Geocaching Australia

I bought a pen/torch combination which was supposed to enable you to write an “invisible” message which would be visible using the torch. I assume the ink fluoresces when subject to the “UV” component of the torch’s light. This worked fine when I tried it inside.
I had planned to use this in a multi-cache where I put a card in the WP1 cache which had the WP2 cache’s coords written using the “invisible ink” so that finders had to use a “UV torch” to read the coords.
However, when testing out the idea, I’ve found that the coords are faintly visible in daylight, presumably due to the UV component in sunlight. In fact I found them quite easy to read when in shade, but with bright sunlight nearby. This was true using different varieties of white card or paper.
Has anyone found a way to use this idea in such a way that the coords cannot be read without using a “UV torch”?

I wrote the coordinates in a spiral pattern, south going one way & east going the other. Yes, you can see that something is written on the paper, but you can't make out what it is until you use the UV light.

Toriaz wrote:I wrote the coordinates in a spiral pattern, south going one way & east going the other. Yes, you can see that something is written on the paper, but you can't make out what it is until you use the UV light.

Thanks for that idea - I'll give it a try.
I assume you write it while illuminating your writing with the UV torch, otherwise you could end up with a real mess!

Bunya wrote: I assume you write it while illuminating your writing with the UV torch, otherwise you could end up with a real mess!

Yep. Although my handwriting on that cache has been criticized, so maybe it didn't make difference for me .

I really must re-do the waypoint for that one soon, if my writing is really that bad .

Bunya wrote:I bought a pen/torch combination which was supposed to enable you to write an “invisible” message which would be visible using the torch. I assume the ink fluoresces when subject to the “UV” component of the torch’s light. This worked fine when I tried it inside.
I had planned to use this in a multi-cache where I put a card in the WP1 cache which had the WP2 cache’s coords written using the “invisible ink” so that finders had to use a “UV torch” to read the coords.
However, when testing out the idea, I’ve found that the coords are faintly visible in daylight, presumably due to the UV component in sunlight. In fact I found them quite easy to read when in shade, but with bright sunlight nearby. This was true using different varieties of white card or paper.
Has anyone found a way to use this idea in such a way that the coords cannot be read without using a “UV torch”?

I had to try a few different UV pens, and some different types of paper before I found the right combination that wasn't readable in sunlight. I also laminated the paper and the heat made some pens show up! Very frustrating.

I found the pens that fluoresced in alternate colours were the worst

Pacmania wrote: I had to try a few different UV pens, and some different types of paper before I found the right combination that wasn't readable in sunlight.

Are you going to let us know the magic combination, please?

I can't actually remember . I think it was a no name brand pen, and better quality laser paper. The text wasn't that bright under UV which is probably why it was invisible under sunlight. The brighter pens (e.g. ones that glow red) were not very good.

Could you write the WPT's on something that can't be moved? Or place them in a tunnel or under a bridge, under a park gazebo in the shade/dark. Somewhere that is in the shade and fixed in place that can't be taken into the daylight? Sounds fun, good luck with it.

Bunya wrote:......
However, when testing out the idea, I’ve found that the coords are faintly visible in daylight, presumably due to the UV component in sunlight. In fact I found them quite easy to read when in shade, but with bright sunlight nearby. This was true using different varieties of white card or paper.
Has anyone found a way to use this idea in such a way that the coords cannot be read without using a “UV torch”?

The only way to prevent some-one using the UV in sunlight to read the information is to prevent the message from reaching sunlight. For example, written on a tag that is attached inside a tunnel/cave; or the tag is found by following fire-tacks as part of a night cache.
Cheers