Turn Your iPhone Into a Black Light

By Jeff Bertolucci

This simple hack lets you turn a smartphone's flash into a black light. It's a fun and easy project that kids will enjoy, particularly around Halloween.

Turn on a black light in a dark room, and you'll see a dim, purplish glow. But, the bulb also produces ultraviolet light, which you can't see.

As you know, certain things glow under black light -- white T-shirts, fluorescent posters, toothpaste, and petroleum jelly, just to name a few. Why? Phosphors in these objects emit visible light in response to UV radiation from the black light. (Interestingly, the phosphors in white T-shirts come from the detergents we use to wash them.)

Here's an oldie-but-goodie phone hack that's fun for Halloween, takes only a few minutes, and probably won't cost you a dime. We're going to turn a smartphone's camera flash into a black light!

OK, so maybe it's not the world's greatest black light -- call it Black Light Lite -- but it does work. Here's how to do it.

Project supplies

Step 1

Cut off a small piece of tape and cover the flash.

Apply the tape

Step 2

Color the first layer of tape -- just the part covering the flash --with blue Sharpie ink.

Blue Sharpie


Some instructions for this hack -- and there are many online -- say to repeat this step twice, meaning a second layer of tape covered by a second layer of blue ink. I did this hack twice -- once with a single layer of blue-inked tape, and then with a second layer -- and didn't see a difference in the black light effect.

Step 3

Apply a second layer of tape. Use the purple Sharpie to draw over the blue-inked area.

Purple Sharpie

Step 4

Turn out the lights (if it's nighttime) or find a windowless room and shut the door. Turn on the phone's flashlight and enjoy the eerie, bluish-purple glow.


On the iPhone, swipe up from the bottom of the screen and tap the flashlight icon in the lower-left corner. If your Android phone needs a flashlight app, you'll find plenty of free choices, including Smart Flashlight and Flashlight HD LED.

Let the glowing begin! Here's an example of neon-colored highlighter ink -- a yellow Sharpie, in fact -- glowing under my iPhone 6's black light.

Neon-colored highlighter ink

White paper glows too, so I created a paper airplane for the occasion.

Paper airplane under black light

Halloween supplies are often black-light friendly.

Halloween supplies

The Skull of Doom glowed as well.

Toy skull under black light

What else glowed? A mouthful of toothpaste, but that photo (of me) was so disturbing that I chose not to upload it here.

Again, don't expect the vibrant glow you'd get from a dedicated black light, but this simple hack is easy and fun, particularly with kids.


If young children are involved, do the taping and inking yourself while the youngsters watch. One slip of the Sharpie, and you'll cover the camera lens with blue or purple ink.