24kupi UV Pen with Invisible Ink Review — Does a $3 Pen Actually Work?
The UV Pen is the cheapest product on 24kupi at just $3. It's a pen that writes with invisible ink — the text is only visible when you shine the built-in UV LED on it. It's the most low-tech cheating device they sell. I bought a 3-pack bundle ($8) to test.
How It Works
One end of the pen writes with invisible ink (clear, leaves no visible mark on paper under normal light). The other end has a small UV LED light. Press the button, shine it on the paper, and your notes appear in a glowing blue-purple color. Release the button and the text becomes invisible again.
The Idea
The intended use case: before the exam, you write formulas and key facts on your arms, hands, desk surface, or a blank piece of paper using the invisible ink. During the exam, you quickly shine the UV light to read your notes, then turn it off. Nobody can see the writing without UV.
Does It Actually Work?
Technically, yes. The invisible ink is genuinely invisible under normal room lighting. The UV LED does make the text glow and become readable. In a dark room, the effect is dramatic.
However, in a well-lit exam room (fluorescent overhead lights), the UV glow is much weaker. You have to hold the light very close to the paper (within 2-3 inches) and the text is faint. Reading more than a few words requires holding the UV light steady while reading, which looks suspicious.
Practical Problems
- The UV light is visible. The LED emits a distinct purple glow that anyone sitting nearby can see. In a quiet exam room, repeatedly shining a purple light at your desk is not subtle.
- Writing on skin fades. Invisible ink on paper lasts for months. On skin, it fades within a few hours — especially if your hands are sweaty (which they will be during an exam). By the second hour, my arm notes were barely readable.
- You can't write much. The ink is clear, so you can't see what you're writing as you write it. Without visual feedback, your handwriting is messy and overlapping. Fitting useful notes in a small area is difficult.
- Paper wrinkles. Even though the ink is invisible, pressing a pen on paper leaves indentations. A proctor who picks up your "blank" paper might notice the texture of writing under their fingers.
Compared to Just Writing Small Notes
Here's the irony: a tiny piece of paper with small handwritten notes (the classic cheat sheet) is probably more effective than invisible ink. You can read a cheat sheet without a special light, you can write more legibly since you can see what you're writing, and it doesn't emit a glowing purple beacon.
Pros
- Incredibly cheap ($3 per pen)
- No electronics — passes through any metal detector
- No batteries to worry about (the UV LED runs on internal button cell)
- The science works — invisible ink is genuinely invisible
Cons
- UV light is visibly purple — not subtle
- Ink fades on skin within hours
- Can't see what you're writing (clear ink)
- Paper shows writing indentations
- Weak glow in bright exam rooms
- A regular cheat sheet is honestly more practical
Final Verdict: 3/10
For $3, the UV pen is a fun novelty that technically works. But in practice, the visible purple glow, the fading ink on skin, and the difficulty of writing blind make it less practical than a simple handwritten cheat sheet. It's the kind of thing that sounds clever in theory and falls apart in a real exam room. Buy it for fun, not for your finals.