[SML] LED Blacklights

[Joe D]

Sorry if this is a repeat, but I cannot see my posting in either the archive, or in my in box. There has not been an error message either.  I have confirmed that my subscription settings are set for me to see my own posts, but regardless of that, it should be in the archives.  

I first posted this from my normal e-mail, but those seem to disappear into the aether.  So, here is the original posting, plus an update sent from my Yahoo account.

I have recently been investigating LED black lights.  I have read the comments that these really aren’t black lights, since the light they put out is visible.  So, they are ALMOST UV fixtures.  They seem to be relatively cheap, so I ordered a 16ft , $16 strip of UV LEDs just to try them out.  However, the stuff from China sold on E-bay is not exactly known for its high quality or accurate descriptions.  And of course, the seller’s know little about the product details.

This week I have also been playing with a cheap RGBW (Red, Green, Blue, White) LED strip.  I was pleasantly surprised at how bright they have become since I last saw some a few years ago.  Of course there were a few drawbacks. The dimming on the cheap IR controller is not smooth at all, and can only go down to perhaps 20% brightness.  There is a flashing mode, but really it is not a strobe effect since the light is on about as long as it is off, so you can’t get that frozen motion effect.  Here is a link to the one I am working with right now, which included the controller I am describing.
http://www.ebay.com/itm/400729668965?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
It is on E-bay as model "5050 SMD 300leds Strip Light" sold by katewu20152011

I was also intrigued at what I saw when I set it to a pure blue color. Though, the light is certainly quite obviously blue to the eye, and not a black light, It caused many items to fluoresce.  And that got me wondering where the sweet spot is for LED based UV lights.    I have done a little reading, and found that there are actually, true, LED based black lights that are invisible to the eye, but the commenter also said they were too expensive to use for theatrical effects.

Of course, the actual wavelength is not information the e-bay seller puts in their description.  Though, some have replied when I asked.  But, I really don’t know at what point the visibility of the blue light is so little that it is negligible.  Can anyone comment about the actual wavelength I should be seeking?  Should I expect better fluorescent glow from shorter wavelengths, or am I only gaining better invisibility of the light itself?

Here is my follow-up message;

I have the following info from Wikipedia;

  Ultraviolet (UV) light is electromagnetic radiation with a wavelength from 
  400 nm to 10 nm…. Though usually invisible, under some conditions 
  children and young adults can see ultraviolet down to wavelengths of
   about 310 nm,[1][2] and people with aphakia (missing lens) can also see
  some UV wavelengths. 

So, it would seem that I need to get to 400 nm for true black light effects.
They also add;

  Light-emitting diodes (LEDs) can be manufactured to emit light in the 
  ultraviolet range, although practical LED arrays are very limited below 
  365 nm. LED efficiency at 365 nm is about 5–8%, whereas efficiency 
  at 395 nm is closer to 20%, and power outputs at these longer UV 
  wavelengths are also better. 

So, perhaps 390nm should be my goal. Though, as I said before, I don't have a lot of confidence in the specs from these vendors that sell the cheap stuff from China.  If the end user cannot verify the wavelenght, they certainly can get away with deliberately mis-quoting the specs.  Though, perhaps if I can see the light, it is not UV?

One final note;

I also recall hearing that during WWII, the military recruited older men, who had had cataracts removed. Since the implanted lenses of the day were not able to block UV light, they could see UV light that was used to send Morris code signals.  

At age 50, I notice that true UV light tends to look hazy to me, so I am guessing that my older natural lenses must fluoresce to some degree, and cloud my vision a little. It never did that when I was younger.

Joe Dunfee
jdunfee12 at yahoo.com
305-999-1759