You want to dress up for a fancy dress party, and you want to arrive in costume, but you need to cross town without drawing attention to yourself.

Enter the Invisible Costume.

Step 1: The Gimmick

This Instructable uses UV LEDs built into a hat of your choice to reveal facial decorations drawn in UV security markers.

Health and safety

I have been unable to find official HSE data on the use of UV LEDs near the face. Since strong UV light can cause a semi-permanent to permanent condition known as "welder's eye", I would strongly advise that anybody who uses this project does so with their eyes shut when the LEDs are switched on.

This advice is from The LED Museum :

Most sunglasses and some prescription eyeglasses have some degree of UV protection already; to determine if yours do or not, aim the flashlight at something that clearly fluoresces (such as those neon green & orange stickers that occasionally show up on your mail, day-glow orange price tags, or painted portions of a blacklight poster), positioning the light so the object in front of it glows (fluoresces) brightly from the ultraviolet light. Then put the glasses between the face of the light and the target. If the glow stops or dims dramatically, the glasses are stopping the UV and you can use them as eye protection at least for short-term use.
However, if the glow remains or dims only slightly, look for another pair.

In any case, never look directly at the LEDs without eye protection - not even for a minute.

Symptoms of UV exposure may not appear immediately; they can be delayed by 30 minutes to several hours. They can include a burning or "sand in the eyes" sensation, and a hazy look around light bulbs and other bright objects. Mild cases are self-correcting within 24 to 36 hours; however if despite my warnings you still stared at this light from close range for more than a few minutes, it might not be a bad idea to see an eye doctor and tell him you've been exposed to UVA radiation.

DISCLAIMER: I shall not be held responsible for any damage or loss of eyesight arising from your use or misuse of the information on this page. Thank you for listening.

UV security markers are not designed for use on skin, so users should also check for sensitivity by applying the marker to the inner wrist and waiting 24 hours to see if there is any irritation or allergic reaction.

Step 2: What You Need.

You are going to set up a bank of UV LEDs in the brim of a baseball cap (or other hat of your choice) to shine on your face at the press of a button.

You will need:

A hat with a brim. I'm using a baseball cap, but you may prefer another style of brimmed hat.
UV LEDs. I'm using ten I purchased from fellow-Instructabler Phenoptix .
Wire
Aluminium foil
Card (cereal packet is fine)
Glue stick
Black electrician's insulation tape.
A normally-open push-switch (I used one I had in my bits box. I think it came from a VCR control panel).
Battery clip
Double AA battery-holder.
Optional - resistor. Many Instructablers swear by putting a protective resistor in series with an LED. These LEDs won't be switched on very much, and only have a 3V supply anyway.
UV security pen (the kind of thing used to mark your electronic goodies in the hope that the police can trace them after they've been stolen. You can also buy "secret message" pens in stationary stores that use UV-visible ink as well).
Something to remove the security pen!

Step 3: The Mounting for the LEDs

Cut a crescent of card that matches the outline of the brim of your hat.

Cut two crescents of foil that are slightly smaller than the card crescent.

Use the glue to fix a piece of foil to each side of the card. It is vital that the two pieces of foil do not touch at all, and you should also try and make sure the glue-stick does not provide a potential short either.

Split the end of the speaker lead, strip the ends and tape one strand to each side of the card, touching the foil.

Step 4: Fixing the LEDs to the Mounting.

Lay the card-and-foil crescent on your workspace. You may need to tape it still for a while.

Slot your LEDs "astride" the card, with one leg above and one below. Make sure you put every LED the same way round!

I left some distance between the card and the actual LEDs to allow for changing the angle they shine.

Tape the upper legs in place, turn over and tape the second legs in place.

You should now check that all the LEDs are working - a couple of mine weren't, but pressing the tape more firmly in place sorted that.

Step 5: Fix the Mounting to the Hat.

Using more of the insulation tape, fix the mounted LEDs under the brim of the hat.

I lined the entire underside of the brim in black tape to make everything less obvious, and fed the lead through the inside of the hat and down my back.

I also added small pieces of insulation tape to the front-facing sides of the LEDs, both to try and conceal them, and to cut down on stray UV.

The lead goes down my back and into my pocket, where I attached a small battery pack and a push-switch (so the LEDs only stay on while I press the button).

Step 6: Arrange the LEDs

Angle the LEDs to shine over as much of your face as you can.

This would have been so much easier with a friend - I spent ages changing the angle, taking a photo to check, changing again.... _frustrating

In an ideal world, I would have sat there with the hat on whilst somebody else spent thirty seconds pointing the LEDs the right way. Unfortunately, Kitewife was busy making bread, and the boys had found something on TV that was much more interesting than standing in a cold shed with their father.

Step 7: The Invisible Costume...

OK, remember in Step 2 where I said to make sure you had something to take the UV pen back off your face?

Guess who forgot?

Fortunately, I remembered just after I drew on my hand (checking for sensitivity) but before I drew anything on my face.

Security pens are solvent-based, so they need more than plain soap and water to remove them. Oddly, baby-wipes are ideal, but it's quite a while since we had nappy-changing in the family, so we didn't have any wipes handy.

You may not want to use a security pen, and prefer instead to use proper UV make-up. This requires two layers of make-up (a base-layer that matches your own skin-tone and the UV layer on top), but can be very effective if you are planning a Hallowe'en costume. You could draw a skull or cadaver face that you flash at unsuspecting passer-bys as you walk from house to house.

Anyway, there you go. Enjoy.

Since making this hat, I have come across a UV Body Pen that would be perfect.

Going back over this project, I think the one thing I would change is the hat - a wider-brimmed hat (such as a panama or ten-gallon hat) would have allowed me to space the LEDs out more efficiently. Still, live and learn.