[car]

I've built 12V mercury vapor UV-C (254nm) lights for fluorescent mineral hunting, and that wavelength is quite harmful, requiring skin and eye protection. Mercury vapor lamps produce a spectrum of wavelengths, also in the visible spectrum, which gets filtered out since it distracts.

According to this [1] article, the 222nm range is safe for exposure, but the Krypton-Chloride bulb in the far-UVC lamp does also produce harmful wavelengths (256nm), therefore a filter is absolutely necessary. Thankfully simple plastics should work fine for that.

I would still be extremely careful deploying these lights in occupied spaces.

Edit: Come to think of it, filtering the harmful UVC (256nm) from KrCl excimer lamps with acrylic would probably also block the far-UVC. Which makes me wonder what material the filter is. Regular glass stops UVC, which is why UVC lamps are usually quartz or special glass formulations.

"What really needs to be understood is that an unfiltered 222nm Far-UV peak from any KrCl excimer lamp emits a wide band of wavelengths starting at 200nm, past the human safe zone of less than 230nm, all the way to the end of the UVC spectrum at 280nm -- with a very worrisome second harmonic peak at 256nm."

"the 222nm excimer lamp's second harmonic peak at 256nm exclusive to KrCl Far-UVC lights should be treated no different than the well-established carcinogenic hazards involved when using 254nm mercury-line UVC germicidal bulbs."

[1] https://www.prweb.com/releases/222-nm-far-uvc-cancer-risk-wi...

[Modified3019]

What took me by surprise (but really shouldn’t have) with my set of UV A, B, and C flashlights, is how much of the light can get reflected. Pointing at a rock and seeing a spot on my shirt light up was educational.

[vbelenky]

The filter is a hafnium oxide interference filter! There are some absorption-based filters being studied in academic settings (as they'd be potentially cheaper to manufacture) but none are used commercially as far as I know

Honestly there aren't that many commercially available unfiltered KrCl lamps out there. I'm only aware of one and it's stupid expensive. Every other lamp you can actually buy is filtered, although some filters are a bit worse than others (though still pretty safe). Any lamp module that uses the Ushio Care222 emitter is certainly very well filtered because Ushio integrates the filter into the module.