Can you make a blacklight with your phone and markers?

Somewhere on the internet, whether it were Tumblr, or Facebook, or Twitter, or Pinterest or… you know just somewhere, I saw this little post about turning your phone camera into a black light. I know, I know, I thought the same thing. “That’s not how blacklights work.” You can’t just color over your flash and make it do, whatever it is that blacklights do.

And that right there was the thing. How do blacklights work? What makes things shine in their awesome presence. The more I thought about not really knowing, the more I thought, “Well maybe it does work.” So I turned to Google, and what a found was just a long list of instructables; and nothing debunking them.

Could this be real, is my (albeit limited) understanding of blacklights wrong?

I was still pretty confident, but luckily I work in a place that not only has the right colors of marker but gives me access to a blacklight as well as a strain of E. coli that has been engineered to fluoresce under said blacklight.

So I gathered my supplies, picked an instruction (They all were pretty much the same.), and went to work.

step 1
Cover your flash with tape and color it with a blue sharpie.
step 2
Repeat step one.
step 3
One more time, only this time use purple sharpie.
my blacklight
Supposedly you now have your own blacklight!

And when I took my first picture I was pretty impressed by the results.


I mean just look at me! That’s totally a blacklight glow!… Or is it?

To test if this was real or just some facsimile I had to turn to my positive control.

ecoli under blacklight
My lab’s E. coli O157:H7 strain under true blacklight.

And now for the big reveal!

my picture of ecoli
The same plate with my “blacklight.”

So, yeah that’s a little anti climatic. So why did it look so good in the picture of me, but utterly unimpressive when it came to the actual flourescence?

Well what I thought at the beginning of this “That’s not how blacklights work.” was kind of right. So, what then? Why do things shine in blacklight, and how is that different then what I saw in my selfie?

Like most light based questions it comes down to wavelength; blacklights emit UV whereas your camera phone flash emits white light.

The white light that is in the LED on your phone is made by combining blue light (450-470nm wavelength) with phosphor. The phosphor changes part of the blue light to yellow light (580-590nm wavelength). This combination of blue and yellow together create white. On the other hand Ultraviolet wavelengths that are used for blacklights, and cause fluorescence range from 400nm wavelength (UV-A) and lower. So, no matter how many color filters you want to put over your flash, you won’t be able to change your white lite emitting phone into a fluorescence revealing black light.

I’ll be honest and say I am not an optics expert by any stretch so if this is wrong feel free to hit me up in the comments, but basically the way I see it; covering your phone camera’s flash in marker only serves to wash out your pictures in said color. Sure it gives a fun faux blacklight, but in the end your phone’s LED just can’t produce UV in the way a real blacklight does.

Chris T.

Chris is a microbiologist with a passion for nature. He has a degree in Natural History and spends his time taking pictures of mushrooms, riding his bike, painting, and watching tv.

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  1. Huge kudos for testing this! Very interesting post. I’m intrigued by the slight effect you DID get from your phone mod. Still kind of cool but not black light.

  2. Yeah, I was intrigued by it a little myself. I think it may be that the flash still blows things out, like a normal flash, but with the blue and purple filters we interpret it as a glow instead of washout.

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