You can create a DIY blacklight effect using your phone’s flashlight, some clear tape, and a couple of permanent markers. It takes about two minutes, costs almost nothing, and produces a violet glow that can make certain materials fluoresce. It’s not true ultraviolet light, but it works well enough to be genuinely useful for a few fun applications.
What You Need
The supplies are simple: your smartphone, clear sticky tape (standard office tape works fine), a blue permanent marker, and a purple permanent marker. That’s it. The markers and tape act as a crude color filter, blocking most of the white light from your phone’s flash and letting only the violet end of the spectrum through.
Step-by-Step Instructions
Start by placing a small piece of clear sticky tape directly over your phone’s flashlight. Use the blue permanent marker to color over the tape right where the light shines through. Cover the area completely so no white light leaks around the edges.
Next, stick a second piece of tape on top of the first layer. This time, color over it with the purple permanent marker. Then add a third piece of tape and color it purple again. You should now have three layers total: one blue, two purple.
Turn on your flashlight in a dark room. You should see a dim violet glow instead of the usual bright white. If too much white light is still bleeding through, add another layer of purple-colored tape. If the light is too dim to see anything, try peeling off one layer and testing again.
Why This Isn’t True UV Light
Smartphone LED flashlights emit light entirely in the visible spectrum, between 400 and 750 nanometers. They produce no significant ultraviolet or infrared components at all. Real UV blacklights operate around 365 nanometers, which is below what the human eye can see. Your phone physically cannot emit light at those wavelengths.
What the tape-and-marker trick actually does is filter out most visible light colors (red, green, yellow) and allow only the violet-blue portion to pass through. This puts the output somewhere around 400 to 420 nanometers, right at the boundary between visible violet and ultraviolet. That’s close enough to make certain fluorescent materials glow, but it’s far weaker and less selective than a real blacklight.
What It Can (and Can’t) Do
The DIY phone blacklight works best on materials that contain optical brighteners, which are chemicals added to absorb near-UV energy and re-emit it as visible blue-white light. You’ll find these in a surprising number of everyday items.
- White laundry. Clothes washed with certain detergents will glow noticeably under the filtered light. The brighteners in detergent are specifically designed to fluoresce, which is what makes whites look “whiter than white” in daylight. Some detergent brands fluoresce intensely while others barely react at all.
- Highlighter ink. Crack open a yellow or green highlighter, dab some ink on paper, and it will glow under your DIY light. This is one of the most dramatic demonstrations.
- Tonic water. The quinine in tonic water fluoresces a bright blue under near-UV light. Pour some into a clear glass in a dark room for a striking effect.
- White paper. Most white printer paper contains optical brighteners and will glow faintly.
- Certain fabrics. Light-colored materials, particularly whites, cyans, and light yellows, tend to show the most visible fluorescence. Darker fabrics generally won’t react.
Where this trick falls short: don’t expect it to reliably reveal security features on currency, detect pet urine stains on carpet, or check for scorpions. Those applications require a true UV source in the 365-nanometer range. Your filtered phone light simply doesn’t produce enough energy at the right wavelength to trigger those responses with any reliability.
Tips for Better Results
The room needs to be as dark as possible. This matters more than the quality of your filter. Because the violet light coming through is dim, any ambient light will wash out the fluorescence you’re trying to see. Close the blinds, turn off all lights, and give your eyes 30 seconds to adjust.
Hold the phone close to whatever you’re testing, within a few inches. The filtered light is weak, and intensity drops off fast with distance. You’ll also get better results with fresh marker ink on the tape. If the color looks patchy or uneven, redo the layer. Gaps in the coloring let white light through, which defeats the purpose of the filter.
If you want a step up without buying a full blacklight, small 395-nanometer LED flashlights are available online for a few dollars. These produce light just below the visible threshold and will outperform the phone trick significantly. They’re strong enough to reveal fluorescence in fabrics, detect optical brighteners, and produce the classic blacklight poster glow.
Is the Light Safe?
The filtered violet light from your phone poses no meaningful risk. Since the phone’s LED doesn’t produce any actual ultraviolet radiation, there’s no concern about UV exposure to your skin or eyes. Violet light in the 360 to 400 nanometer range is a normal part of sunlight that humans are exposed to daily. The intensity from a phone flashlight filtered through multiple layers of tape is far lower than what you’d encounter spending a few minutes outdoors. You’re essentially just looking at a very dim purple light.