching in and taking on this guys jobs just so he can keep the supply of paints coming for those NL from nostramo 😂
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In conclusion, this offers some interesting cultural possibilities as a result of biology. The dyes might even explain how you can read anything in a dark place like Nostramo.
