When caching at night the color in most things tends to go away. The blue light of the Moon doesn’t reveal a lot of color. There are two parts of the light spectrum that make for interesting cache experiences. Those are infra-red light and ultra violet light (it’s actually near ultra violet but we’ll let the science slip for a moment).
A little over a year ago Groundspeak added the UV attribute that can be assigned to geocaches. Since that time we’ve seen UV enabled caches grow. In a typical UV scenario a hint or coordinates are written in an ink that is UV reactive. The UV clue is sometimes written on the back side of a clue that is written such that it’s visible in white light. A cacher that is unaware of the UV attribute might be tricked by this approach.
In order to protect the UV inks from the elements we recommend placing them in non-UV blocking laminate. We carry non-reactive paper and self-sealing laminate pouches for this very reason. There is another way to provide clues using UV light and that is with UV reactive monofilament line.
We recommend creating eight segment digits like you see on a digital clock where some segments are not UV reactive and others are. Under white like the two lines are very hard to differentiate. Turn on a UV light and the correct coordinates pop out. In the pictures for this article the UV line is protected inside a sealable container. This is mostly for protection from the Sun’s rays. UV reactive features will degrade over time when exposed to sunlight. The UV line does not need to be protected from the elements, water won’t hurt it. You’ll also want to make sure that the cacher can’t see the backside of the coordinate segments. If they can see the backside they can see which segments use which kind of line and deduce the coordinates from that information.
The only special equipment you need to find a night cache with UV clues is a UV flashlight. There are a variety of UV lights available from small button lights to larger 3-in-1 lights. If UV caches are growing in your area then you’ll want to add one of these lights to your pack.
Caches that use infra-red (IR) light are harder to create and require special equipment to find them. An IR cache needs two components, an infra-red light source and an infra-red capable camera. Any remote control where you have to point it at the TV uses an infra-red LED. You can either take apart an old remote control or you can buy IR LEDs for about 50 cents each. We opted to buy them for our Lunar Lander BFL Boot Camp cache.
Now that we had an IR light source we needed to figure out what to do about a camera. I learned a while ago that webcams will pick up IR light but those aren’t too convenient to take into the field. I suppose if you had a little netbook that might work. I discovered that my BlackBerry makes for a decent enough IR camera for what we had planned. The facetime camera on an iPhone might work if you are patient. Many cameras have a built in IR filter. You can test your camera by viewing the front of a remote control through your camera as you push the buttons. If you see a light then your
camera will work to view IR light. You can also use cameras that have a “night vision” mode. As you can tell it’s not easy finding a way to view infra-red light. If you choose to build and IR cache it will be important to provide sufficient details in the description relative to your difficulty rating to allow people to find the cache. If you set your difficulty rating at 5 then you might not need to provide too many clues on how to solve the cache.
The trick to making and IR cache is finding plastic that is opaque to white light but easily transmits IR light. Black or smoked plastics work best. I used multiple layers of a red plastic. It is very hard to pass white light through multiple layers but the IR easily passes.
We’ve just described a few ways that we have used ultra violet and infra-red to create caches. Hopefully these will be your starting point when you build your own creative night cache.