Thinking about blacklights

[lonesome_dave]

With Halloween approaching in the U.S. (and a few other places) I'm noticing the blacklights popping up here and there for spooky night lighting effects. Got me thinking about my early experiences with blacklights and how they helped spark my interest in science.

When I was a young man many moons ago everyone had to have a fluorescent blacklight in their room to light up those great posters and paraphernalia. Even without any fluorescent items, the deep violet glow (mostly the visible 405nm line) of the tube was magical. I had a lot of fun experimenting with materials looking for fluorescence and phosphorescence.

 

Spooky Action at a Distance
If I can steal that famous remark here I want to point out the first thing that fascinated me about blacklights. When viewed from a distance in the dark, a well-filtered blacklight creates the weird effect of the eyes not being able to focus on the light. If the light is rather dim it may disappear completely when you look right at it and then reappear when you look to either side of it like a chimera or ghost. This effect is only observed on blacklights that are well-filtered so only the deepest violet passes through the glass.

 

My understanding of this effect is that the central focussing area of the eye (fovea) only contains cones that are not sensitive to deep violet while the rods in the periphery can detect that wavelength. So, the deep violet only appears as a spooky aura around the light particularly when it is dim. (Someone else here on UVP has a better explanation for this effect I'm sure).

 

 

 

Back in the day the standard blacklight was the 40W 48-inch tube that would light up your whole room. The 20W and 15W versions could also be obtained at most hardware stores. These days it can be hard to find the actual glass fluorescent tubes as they are being replaced by LED strips inside a plastic tube. Most of the plastic blacklight tubes I've come across use strips of 395nm LEDs which don't put out much UV below 380nm. The original glass fluorescent tubes provided lots of UVA. The few LED strips I've seen that claim to use 365nm LEDs don't bother to filter out the stray visible light (some of which is caused by the clear plastic envelope fluorescing).

 

The compact fluorescent (CFL) blacklights seem to have a poorer filter glass that lets considerable visible light through, destroying the eerie glow effect of the better filtered tubes. Looking at the CFL blacklights that I have through a simple transmission grating reveals all the major visible lines of mercury. If anyone on UVP has identified a CFL blacklight (E27 screwbase) with good filtration it would be nice to hear about it.

 

The most widely available blacklight glass tubes today are the little 4W and 6W tubes and a 17W tube still available at many Walmarts. The latter is a slimmed-down version of the 20W tube with the smaller diameter of the 15W tube but the 24-inch length of the original 20W tube (see top of photo above). The tube and fixture are sold together for about $12-- about what you would expect to pay for just the tube. This is in fact the only glass fluorescent tube still sold by the Walmarts I've visited. On ebay you can still find some of the larger size blacklight tubes if want to risk having a long slender glass tube filled with toxic gas shipped to your home.

 

Just to clarify for those unfamiliar with these tubes, they are called fluorescent tubes because most of the light emitted comes from the fluorescent powder coating the inside of the glass. This material lights up when struck by the UV light given off by the mercury vapor discharge inside. The blacklight tube uses a UVA phosphor and a filtered glass envelope (Wood's glass) to block most visible light and pass mostly UVA from the special phosphor along with the 365nm UVA line of mercury.

 

White and Blacklight tubes in VIS and UVA (Baader U on quartz lens, full spectrum conversion)

 

The image above reveals the UVA phosphor in the blacklight tube. Just like the white phosphor in the regular tube, it is excited primarily by the UVB and UVC radiation from the mercury discharge inside.

 

These two-pin tubes use hot cathodes which are tungsten filaments at each end that heat up enough to initiate a discharge of electrons using only line (mains) voltage. The longer tubes might use a ballast that provides a kick-start voltage to get the discharge going. You could still light them up with high-voltage (3KV-10KV) neon sign transformers after the filaments burned out. This would be known as cold-cathode operation.

 

Yes, you can get those little 365nm LED lights with good filtration these days but they don't compare to the raw beauty of a 24 inch glass tube with a mysterious deep violet glow. I see Walmart is now also stocking the LED version of these so I expect to see the authentic glass versions with the good filtration and strong UVA output (and dangerous mercury vapor) to be phased out for safety & practicality reasons. So, get one while you can. Don't be the only kid on the street who doesn't have one!

 

[Doug A]

Very interesting and Informative post. I had a small blacklight as a youngster. It was about a 6" tube in a substantial metal housing, with polished reflector. Had a lot of fun with that light. Shined it at everything. 

 

Now I need to pick up a few fluorescent blacklights, before they become extinct. Might be interesting to try some UVIVR and compare it to the 365 nm Nemo led torch. They should look totally different. 

Thanks for sharing,

Doug A

[dabateman]

I think most of the Led black light replacements will be 405nm bulbs.

Hopefully not too many will be 365nm, unless indicated.  But we will have to see. 

Some 405nm Led bulbs have even been marked as UVC, which is sad. Especially those corn bulbs. 

 

[lukaszgryglicki]

405nm UVC? LOL

This is why I hate marketing.

 

[lonesome_dave]

Like dabateman says, the LED strip blacklight sold at my local Walmart (and marked on the box as 365nm) is actually using 405nm LEDs as measured by my monochromator. Not marked as UVC at least.