Have purchased some ebay fake sun glasses. Let’s put aside that I’m a bad man for a minute.

How could I test the UV blocking abilities using just stuff found in my home? Is there a way?

stick them to your arm and go out in the sun.

IIRC it’s quite difficult to make see through plastic for glasses that isn’t reasonably UV protective.

Polycarbonate (which I think is what sunglasses are made from) doesn’t transmit UV, whether you buy genuine or fake ones.

What grum said. UV protection isn’t a special ‘thing’, it’s just an inherent property of most generic plastics.

Which rather begs the question, why are there these scare stories in the press about the risks attached to buying cheap sunglasses which don’t block UV?

Lazy journalism, its easy enough for Oakley to send round a press release saying kittens may be dying needlessly if you buy cheep sunglasses, it gets published as fact,

A bit like the ‘look whats new’ sections of most mags, the journos often dont even have the product yet.

If you actually want to test it- do you own an LED light? They throw tons of UV. Find a big roadsign, shine light at it to see what happens- modern roadsigns are UV-reactive which is why you can see them light up even when your light isn’t lighting up anything else near them. THen, shine through the glasses, and see what happens. Science!

But don’t bother really, they’ll be uv opaque. UV exposure’s a legitimate concern for some sports- mostly snow and water, where you’ve got no cover and lots of reflection. But if these are for mountain biking in the UK, fahgeddabadit, there isn’t any sunshine and it’s all blocked by the trees anyway. You’ll get more uv exposure just going about your daily business than you will on the bike, generally.

modern roadsigns are UV-reactive which is why you can see them light up even when your light isn’t lighting up anything else near them.

lol, no, that would be because they are highly reflective. 🙂

Find a big roadsign, shine light at it to see what happens- modern roadsigns are UV-reactive which is why you can see them light up even when your light isn’t lighting up anything else near them.

No. Retro-reflective. They consist of tens of millions of microscopic glass spheres bonded to a reflective aluminium backing with a clear laminated coating. They reflect light from different angles because most light sources have a light spread beyond the bright centre cone, and because they’re spheres, or lenses, the signs will reflect back a significant proportion of the light falling on them.
Try shining a laser on a reflective sign; it won’t shine until the laser beam is pointing directly at the sign, because it’s a coherent light source with no scatter or diffusion.
UV has nothing to do with it; try getting a true UV light source, it might reflect back from a road sign, but you’ll barely see it because the light is almost invisible to the human eye.

Reflectivity is light reflected from a source to a surface and returned to its original source. For traffic signs and vehicle operators, the light source is a vehicle’s headlights, where the light is sent to the traffic sign face and then returned to the vehicle operator. Traffic signs are manufactured with retroreflective sheeting so that the traffic sign is visible at night. Reflective sign faces are manufactured with glass beads or prismatic reflectors imbedded in the sheeting so that the face reflects light, therefore making the sign appear more bright and visible to the vehicle operator. According to the National Highway Traffic Safety Administration (NHTSA)[where?], the Traffic Safety Facts 2000 publication states the fatal crash rate is 3-4 times more likely during nighttime crashes then daytime incidents.

A misconception many people have is that retroreflectivity is only important during night-time travel, however, in recent years, more states and agencies are requiring headlights to be used during inclement weather, such as rain and snow. According to the Federal Highway Administration (FHWA)[who?]: Approximately 24% of all vehicle accidents occur during adverse weather (rain, sleet, snow and fog). Rain conditions account for 47% of weather related accidents. These statistics are based on 14 year averages from 1995 to 2008.

what cheshire said, its an innate property of polycarbonate, nothing to do with special coatings or giving polycarbonate a fancy trade name.

Hmm, I got that straight from a mate of mine who makes them. Suppose he could be wrong… But how can it just be retroreflectives, if it’s reacting to nonvisible light? (you get the same with HIDs, the retrofit in my motorbike used to make the bloomin things glow with its sidescatter)

Hmm, I got that straight from a mate of mine who makes them. Suppose he could be wrong… But how can it just be retroreflectives, if it’s reacting to nonvisible light? (you get the same with HIDs, the retrofit in my motorbike used to make the bloomin things glow with its sidescatter)

We’re talking about two different things here; polycarbonate plastics block UV light, which is invisible, but allow through visible light, and infra-red, for that matter. Retro-reflective materials reflect back any and all light that shines on it, but the UV/IR components are totally irrelevant because the human eye cannot perceive them. LED lights do give out a lot of UV, but it’s invisible, the visible white light component is very intense, though, which is why reflective materials reflect back so intensely, as do HID Xenon headlights. Conventional incandescent headlights are much yellower and dimmer, so won’t reflect back such an intense amount of light from a road sign.
Simple optics, really, the brighter and whiter the light source, the brighter and whiter the light reflected back towards the viewer, and the brighter the outer ‘cone’ surrounding the centre hot-spot, the brighter the light that will reflect back although the main light is pointed away from the sign. That’s how come a roadsign works even with dipped headlights.

Yep, but what I’m talking about here is an effect where there’s no/practically no visible light- you could look right at the headlight from these angles and it was no brighter than, say, looking at the wheel. But a sign on the same angles would be as bright as the sun.

(that’s how the subject came up with us, I noticed it when I was setting up the headlight and said “isn’t that weird”, he said “Nah, the text on the sign is reacting to the UV, it’s supposed to do that”)

Hmmm, odd, that. I’m stumped, tbh. It’s the first I’ve heard that signs react to UV. It’s entirely possible that newer coatings on the reflective sheet have a fluorescent component built in, in order to show up more clearly with new headlights having a greater UV content, like all-LED ones on Audi and now even some of the cheaper Seat Leon cars.
And I’ve seen that some manufacturers have been experimenting with UV supplementary lights, along with IR, and special cabin equipment, to improve poor visibility vision.
Interesting, I might do a bit of digging around, see if I can find anything about this; it’s the kind of techy stuff that fascinates me, has no practical use, but I’m interested anyway. 😀
Geek, ain’t I?

“Nah, the text on the sign is reacting to the UV, it’s supposed to do that”

Sounds like your mate is talking about fluorescence, in this case the sign material (many signs have fluorescent properties these days) absorbs non-visible radiation (UV) and then transmits it as visible radiation (light). Same as why a yellow fluoro hi-viz vest seems to ‘glow’ yellow in daylight but indoors or under normal car headlights/street lights at night (when there is pretty much zero UV about) it’s just regular yellow.

If modern bulbs are emitting UV, it’s invisible to the eye, and won’t light anything up, but will be reflected as visible light by the fluorescent material.

Aye, exactly that.

LEDs are a little bit like lasers in that they produce a narrow wavelength of light. They won’t produce much UV since that is the harder end of the spectrum to produce. We had red LEDs way before blue and white ones.

It is to do with the energy between the band gaps. The blue end is large so requires more energy to excite the electrons. The energy is around the point when bonds start breaking so if you are not careful the thing breaks down. Did some work with UV LEDs and they are still relatively new tech.

Bank notes are tested with a UV light in shops, try taking the glasses to a shop and putting a note behind the lens whilst holding it under the light to see if it still glows.