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[Note: this question was posted on Sandbox]

What would be differently perceived from a person with broader visual spectrum than normal?

So I've seen this question, especially the accepted answer, and it certainly is not what I'm looking for. The question asks for a man with the eyes of martial eagle. The answer, likewise, considers the acuity of a martial eagle's eyes, or any predatory bird in general. Likewise, most answers account for increased fidelity of the image from those eyes. What I'm looking for is more specific on the effect it has to have a broader visual spectrum.

In my scenario a particular person has a visible light spectrum of ~300nm to ~800nm, and is living his daily life among normal humans. How he acquires the ability is not important, and he has no difficulties dealing with processing the information, etc. This person lives in a normal human city at around 2000s, and does sometimes travel. Due to his acuity of vision happening to overlap with that of a normal person, he can easily distinguish normal human color, and also beyond.

Therefore the scope of this question is what is normally invisible for humans, but visible from the perspective of this person in:

  • normal, daily urban life environment. Especially concerning day-to-day interactions
  • in a medical setting - the person in question is a doctor
  • in a forest
  • when observing stones or geographical formation normally found in tours through areas with a relatively active volcanic, for example Ijen Crater

Here's what I know and some that I don't so far.

  1. Near ultraviolet, that is on the spectrum of around 300nm-400nm is what is normally visible to birds, insects and fish. Some people with a condition known as Aphakia or missing lens, or those who possess artificial lenses can see in this spectrum, and they describe it as whitish blue or violet (I found it here and there; this and this are a few of the cited sources). Still that's as far as I know. Perhaps the most interesting thought I have is that this person can probably see sunblock as a translucent layer over the skin of the wearer, instead of transparent as we see it, but I can't be sure. Also I know that some stones are fluorescent, but most sites (almost all I found) only describe them as minerals that absorb UV lights and emit visible light. I am not interested in this. What I'm interested in is what the stones would look like for this person.

  2. Near infrared, that is on the spectrum of around 750nm-1400nm. But apparently normally used in near infrared spectroscopy is light on the range of 780nm-2500nm. Previously I expected him to be capable of full night vision, but apparently this is not the case. Still, I have no idea on whether or not he would fare better at night than a normal person, or, how much better he is. Also I once found it in the net that a fart is basically hot air, and probably visible at infrared, but I am not sure on what spectrum a fart would be visible.

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    $\begingroup$ Youtube for "full spectrum camera". Example: youtube.com/watch?v=RpYRraBniOA $\endgroup$ – sdfgeoff Jan 15 '18 at 19:41
  • $\begingroup$ @sdfgeoff that's a very helpful link! Anyway after watching the video I realized that it only spans as far as near infrared, but not ultraviolet. Moreover the spectrum is auto-adjusted (if I heard it right) so I assume the color is also adjusted so that our eyes can see it. This still partially answers the forest scenario though, so upvoted. $\endgroup$ – Hendrik Lie Jan 15 '18 at 19:57
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    $\begingroup$ you might want to consider that colour recognition and even perception has a lot to do with training. For example, native Russian speakers are better at distinguishing hues of blue than native English speakers because Russian has distinct words for blue and light blue. Tailors working with business suits can see hundreds of shades of grey, black, and blue. You need to justify why it makes sense for your character to see additional wavelengths. $\endgroup$ – Olga Jan 15 '18 at 20:17
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    $\begingroup$ I believe that if your character had full night vision they would just see the back of their own eyelids whenever they closed their eyes. Should make sleep difficult. $\endgroup$ – Renan Jan 15 '18 at 20:23
  • $\begingroup$ @Olga actually, the character can see additional wavelengths not because he needs to see more, but just because he can due to in-story settings. $\endgroup$ – Hendrik Lie Jan 16 '18 at 7:59
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I have aphakia, so I can address the near-UV end from personal experience. For the near-IR, I can only reason from other sources.

In daylight, subtle effects like the patterns on flowers mentioned in another answer are completely dwarfed by the giant UV source in the sky. Somebody seeing into the near-UV sees sunlight as brighter than others do, to the point of being painful at times. On bright sunny days, or bright winter days with snow cover, I have to wear protective lenses and/or squint to block out a lot of the light. Near-UV vision is a negative in broad daylight.

Near-UV shines, so to speak, in moonlight. Moonlight is just reflected sunlight, but the full moon is only about 1/400,000 the brightness of the sun, so someone who sees into the near-UV can see by moonlight without it being painful. My personal experience is that moonlight makes everything brighter (compared to what I see through glasses) and whiter/bluer. I've never looked for, or noticed, flower patterns or vole urine trails, so I don't know if that's farther into the UV or bees' and predators' eyes are different in this regard.

I would expect a human who can see into the near-UV without the loss of visual acuity that comes from aphakia to see well by moonlight, preferring moonlight to low levels of artificial light. (For example, when walking outside he'd probably prefer not to carry a small lantern.) Light-colored surfaces will be especially visible; he'll probably trip over dark obstacles the same as anybody else.

At the other end of the spectrum, near-IR vision will see thermal signals, the hotter the brighter. This is the principle used by night-vision devices that help military people see opponents. It's not so much an optical "seeing" like near-UV but, rather, heat made visible in near-IR. You'll see a person but not the details of what he's wearing, for instance. (Near-IR vision won't help you see stuff that's the same temperature as its surroundings.) I speculate that a very hot object could "blind" you in the way that bright sun "blinds" you in near-UV, and that near-IR vision will brighten things overall in daylight, particularly on a warm sunny day.

