Can You Use Your Phone Flashlight as a UV Light?
Our customer asked:
Can you turn your phone's flashlight into a UV light?
TL;DR
No. You cannot turn a phone flashlight into a real UV light. The LED in your phone produces essentially zero ultraviolet, and tape, marker and apps cannot add a wavelength that the LED never emitted. The tape-and-Sharpie trick gives you dim violet-blue visible light. That will make a highlighter glow. It will not reliably show pet urine, scorpions, or the security thread in a $20 bill.
I run product innovation at LUXPRO, and I get asked this a lot. I understand why. The hack costs nothing, and a real UV flashlight costs money. So here is why the physics says no, what the trick is good for, and when it makes sense to spend $17.
Why your phone LED has no UV in it
White LEDs are not white at the source. Inside your phone's flash is a tiny blue LED chip, emitting at roughly 450 nm, coated in a yellow phosphor. Some blue passes through, the phosphor converts the rest to yellow and red, and your eye blends it into white.
Look at the output of that LED on a spectrometer and you see a sharp blue spike, a broad hump through green, yellow and red, and then almost nothing below about 420 nm. Ultraviolet starts at 400 nm and goes down from there. A UV flashlight like our LP32UV emits at 385 to 390 nm. A phone puts out essentially nothing there.
That is deliberate. UV output in a phone flash would waste battery, it would yellow the plastics around the lens over time, and it would do nothing for your photos. Phone makers engineer it out.
What the tape-and-marker trick really does
The popular hack: put clear tape over the flash, color it blue with a marker, add another layer of tape, color it blue again, then add a purple layer. Turn on the light and you get a moody violet beam. It looks like a black light.
A filter can only subtract. The marker ink blocks the green, yellow and red parts of the beam and lets the blue-violet part through. You have not created ultraviolet. You have thrown away about 90 percent of your light and kept the bluest 10 percent.
So why does a highlighter glow under it? Highlighter ink, tonic water, and the optical brighteners in white paper are unusually easy to excite. Their absorption band reaches well up into visible blue light. A deep blue light will make them fluoresce a little. It is a real effect, and it is a fun demo for kids. It is also about the only thing the trick can do.
Phone hack vs. a real UV flashlight
| Target | Phone + tape + marker | Real UV flashlight (385 to 395 nm) |
|---|---|---|
| Highlighter ink, tonic water | Faint glow | Strong glow |
| Dried pet urine | No usable result | Visible yellow-green in a dark room |
| Scorpions | No | Bright blue-green from several feet |
| Currency security thread | No | Yes |
| Uranium glass | Maybe a hint, easy to fool yourself | Vivid neon green |
| AC / engine leak dye | Weak at best | Yes |
| Charging glow-in-the-dark tackle | Slower than just using the bare phone light | Seconds |
The danger with the hack is the false negative. You sweep the carpet with a taped-up phone, see nothing, and conclude the carpet is clean. The carpet may not be clean. Your light simply cannot excite the stain. When you are about to sign a lease or buy a used couch, a test that always comes back clean is worse than no test at all.
What about UV light apps?
An app can change the color of your screen. It cannot change what the LEDs in the screen or flash are physically able to emit. A purple screen is visible purple light. Every black light app in the app store is a novelty.
Can a phone camera see UV?
Not in a useful way. Phone cameras sit behind filters that block UV and infrared so that your photos look right. A phone is very good at photographing the visible glow that a real UV light produces. If you are documenting pet stains for a landlord or a repair on an antique, hold the UV flashlight in one hand, shoot with the phone in the other, and turn the phone's own flash off. We put a wrist lanyard on the LP32UV for this reason.
The cost of doing it properly
Thirty years ago a portable black light meant a fragile fluorescent tube and a ballast. UV LEDs changed that. Our LP32UV is $16.99 with batteries in the package. It has 32 UV LEDs at 385 to 390 nm, an aluminum body, a tail switch and an IPX4 splash rating. Three rolls of tape and a pack of markers cost about the same.
It is a simple light. It does not have a filtered 365 nm emitter like the specialist lights that inspectors and mineral collectors use. If you need that, I point you to the right ones, including competitors, in my review of the 7 best UV flashlights. For pet stains, scorpions, cash, collectibles and tackle, the LP32UV does the job, and a phone will not.
Frequently Asked Questions
Q: Does the blue and purple Sharpie trick work at all?
It makes highlighter, tonic water and bright white paper glow faintly, because those react to visible blue light. It does not produce ultraviolet and will not reveal urine, scorpions or security features.
Q: Is there any phone with a built-in UV light?
No mainstream phone has one. A few rugged niche handsets have added a UV LED as a feature, which shows that a standard flash cannot do it.
Q: What can I do with a real UV flashlight once I have one?
More than most people expect. Here is my list built from real customer use: What is a black light flashlight used for?
Q: Is a real UV flashlight safe to use around the house?
Yes, with sensible handling. Never look into the beam, and do not hand it to young kids as a toy. Full details: Are UV flashlights safe?