I've read that people who are Aphakia (do not or no longer have a natural lens) may be able to perceive ultraviolet light. A mainstream media writeup here and another here plus a personal experience that sounds familiar to what I'm seeing. An Eye Surgeon recently wrote about blue-violet color changes after Crystalens implants (PDF) and his experience is that only 3% of patients have experienced (or mentioned!) this phenomena ... but some people may just have more sensitive photoreceptors, so the vast majority of the patients would not see this.
The natural lens becomes increasingly yellow with age (which basically provides a Blue-Blocker filter) as seen in the image below. So I enlisted a few kids in the neighborhood to "volunteer" for me, and (compared to adults), they seem to see a bit more color glow on the black light, but nothing close to the huge difference I see with a UV filter or polycarbonate glasses.
Unlike many IOL's, the Crystalens transmits some UV light and I suspect
this may have been intentional by design in order to provide the
clearest possible vision, especially in low-light.
One medical literature review says
the use of blue-blocking IOLs is not detrimental in visual acuity, color perception, and contrast sensitivity but another says blue-blocking IOLs decrease photoreception without providing significant photoprotection ... so even the experts don't agree. My
eye-doctor-wanna-be opinion is Dr. Mainster is probably right - do not blue-block.
Several months after I wrote the above, out came this NY Times piece (related papers here and here) about aging eyes causing health woes.
Here's Wikipedia's writeup on Color Vision that talks about the role of the Short, Medium, and Long cones in color perception. So perhaps a small percentage of people (myself included) have increased sensitivity in the S cones to light around 400nm (or shorter) ... but normally, the natural lens would filter that out - mine is now gone. It's worth noting that the "violet glow" is similar with both eyes. On a related note, I tested my color perception with an online FM100 Hue test and scored an 8 (seems pretty good) with the few errors in the green-blue area. I re-took the test (only takes a few minutes) in early/2018 and got none wrong - perfect score!
I actually noticed this the day after my first Cataract Surgery with my son's Colorado Rockies shorts (see below) and it adds a faint blue/violet sheen to the outdoors - indoors, most UV light is filtered. It does make sunrise/sunsets look even more awesome!
So being the over-analytical engineer (!), I've done various "tests" of my UV vision. It's a little harder now that both eyes have been done (which I'm very thankful for from a visual acuity perspective), since I can not compare/test myself with and without natural lens. I can see much better than others in a dark room lit only with a "black light" ... although ironically, other people can see fluorescence (stuff that glows in the dark) better than me because it stands out more - i.e. the rest of the room is dark for them!
Another test would be using a (scientific grade) prism (or diffraction grating) to split sunlight into different wavelengths and compare what I can see versus other people. Probably the best test would be getting access to Monochromator which generates selective wavelengths and quantify what I can see at the shorter end of the visual spectrum and into the Ultraviolet.
I do see this violet glow in other situations, although not as visible (pun intended) as the Colorado Rockies shorts. Two examples are there is a ever-so-slight blue tinge on asphalt (but not everything per Cyanopsia) there is a slight "white haze" on greenery - both of these seem somewhat similar to pictures I've seen taken with UV capable cameras. Both of these go away when I put a UV filter in front of my eye. And before I had the natural lens removed from my second eye, I could perceive this difference with the already operated-on eye.
A number of people (most who are aphakic - i.e. no natural lens) who have read my writeup and visual examples of seeing ultraviolet light have written to say they have similar ability. Some related interesting tidbits include during WWII, the British used aphakics for signaling using UV lights ... since only they could see it. When the initial telescopic observations of Venus were done, a small number of people said they could "see more" ... which was later confirmed when UV sensitive equipment became available. Walter Scott Houston, a fairly noted amateur astronomer, wrote columns for Sky & Telescope magazine for several decades ... and after his cataract surgery, he was able to see "more" of the night sky with is new Ultraviolet vision. Another example is Claude Monet's painting being influenced by his cataract and subsequent UV vision after removal - PDF.
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That's a good question in terms of the actual mechanism of UV vision ... and I actually have not done any spatial resolution tests which would require an eye test on a chart only seeable in the UV spectrum ... seeing the purple'ish ting on a pair of "black" shorts doesn't count!
Conversely, if a normal sighted person looks at the flames with the UV filter (not the "UV-pass"), they don't see any difference ... but for me, the purplish hue become blue'ish. Finally, when stacking the UV filter to the Wood's Glass, I see the flames become very dim dark purple - probably similar to what the normal sighted person sees with just the Wood's Glass.
2016/02 Update: I get all sorts of Emails from folks who also see UV, and in Feb/2016, HankR wrote me. He had an IOL put in one eye back in 1994 (apparently, UV blocking was much more commonly used after that). He still has his natural lens in the other and writes "Comparing 405nm for example is stunning -- faint violet with natural eye, brilliant purple with white center spot with operated eye (and yes it's scary). Turns out he is a pretty technical dude and bought a 5-pack of some ZWB2 "visible-light filters" (VLF) for UV flashlights (discussed in this forum post and excellent review here) and graciously sent me a couple to test.
I tried my "natural gas test" and compared to the Wood's Glass, the VLF is a slightly paler purple, but the flames are still quite visible to me ... versus hardly at all for a (willing!) teenage volunteer. A more noticeable difference for me was when super-imposing the UV filter. Instead of a very dim dark purple with Wood's Glass, I could barely see the flames with the VLF ... and in fact, had to get much closer to make out an extremely pale grey. The teenagers saw no difference with the UV filter ... as expected. I also tried with my 400 and 365nm flashlights - the VLF knocked down the lights from the 400nm quite a bit more - less of a difference on the 365nm flashlights. So my conclusion was that the VLF is a pretty effective filter and better than Wood's Glass at filtering visible but letting UV pass through. I actually did these tests before I analyzed the spectrum transmittance chart in the posts above ... so it was cool to see that those support my observations.
Because this flower closes up at night, I had to clip one and bring it inside to a darkened room to use my UV flashlight to illuminate it - this was a better way of viewing it. One minor differences I see compared to Dr. Schmitt UV picture is that the dots in the center are glowing purple to me rather than yellow ... but he pointed out that Gazanias come in many variations and their UV patterns vary quite a bit. Also, on the "leaves", I see the white at the tips and the black in the middle ... but I actually see an dark-orangish color bordering the black where the purple is. However, if I use the Wood's Glass, that goes away. This leads me to believe that the flower may be fluorescenc'ing from the UV light into the visible.