Another "black light" that isn't

PhotonWrangler

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While looking through Wally World recently I stumbled across an A19 LED filament "black light" bulb. Being a UV nerd I picked one up to see if it would be useful for fluorescent mineral stuff. Their P/N is GVLA198K. There is also a TCP P/N WF120008.

It turns out that there's NO UV coming out of this bulb at all - just a dim blue glow. It wasn't able to fluoresce anything and my UV detector card said "nope."

I wanted to see if the dark blue envelope was passing UV by shining a known-good UV flashlight through it and that worked fine. This tells me that the problem is with the choice of LEDs themselves. I'll probably crack this open to see what it's using.

The bulb isn't even useful as a blue lamp since it's output is so dim. Don't waste your money on this one.
 

LEDphile

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While not UV, the 450nm royal blue emitters will excite many fluorescent pigments in a manner similar to a blacklight. As that's usually the LED chip of choice for making white LEDs (in combination with a yellow phosphor), I could easily see some being used as the basis for a consumer "blacklight" bulb.
 

snakebite

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likely its the typical blue leds without the phosphor.
this would explain its uselessness.
one year i put up a 8' ho blacklight for halloween.
had folks posing under it to get picture taken.
lots of everyday stuff glows brightly under a good blacklight.
 

Dave_H

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While not UV, the 450nm royal blue emitters will excite many fluorescent pigments in a manner similar to a blacklight. As that's usually the LED chip of choice for making white LEDs (in combination with a yellow phosphor), I could easily see some being used as the basis for a consumer "blacklight" bulb.
I have a blue-LED flashlight which pre-dates most white-LED flashlights and noticed even at the time it tended to light up day-glow tags, posters etc. although not nearly as well as real UV. I suspect this effect allows vendors of dubious "black lights" some latitude in their claims.

Coloured filament bulbs in one big-box store typically sell for $4-$5 but the "black light" versions go for $11-$12; all that for a coat of clear purple lacquer. The claim of "lights up black" is almost funny (sort of like waking up dead) but not unique as some automotive white-LED products apparently "light up clear".

Dave
 

PhotonWrangler

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"Lights up black" - lol. Yeah I know exactly which bulb you're talking about as I have one of those also. There are a number of bulbs in the $9-$12 range that are near-UV, probably in the 395-405nm range, that will fluoresce things but at reduced contrast because of the visible purple light they put out.

I suspect that this house-branded bulb from Wally World is using white LED filaments instead of blue ones as it doesn't fluoresce anything at all, not even a little bit. I will be cracking this thing open to confirm this.
 

Lark Hunter

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Coloured filament bulbs in one big-box store typically sell for $4-$5 but the "black light" versions go for $11-$12; all that for a coat of clear purple lacquer.

Dave
Walmart and friends didn't even expend the effort to raise the price on this one; I (unfortunately) purchased one to replace a burned out blacklight CFL, and recall the price being about $4.50

^ My guess is going to be ~5000k LED white filaments sulking behind that purple filter. It's pretty sad when an incandescent blacklight bulb puts out more UV than this creation. Gahhbage!
 

PhotonWrangler

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Lark Hunter, you called it right. I cracked it open today and as expected I found phosphor-white LED filaments. I carefully screwed this into a plug base adapter and powered it up, and it lit up a bright cool white, maybe 5000-6000k. This explains why there's no UV coming out of the bulb whatsoever. What a manufacturing blunder.

GVLA19BK_bulb.jpg
 

Dave_H

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Alternate source of near UV are LED "germicidal"/"disinfectant" A19 bulbs though I'd hesitate to call them "cheap".

One I got on sale for $7-$8 was the Globe Electric 35360 which can toggle between regular visible and UV by light switch; neat, but UV has lots of visible. Perhaps a good filter (405nm) would make it more useful.

Dave
 

PhotonWrangler

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Alternate source of near UV are LED "germicidal"/"disinfectant" A19 bulbs though I'd hesitate to call them "cheap".

One I got on sale for $7-$8 was the Globe Electric 35360 which can toggle between regular visible and UV by light switch; neat, but UV has lots of visible. Perhaps a good filter (405nm) would make it more useful.

Dave
I was in my local "Green Walls" pharmacy the other day and found that they were closing out those A19 Globe "UV" (405nm) bulbs for $1.50. They were also closing out that suction-mount disinfection LED array for $2.00. For those prices Imy curiosity got the better of me so I snapped up one of each of them.

The documentation on the Globe A19 bulb does indeed state that the wavelength is 405nm, which is NUV. (The receipt called it a UVC bulb - hah!) It looks decidedly blue to me - not even violet. While this could be due to fluorescence of the plastic globe, I'm not sure.

The other unit with the suction cup mount has four LEDs in white plastic cases along the periphery and a single brass-colored metal cased LED in the center which might be UVC. I will be testing this for UVC content later.

**Update**

The UVC LED suction cup unit does indeed have a real UVC LED in the center, surrounded by four UVA-ish LEDs. This could be a cheap source for harvesting UVC LEDs while these units are on closeout.

On the Globe A19 bulb, there's a little bit of UVA content but absolutely zero UVC output. Thsi confirms that while it could kill some types of bacteria, it won't kill (deactivate) viruses. The wavelength is too long for that.
 
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Dave_H

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The UVC LED suction cup unit does indeed have a real UVC LED in the center, surrounded by four UVA-ish LEDs. This could be a cheap source for harvesting UVC LEDs while these units are on closeout.

On the Globe A19 bulb, there's a little bit of UVA content but absolutely zero UVC output. Thsi confirms that while it could kill some types of bacteria, it won't kill (deactivate) viruses. The wavelength is too long for that.
My conclusion also for A19. It has way to much visible light to be useful for fluorescing things.

What brand/model is the "suction cup" device?

Dave
 
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