Russell said: >> UV LEDs producing even a few mW of power have until recently cost >> hundreds of dollars. Seam said > Hmmm again. I recently bought some UV LEDs on eBay for about US $0.50 > each. Supposed to be 385nm wavelength. When I tried one, it produced a > faint visible glow - whiteish blue if I recall correctly, but it > definitely could make UV-sensitive materials fluoresce (like the > security bands in US paper currency). Have I been duped or has there > been a recent breakthrough that makes these kind of UV LEDs cheap? It > also seems to me that I've seen tiny "blacklight" pens for several > years now which are intended to check for valid currency. These seem > to be using LEDs. Am I wrong? The output of these devices definitely > is not bright blue light. or, Russell could be wrong :-). But, it's all relative and I should have added even more detail. David said: > I've use these ones from Element14 before: > http://au.element14.com/vcc-visual-communications-company/vaol-5guv0t4/le= d-5mm-ultraviolet-160mcd-405nm/dp/1712756 I note in that data sheet that one application is "germicidal" use. I think they are being a bit naughty. They would probably give some living things a harder time but they are well away from what is usually deemed "germicidal" wavelengths. There's UV and there's UV and there's UV and ther's sort of almost UV - and I could hav ebeen more specific. UVA 320- 400 nm UVB 290 - 320 nm UVC 190-290 nm The A/B and B/C bounday figures vary by about 10 nm between sources. All seem agreed that uVA starts at 320 nm - but even that is arbitrary. Dave's LED datasheet -> http://vcclite.com/wp-content/files/VAOL-5GUV0T4-LED-5mm-UV.pdf is listed at 405 nm in the heading and has bins of 400-405 and 405-410 nm. Neither is quite UV by definitions above BUT the light is energetic enough to cause fluroesnce in many materials and to distress some bugs. Suntanning is generally caused by YVA range light. Sunburning is generally caused by UVB. Don't take that as gospel - UVA can burn you - BLUE light can burn you - some parts of you more than others. This extremely good (IMHO) overview of UV safety gives a good picture of relative dangers at various wavelengths WELL WORTH LOOKING AT. https://www.case.edu/ehs/Training/UV/UVsafety.pdf It explains what wavelengths are stopped where and by what and the effects. The real nasty is UVC which is emitted by tyhe sun and by true "germicidal" lamps. However, solar UVC is absorbed by the atmosphere. UVC from eg uncoated mercury tubes can be hazardous tens of metres in air from its source. They say: .. Germicidal lamps or low pressure mercury lamps are commonly used in laboratory. They are usually in housed fixtures such as a biological hood, but may not always limit exposure to the eyes or skin. The UV wavelength of these lamps is between 200-280 nm and work by breaking DNA which removes the capability of organisms to reproduce or kills them. Unprotected persons should not be in a room where a lamp is active. Hood lamps should be turned off after sterilization time has ended. Russell --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .