What Glows Under a Black Light? Halloween Ideas
Our customer asked:
What actually glows under a black light, and how can I use one for Halloween?
TL;DR
Tonic water glows blue. Highlighter ink, neon costumes, white fabric washed in regular detergent, UV face paint and petroleum jelly all light up under UV. Glow-in-the-dark props charge in seconds under a UV flashlight. A handheld UV flashlight is best for spot effects: a glowing prop, a hidden message, a "reveal" in a haunted hallway. For lighting a whole room, you want a UV tube or bar instead. One rule: never shine it into anyone's eyes.
I run product innovation at LUXPRO, and UV is the one category where customers teach me more than I teach them. Most of our UV lights get bought for pet stains and scorpions. Then October comes around, and people start writing to us about haunted hallways and glowing punch bowls. Here is what our team and I have found works.
What glows under a black light
| Item | Glow | Halloween idea |
|---|---|---|
| Tonic water | Bright blue (from the quinine) | Glowing punch or a "potion" jar on the snack table |
| Highlighter ink | Vivid yellow, green or pink | Draw on balloons or dark paper; snap a highlighter's ink tube into water for glowing jars |
| White clothing washed in normal detergent | Blue-white | Ghost costumes, skeleton prints and sheet ghosts |
| Neon and "UV-reactive" face paint | Intense neon colors | Skull faces and glowing costume details |
| Glow-in-the-dark props | Their own glow, charged by UV | Charge them with UV right before guests arrive (see below) |
| Petroleum jelly | Pale blue-white | Smear a "slime trail" on a dark surface for a haunted hallway |
| Invisible UV ink pens | Shows only under UV | Hidden messages and clues for a kids' scavenger hunt |
| Vitamin B2 dissolved in water | Yellow-green | Another glowing jar color, for decoration only |
What does not glow (Halloween edition)
- Fake blood. Most stage blood absorbs UV and looks dark, just like real blood. The glowing blood on TV crime shows comes from a chemical spray, not a flashlight. If you want glowing gore, look for fake blood labeled UV-reactive.
- Most black and dark fabrics. That is good news: a black backdrop makes everything else pop.
- A phone flashlight with purple tape over it. It will not make any of this glow properly. Here is why.
Flashlight or black light tube?
This is where people get disappointed, so I will be straight with you. A handheld UV flashlight lights a patch a few feet across. It is perfect for spot effects:
- Revealing a hidden message on a door as guests walk past
- Making one prop in a dark corner glow
- A "flashlight tour" of a haunted hallway where kids take turns finding glowing clues
- Charging glow-in-the-dark decorations
I will be honest: it is not the right tool for washing a whole room in UV. For that, use a UV tube or LED bar mounted high on the wall. What I recommend is both: a bar for the room and a flashlight for the surprises.
The trick for glow-in-the-dark props
Here is a trick I love. Glow-in-the-dark paint and plastic store light and release it slowly. UV charges that pigment far faster than ordinary room light. One of our customers uses our UV light to charge glow fishing lures at sea, and told us deck lights were "not as effective as true UV charging." The same physics works on plastic skeletons, and it is my favorite Halloween use for one of our UV lights. Sweep a UV flashlight over your glow props for ten to twenty seconds just before trick-or-treaters arrive, and they will be at their brightest when people walk up.
Which light to use
For a kids' scavenger hunt or hidden messages, the LP9UV is small, has nine 385 nm UV LEDs and a wrist strap, runs on three AAA batteries, and costs $9.99. For lighting up a porch prop or a corner of the room, the LP32UV has 32 UV LEDs across a wider head for a broader wash of UV, for $16.99.
Once Halloween is over, the same light finds pet stains, checks cash and spots scorpions. Here is the full list of everyday uses: What is a black light flashlight used for? For our Halloween flashlight roundup, see 8 Best Flashlights for Halloween.
The one safety rule
Never shine a UV light into anyone's eyes. A UV beam looks dim, so people do not blink or look away the way they would with a bright white light. That makes it easy for kids to stare into it, or to shine it at a friend's face without realizing it is a problem. I am happy for kids to use it for the hunt with an adult nearby, and keep it pointed at walls, props and the floor. More detail here: Are UV flashlights safe?
Frequently Asked Questions
Q: Why does tonic water glow under a black light?
It contains quinine, which absorbs UV light and gives it off as visible blue light. Diet and regular tonic both work.
Q: Will my white costume glow under UV?
Probably, and it surprises a lot of our customers. Most laundry detergents contain optical brighteners that glow blue-white under UV. A costume washed in regular detergent usually glows more than a brand-new one.
Q: Do glow sticks need a black light?
No. Glow sticks make their own light through a chemical reaction. A black light can make the plastic tube itself look brighter, but it does not recharge a glow stick.
Q: What's the best black light for a Halloween party?
I recommend a UV tube or bar for washing the room, and a UV flashlight like ours for spot effects and reveals. A flashlight alone will not light up a whole room.