U-340 and 365nm Torch

[Cadmium]

I will post this once again, and try to keep it simple.

 

Before I go further I must remind you that UV light is dangerous to your eyes, and you should NEVER look into a UV torch, filtered or not.

You must wear special UV blocking eye protection when using a UV torch.

 

People often get Hoya U-340 filters for their Convoy torches, and think they see visual light, but they are looking at something that is fluorescing.

You can't see any light with your eyes through a U-340 2mm thick filter.

(technically, you can see some 700nm range red light with your eyes through a U-340 filter, you can see this if you view an incandescent bulb through a U-340 filter,

however, a Nichia 365nm UV LED emits no such 700nm visual light)

 

If you look at the front of a U-340 filtered Nichia 365nm torch, wearing UV protective glasses, you will not see any light, the filter will look black,

but shining that same torch at something will show light, and often blue light.

 

I think that says it all. And below is a photo showing this which you can test your self using any camera that has a visual only barrier filter on the lens to prevent any UV from entering the camera.

 

I know how confusing this can be to people. I understand. But the reality is that you are not seeing anything above 400nm coming out of a Nichia 365nm torch that is filtered with U-340.

For the price, and the quality, Hoya U-340 2mm is the best filter glass for blocking 400nm+ visible light.

 

Here is the example:

[dabateman]

I would add that a correctly working Convoy will not emit light above 400nm.

I have one that does and would not call correctly working.

I have one strong bright correctly working Convoy and one dimmer not correctly working one which emits light above 600nm.

 

[colinbm]

Good work Steve

[Cadmium]

This is the kind of thing that happens: "unless my eyes can see 365nm this filter doesn't work..."

To be honest, it can be a bit upsetting for me.

I completely understand how someone can have that reasoning. The situation is deceptive to the eyes.

It is difficult to explain all of this to someone. Even pictures such as the one above often don't work to illustrate the situation.

 

Although the Nichia 365nm UV LED doesn't seem to have much visual leak in graphs I have seen, it is obvious that these torches do emit visual violet/blue light when unfiltered.

I think the photo above illustrates that quite well)

All I can say is there is absolutely no visual light transmitted through a U-340 2mm filter (also illustrated quite well in the photo above).

So please, try to understand that.

A U-340 2mm filter on a Convoy looks black, the target is illuminated only from fluorescing.

[ulf]

Although the Nichia 365nm UV LED doesn't seem to have much visual leak in graphs I have seen, it is obvious that these torches do emit visual violet/blue light when unfiltered.

 

Steve, this warning is very good and cannot be repeated often enough!

 

The LED's visual leak is really very small, but the Nichia 365nm UV LED emit very much optical power.

That makes the visual leak well visible, without the filter.

 

The dangerous thing is the high energy in the totally invisible UV-peak.

With the filter attached it is easy to make misstakes.

From a decimeter or two the light several times brighter than the sun, and still invisible.

 

It is painful and dangerous to look straight into the sun, but normally you don't do that as it isn't comfortable.

It is an instinctive reaction to avoid being blinded.

This natural reaction is lacking for UV-light, making it even more dangerous.

 

Here is an example with UV-images of a flower, first illuminated by the sun only and then also by the Convoy S2+ UV-torch at a distance of ca 10-15cm:

Exposure times: with the sun only, 15s, with the torch and sun, 1s.

 

The surroundings in the second image are almost completely black as the sunlight cannot compete at all in brightness with the LED light.

Scary bright!

 

NEVER EVER look into the torch to see if it is on.

Instead point it to something that have fluorescence, like a pice of paper.