cofty, there might be a benefit for humans also. Humans have created night vision cameras to help them detect heat signatures of humans, nonhuman animals, and heat from human-made sources, in order to see those entities at night. If a human could naturally see heat signatures of dangerous wild animals and of enemy soldiers at night, that could help the human to be safe. Seeing part of the UV spectrum might also help a human.For example help the human to find certain food sources and certain useful minerals which emit UV rays.
After writing the above an internet search and found the following fascinating articles.
https://wonderopolis.org/wonder/Why-Can%E2%80%99t-We-See-Ultraviolet-Light says the following.
"Is UV light invisible to everyone? Actually, no. People with a condition called aphakia can see UV light waves. Those with aphakia are missing an eye lens, often due to surgery or genetics. The lack of this lens enables them to see beyond the visible spectrum of light, but it also causes blurry vision and farsightedness.
Additionally, some animals can see UV light. Scientists have known for a long time that bees have this ability. Many birds and reptiles can, too. Until recently, experts believed that UV light was invisible to all mammals. However, a recent study found that most mammals probably can see these waves of light, including dogs, cats, and reindeer. It found that the lenses in their eyes allow UV light to pass through.
How about the other end of the spectrum of visible light? Red light has the longest wavelength commonly visible to humans. Light with longer wavelengths than red is called infrared. Scientists once believed no one could see infrared light. However, experts today think many humans can do so, especially if more than one infrared photon hits the eye at once.
How do you think the world might look different if you could see UV light? Would you see a deeper shade of violet? Maybe it would be a whole new color altogether!"
See also https://www.bbc.com/future/article/20150727-what-are-the-limits-of-human-vision . Besides mentioning the ability of seeing UV when the lenses are removed it also says the following.
"A study in 2014 pointed out that, in a manner of speaking, we all can see infrared photons, too. If two infrared photons smack into a retinal cell nearly simultaneously, their energy can combine, converting them from an invisible wavelength of, say, 1000 nanometres to a visible 500 nanometres (a cool green to most eyes). "
It also says the following. "Jameson knows what she's talking about, given her work with "tetrachromats", people who possess apparent superhuman vision. These rare individuals, mostly women, have a genetic mutation granting them an extra, fourth cone cell. As a rough approximation based on the number of these extra cones, tetrachromats might see 100 million colours. (People who are colour-blind, or dichromats, have only two cones and see perhaps 10,000 colours.)
... In ideal lab conditions and in places on the retina where rod cells are largely absent, cone cells can be activated when struck by only a handful of photons. Rod cells, though, do even better at picking up whatever ambient light is available. As experiments first conducted in the 1940s show, just one quanta of light can be enough to trigger our awareness. "People can respond to a single photon," says Brian Wandell, professor of psychology and electrical engineering at Stanford. "There is no point in being any more sensitive." What are the limits of your vision? "
Regarding infrared vision see also https://www.sciencedaily.com/releases/2014/12/141201161116.htm .
https://www.newscientist.com/lastword/mg24432591-000-super-seers-why-some-people-can-see-ultraviolet-light/ says the following.
"Richard Swifte Darmstadt, Germany
The human retina is sensitive to the ultraviolet (UV) spectrum down to about 300 nanometres, but the lens of the eye filters it out. This adaptation perhaps arose to protect the retina from the more damaging UV. It also avoids the increased blurry effect of having too wide a spectral range, since different wavelengths focus at different distances from the lens.
Artificial lenses are designed to block UV. But people born without a lens, or who have a lens removed and not replaced, sometimes report seeing ultraviolet as a whitish-violet light. One example is the Impressionist painter Claude Monet, who developed bad cataracts in later life and eventually had his left eye’s lens removed. His subsequent works heavily feature bluish colours, often thought to be the result of him seeing UV.
Brian Horton West Launceston, Tasmania, Australia
Normal colour vision ranges from wavelengths of around 380 nanometres (violet) to 750 nanometres (red). Most people can’t easily see light shorter than 380 nanometres because the lens of the eye absorbs it. If the lens is missing or removed, often due to cataracts, light below the violet range isn’t blocked and can be detected down to around 310 nanometres. Without the lens to focus light, these people are far-sighted and need corrective lenses to focus at short distances.
Insects can see ultraviolet light, and some other animals have vision in this range too.
Bob Butler Llangoed, Anglesey, UK
Some years ago, after being admitted to hospital with sepsis, I developed uveitis, an eye inflammation that could have caused permanent loss of vision. The lens of my right eye was removed and replaced with an artificial one. The new lens meant I could see better through this eye than I ever had before.
On leaving hospital, I decided I deserved a pint of bitter. Standing at the bar of my local pub, I noticed that their device for detecting counterfeit banknotes was emitting very bright bluish light. I mentioned this to the barman, who looked at me with a very quizzical expression but made no comment. I then realised that he couldn’t see the light: it was visible through my right eye alone.
It seems that the natural lens in the eye has a filtering effect as a protection against ultraviolet light. I owe the staff of the emergency eye clinic my thanks not only for saving my eyesight, but also for my ability to see UV light."
Since our bodies evolved to be what they are and since our lenses block UV why do human retinas have the ability to see UV (if the lenses get removed)? Perhaps the UV sensitivity came about as a by-product of having violet light sensitivity, and thus also didn't give removed from the gene pool as a result of non-use. [Natural selection can't directly select for something which is never used, though it can indirectly select for it if the feature is a by-product of something which natural selection selects for.] But after writing the above I found an article which claims we do see some UV after all (even with our lenses intact) [evolution is true], for note what https://publichealth.uga.edu/uga-study-finds-people-can-see-uv-light-opens-questions-about-consequences-for-eye-health/ says. It says the following.
'Yet, new research from the University of Georgia found that people can see ultraviolet light, and the health implications may be significant.'
In a study recently published in PLOS One, co-authors Billy R. Hammond and Lisa Renzi-Hammond show that 100 percent of the participants, all young adults, were able to detect an isolated UV peak at 315 nm.
“Every textbook that is written on vision, optometry, ophthalmology, introduction to psychology, sensation and perception all say the same thing, that humans cannot see ultraviolet light. We have now shown otherwise,” said Renzi-Hammond.
... Renzi-Hammond, who studies the intersection of vision and health at UGA’s College of Public Health, says that the team didn’t set out to rewrite the rules on visible light. However, she continued, knowing that the eye can detect UV presents previously unknown consequences.
“From a health perspective, there’s a risk to the retina,” she said. “If you can see the light, it’s getting back to your retina, and in a way that could potentially be damaging.”