disclaimer: I could have sufficed by just remarking: yeah, it works fine, but I can not resist doing a proper test, even for a simple optical filter
In the the double-UV light group buy, this 20mm diameter, 2mm thick ZWB2 filter was discussed as a way to get rid of the large amount of visible light that chinese 365nm leds (and the Ledengin LZ1 365nm is just as bad) produce as an unwanted by-product. Being 20mm diameter it fits in the Convoy tube-style lights as replacement lens. It is for sale on Ali-Express for $16.66 dollar and I bought one to test (with the kind help from BLF-member khas because I can not buy there myself). The above picture shows the filter placed on top of a normal white led flashlight: apart from transmitting UV with a peak at 365nm it transmits in deep red as well (the reproduced colour is not accurate btw), here's the transmittance graph (from the selller's page) of a 1mm ZWB2:
You loose a bit of light (this graph is for a 1mm filter, my thicker 2mm filter reduces the transmittance at 365nm a bit more: to 75%) but apart from that it should be pretty ideal, let's test that!
First the set-up, because how tests are done are always important for interpretation of the results.
The test subject is a 50 euro bank note because it shows several colours of fluorescence at 365nm, less at 400nm. The lights used for testing the filter have the following leds: 1)400nm generic chinese 3535 led, 2)Nichia 276A U365, 3)Ledengin LZ1 365, 4)Royalighting RY-3535P 365nm (the chinese 3535led sold by intl-outdoor).
The yellow glasses are for protection of my eyes during testing, the clear glasses (Bollé impact-protection glasses) is a pretty good UV-blocker, needed in front of my phone camera because it picks up light further down UV than my eyes do, with them what the camera sees is pretty much what my eyes see (with the 365nm leds that is, getting the 400nm correctly imaged was not really possible) :
The left picture was taken without the glasses before the camera, that purple glow is invisible by eye, the right picture was taken with the glasses before the camera, this is quite what I actually see as well (illumination was the RY-3535P with ZWB2 filter).
Mind that in the following pictures the output differences of the various leds, even though that did not differ to great extend, and the slight reduction of output caused by the filter is not captured (I used an auto-exposure on the camera with reduced brightness).
The note with white light:
400nm UV, I used the ZWB2 filter to get a bit more contrast, but for my eyes the blue and green fluorescence of the fibers (the red fluorescent fibers can only be seen at 365nm) is stronger compared to the purple background and of more distinct colour, I can not get this right on image: it is better in real:
Now on to the 365nm lights, the rest of the pictures are reproduced pretty well from what my eyes see, apart that the camera does not pick up the difference between the blue and green fibers as well as my eyses:
Nichia 276A U365 without filter. You can see how good this led is, hardly any visible light to spoil the fluorescence:
Nichia 276A U365nm with ZWB2 filter. The filter still does some good, the minimum amount of visible light is filtered out, and some of the 'upper flank' of the spectrum is reduced as well, removing some purple glow still picked up by the camera in the above picture without filter (but this purple was not seen by eye):
Ledengin LZ1 365nm without filter. This led is known for its very good output at 365nm, but also for its massive amount of white-ish light as well, as can be seen:
Ledengin LZ1 365nm with ZWB2 filter. That fixes a lot!! This filter gets the illuminiation quality on Nichia level :-) :
RY-3535P 365nm without filter. This led has the same problem as the Ledengin LZ1 365nm, good fluorescence but with lots of white-ish light, a bit more even:
RY-3535P with ZWB2 filter. Also here the filter helps out to get the illumination to Nichia level! In my test of this led a year ago I found that the peak wavelength of this led is 370nm compared to 365nm for the LZ1, but you do not see that difference in the fluorescence of a 50 euro note:
This filter does a very good job blocking visible light that comes out of the various 365nm leds that emit in white as well, and even for the Nichia 276A U365, that hardly produces white light it also helps a bit. It is not suitable for 400nm light, it blocks a lot of the usable light and 400nm leds do not have the typical 'white light pollution' anyway. For an optical filter it is really cheap (long ago I used to work in an optics lab, from the major manufacturers any optical filter starts well above 100 euro), but as a flashlight upgrade $16.66 is considerable. Still, for the enthousiast I think it is a good buy. :-)
Did I mention it looks cool in your flashlight? :