Things like remote controls and IR-illumination LEDs built into security cameras operate in the IR spectrum, so somebody with near-IR vision should be able to see them at least in a dark room. I don't know whether they would be too faint to see in a bright room for someone seeing both normal and IR spectra. I was unable to find anything that addresses how near-IR vision would affect what you see in daylight.

You're extending the range in both directions, so we need to consider how these two modifications would interact with each other. In bright daylight, as best I can tell, neither will confer an advantage and at least near-UV confers some disadvantage. (I think near-IR would confer a disadvantage too.) At night, you should be able to benefit from both; the reflection and fluorescence of near-UV is a completely different process from the thermal properties of near-IR. I was unable to find reliable information about how night-vision devices perform under full moonlight, which would give us some hint about whether they interfere with each other.

People who see in the near-UV and near-IR spectra in addition to normal visible light will probably be super-sensitive to sunlight. I would expect them to either spend less time outdoors or wear eye protection akin to sunglasses or chemically-variable lenses. In darker settings and under moonlight, however, they'll do better than normal humans and wouldn't want any filters in the way. If your humans solve their sensitivity problem with special glasses, therefore, they'll need a way to turn it off. (Sure you can just take the glasses off, but that leaves you with something you need to carry around and protect. Photochromatic lenses appeal to people who don't want to carry around an extra pair of sunglasses; your people will have an analogous problem.)

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  • $\begingroup$ Near-IR includes things like remote controls, IR-illumination LEDs built in security cameras, lidars on autonomous cars - those should become visible. Hot objects - I mean, several hundred degrees, not human body of course - will be perceived as light sources too. Overall experience should be the same - brighter environment during the day, slightly better vision in low-light conditions. $\endgroup$ – ZuOverture Jan 17 '18 at 5:43
  • $\begingroup$ @ZuOverture good point, thanks. I'll update. $\endgroup$ – Monica Cellio Jan 17 '18 at 15:08
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    $\begingroup$ Photochromatic lenses (what I believe you're referring to by "transition lenses") also have another problem, in that they generally aren't made as dark as specialized sunglasses. I'm not sure how much of that is down to physics and optics, and how much of it is down to what people want to get (in other words, market pressure), but it's definitely a difference. $\endgroup$ – a CVn Jan 17 '18 at 15:28
  • $\begingroup$ @MichaelKjörling "photochromatic" -- thanks, that's the word I was trying to find. ("Transition" is a brand name over here.) There are physical limits to how dark they can make them; I've asked to have pairs made darker and been told that what I have is the best they can do. $\endgroup$ – Monica Cellio Jan 17 '18 at 15:34
  • $\begingroup$ I've never expected to obtain a first-hand account of aphakia! Thank you very much, this account is very helpful and probably the most accurate and immersive, because you've actually experienced it! At least for near-UV section. For near infrared however, I'm not sure how many this person could see on shorter wavelengths of near-IR, as usually night vision spans to as far as 1000s nm $\endgroup$ – Hendrik Lie Jan 18 '18 at 16:39
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Color is a sensation; it exists in the mind. It is not a physical quantity; it does not exist in nature. Measurements of color (colorimetry) are made by necessity with respect to a "standard observer", that is, a human with no color perception deficiencies and average color perception. It makes no sense to compare color perception across species.

So the only possible answer is that "a person who can see light from 300nm to 800nm" would be able to see some objects in the dark, and some details in daylight, which are not visible to ordinary people. Nothing more can be said.

  • For example, such a person could see quite well in a photographic darkroom. Darkrooms for working with black-and-white material are commonly illuminated with a deep red light, which appears very faint to ordinary human eyes (and is beyond the range of black-and-white photographic material).

  • For another example, many flowers have patterns which are visible only in near-ultraviolet, thus visible to bees, yet appear to have uniform colors in the ordinary visual spectrum.

Images of a *Mimulus* flower in visible light (left) and ultraviolet light (right) showing a dark *nectar guide* that is visible to bees but not to humans

Images of a Mimulus flower in visible light (left) and ultraviolet light (right) showing a dark nectar guide that is visible to bees but not to humans. Photograph by Plantsurfer available on Wikimedia under the Creative Commons-Share Alike 2.0 license.

Normal visible spectrum covers about one octave, from about 380–390 nm to about 700–750 nm; the lower and upper bounds are fuzzy because the cut-off is not sharp, and in certain conditions light can be seen which wouldn't normally be sensed. So for daylight vision the proposed extension is quite small, about one third of an octave at the short-wavelenth end, and about one sixth of an octave at the long-wavelength end.

Humans have two visual subsystems, one responsible for vision when there is plenty of light ("photopic" vision), and the other for vision when there is little light ("scotopic" vision); only one of them can be active at any given time. While the transition from scotopic vision to photopic vision is quite fast (a few seconds), the transition from photopic vision to scotopic vision takes several minutes. The photopic and scotopic vision subsystems do not cover the same spectrum; in particular, the long-wavelength end for the scotopic visual spectrum is around 630 nm, so a person who can see 800 nm light at night would have a significant advantage.

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  • $\begingroup$ Thanks to point out two subsystems of human vision. Also the question asks exactly what are some objects they can see in the dark and what details can be seen in daylight, assuming that they have the means to describe the color. However this answer deserves an upvote $\endgroup$ – Hendrik Lie Jan 16 '18 at 8:09

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