Another "black light" that isn't

[Dave_H]

Do you have one of those plastic UVA/UVC detector cards? They have separate phosphors that only react to their respective UV bands and are useful for determining whether some lights are "NUV" or real UVA as well.

Nothing special at the moment, trying to identify common objects/materials which react
specifically to certain wavelengths.

Dave

 

[JoakimFlorence]

I had noticed some decorative Halloween LED lights that claimed to be "black lights". At first I thought they might be violet emitters (like maybe 405 or 420nm), but on closer inspection I realised it was actually just normal (450nm) blue emitters with a very thin red phosphor covering that made the light color appear purple or "violet".

So this was clearly just for decorative effect, to give the look and appearance of a "black light", but probably without very much real fluorescence.
Although the photons from blue LED emitters are energetic enough to cause fluorescent orange colors to glow.

Actually surprisingly it was very difficult to tell the difference between this light and real violet light from violet LED emitters, just by looking at the light. They both seem the same color and both light sources seem a little bit "out of focus".

I think this approach of using phosphor covered emitters was probably much cheaper than using actual violet LED emitters.
(And it might be possible they worried there could be some potential consumer safety concerns using violet emitters, since this was a directional module, and true violet light can be very strong and energetic but has a low visibility relative to its intensity)

 

[PaladinNO]

*Pats my Fenix LD32 UVC affectionately*

The "blacklight" / UV-market frankly feels like a 50 / 50 scam / dangerous. Actual and proper UVA/UVB/UVC lights are labeled as such, with the appropriate warning labels, whereas cheap "blacklights" have little to no warnings, and oftentimes doesn't even emit any real ultraviolet light to speak of. All you get is a deep blue or violet, maybe dipping below 400 Nm (UVA) if you're lucky.

But I can see how even the fake ones have their uses - it looks cool, is cheap, and carries little to no exposure danger. But the labeling and marketing is mostly just pure nonsense.

Since the manufacturers usually doesn't say (or if they do, they often outright lies), whenever I see purple light nearby, I prefer wearing UV-protective glasses just to be sure. My LD32 UVC is novelty item, but when I use it I wear glasses, gloves and a jacket. I honestly don't know how dangerous the UVC from the LD32 actually is, but I take no chances and I respect it.
...Keeping the light in a black, UV-resistent box with a "hazardous radiation" sign on it is a bit over top, I will admit.

 

[JoakimFlorence]

Since the manufacturers usually doesn't say (or if they do, they often outright lies), whenever I see purple light nearby, I prefer wearing UV-protective glasses just to be sure.

That probably is excessive. The UV from a Wood's glass "black light" tube is almost entirely in the UVA range.
It's probably not a good idea to stare directly at the tubes too long, but otherwise it's nothing to worry about if you are just passing through. If you were going to be around the lighting for more than thirty minutes, some eye protection might not be a bad idea.

How a Wood's glass fluorescent UV lamp ("black light") works is the shorter wavelength UV which is produced by the electric discharge in the mercury vapour excites the phosphor on the interior surface of the tube, which produces a longer wavelength of UV. The Wood's glass filters out most of the visible light, with the exception of a very small amount of violet (up to about 425nm) that makes its way through (mainly this is the mercury line at 404.7nm). The Wood's glass also filters almost all UV below about 360nm. So the UV being given off is almost entirely UVA. Most of the UV from the phosphor is a wavelength centered on about 370nm (mainly around 360 to 380nm). That is closer to what they would call "near visible UV".

UV tubes for skin tanning use a different type of phosphor with a shorter wavelength emission (330 to 370nm). In that case, eye protection is more important. (even though it still falls into the category of "UVA").

My LD32 UVC is novelty item, but when I use it I wear glasses, gloves and a jacket. I honestly don't know how dangerous the UVC from the LD32 actually is, but I take no chances and I respect it.

I would definitely consider some eye protection with those shorter UVC wavelengths. To be fair, the vast percentage of the UVC will be absorbed by the cornea of the eye it will not make it to the retina. But a small percentage could make it through, and this wavelength range is very damaging.
It should also be considered that at lower (visible) lighting levels, the iris of the eye opens more and lets in more light. This can make these lights more hazardous than sunlight, even if the UVC is at the exact same level of intensity.