Torch Filter, U-340 vs U-360, for UVIVF

[Cadmium]

Here is a typical question I get constantly pertaining to 365nm torch/flash-light filtering

 

"Wouldn't the Hoya U-360 filter be more appropriate for the Convoy S2+ 365nm flashlight?

They are very popular with fluorescent mineral collectors who want the flashlight to emit as much 365nm light as possible while filtering out the visible spectrum."

 

The point of filtering the 365nm LED is to cut the emission to below 400nm. That will remove all possible visible violet.

This goes hand in hand with filtering the camera lens with a 400+nm barrier filter, such as 418nm, 420nm, 435nm, etc.,

to prevent any stray reflected -400nm UV from entering the camera and being mixed with the visual fluorescence at the sensor.

2mm thick U-360 (or UG1) cut UV at about 400nm, but not as well as U-340 (and UG11).

U-360 has a slightly higher 'amplitude' at 365nm than U-340 does at 365nm (see graph below), however you might be getting a little bit of visual violet mixed with your fluorescence if you use U-360 (or UG1).

Visually, I see almost no difference between the fluorescence using U-340 or U-360,

however, I know and feel more confident that the U-340 is removing any visual range violet better than U-360, and that is why we use U-340.

 

Any comments or thoughts are more than welcome. U-360 is available, of course, but I just have not seen a reason to use it. Let me know.

 

I will attach a graph below comparing filters for 365nm UV only emission.

[ulf]

The point of filtering the 365nm LED is to cut the emission to below 400nm. That will remove all possible visible violet.

This goes hand in hand with filtering the camera lens with a 400+nm barrier filter, such as 418nm, 420nm, 435nm, etc.,

 

The U-340, 2mm is as you say absolutely the best alternative to be 1000% sure to reject all VIS-light from the Convoy S2+!!

 

That torch with a nominal 365nm LED often have a peak close to 370nm.

The penalty for using a U-340, 2mm over a U-360, 2mm, is an insignificant 20% longer exposure time, due to a slightly stronger attenuation by the U-340.

 

At 400nm the U-340 reach >OD5 where U-360 only attenuate to OD2.

The main question here is if OD5 is needed or if OD2 is enough.

That depends on how much light the Convoy S2+ emit above 400nm compared with the intensity of the fluorescence's intensity, that can be seen by the camera.

 

The Convoy's output at 400nm is less than 1% of the peak intensity and at 410nm much lower.

After filtering with an U-360, 2mm there would be less than 0.01% of the peaks intensity at 400nm.

 

With proper lens filtering at say 420nm, the amount of light reaching the camera at 400nm is very small.

If a GG420, 2mm is used on the lens, it reach OD3 at 400nm.

 

It is likely that a U-360 on the Convoy would not cause any contamination at all in almost all cases.

The U-340 give an extreme safety margin, that can be nice to have to be completely safe from any VIS light.

[dabateman]

Cadmium,

Your first graph makes me wonder, why not 1mm U340?

Or is these a visible leak that pops up that is not in the graphed data?

1mm would look to have similar 400nm cut but more of the 370nm peak, similar to the U360.

When you posted this I was expecting you to have actually made a 20.2mm u360 filter and shown images in a mirror or something to show us that U340 is better. Couldn't you do that to rest your case.

[Cadmium]

Yeah, I find that U-340 (as apposed to UG11) has a visible leak starting in the low/mid 500nm range. You can see it when U-340 1mm is stacked with ANY thickness of S8612.

This doesn't happen with UG11 1mm or with U-340 2mm.

Technically you might be able to get away with it using U-340 1.5mm, but I would not use 1mm.

UG11 and U-340 look the same in graphs, but keep in mind that Hoya provides NO transmission data for U-340 between about 390 and 670nm.

However, this may be only academic when it comes to blocking a Nichia 365nm LED, because I rather doubt the Nichia emits light into the 500nm range.

So the U-340 1mm might work OK?

 

The 'mirror test', as I recall, was a visual test, wearing UV protective eye glasses, and a very quick/short "flash" test, not prolonged, and nothing I ever photographed,

but no mirror test on this one, sorry.

You maight be best to compare the difference in fluorescence strength between the torch filtered with U-340 2mm and U-360 2mm, maybe.

 

[Cadmium]