[SML] More on UV light from LEDs

[Ford Sellers]

-----Original Message-----
From: Joe D 
Sent: Monday, April 13, 2015 10:19 PM
To: Stagecraft Mailing List
Subject: [SML] More on UV light from LEDs

I just received a UV flashlight that is specifically rated at 395 nm...

...   Before I seek a refund from the vendor, anyone have anything else to add?
Joe Dunfee
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Hi Joe,

What do you mean "rated at"?
395 may be the dominant wavelength, but there may be considerable output at other wavelengths.
Dominant means just that... the majority of the output is centered around that number.  
Usually, the tightest binning you can get is +/- 5nm, so the dominant wavelength of this particular LED may be anywhere between 390nm and 400nm... IF they are buying from a tight LED bin.  If not, all they are saying is that 395nm is at the center of their bin, and the color may vary considerably (if the bin is +/- 10nm, the range would be 385-405...a wide range indeed).

Then, add to this the actual quality of the LED.
Usually, LEDs are very specific, but if this company is buying LEDs that are from a small/cheap manufacturer who doesn't use very pure phosphor mixes, or don't use the best curatives, or if the flashlight, or the LED itself has some impurities in their lensing you may get output at other wavelengths, or even some white-ish light in the output... all while still having 395 as their dominant wavelength.

In practice, you would want something in the 365 or lower range to appear invisible to most peoples naked eye.

In our DJ line, we use "UV" LEDs from good led suppliers whose dominant wavelength sits about 405-410.  This is very purple, but does make things fluoresce.  It also allows the Kiddos to "see how bright their UV light is".

In our Pro line, the only "UV" LEDs we use (in our 6-color LEDs) are centered around 395nm.  This is less visible, but still clearly noticeable in a dark room, with no other light on.... it is really more "Hyper-Violet" than "Ultra-Violet". But it does make things fluoresce nicely, and when mixed with the other colors in the fixture, you can get some really amazingly trippy colors that simply do not occur in nature (mixing Red and UV generates a sort of Sangria color that reacts differently to various materials...really fun to play with).

We have looked at making a pro UV fixture centered around 365nm, which is barely visible...but have decided against such a specialty item.  
We have not been able to get our lab to agree that this item would be safe to pass UL certification, the cost per LED is higher, there may be safety issues in our QC department, and we have difficulty measuring the output (I guess we could measure the brightness of a phosphor sheet at a given distance...but how to explain that to a customer).

 There are companies that make UV lights like these... but they are not inexpensive.

I hope that helped, or at least cleared things up a little.

-Ford