How a passport stamp can glow
Fluorescent ink changes ultraviolet light into visible light. Its pigments absorb energy from UV rays and release some of it as light the human eye can see. In ordinary daylight, the ink may blend in with the stamp; under a UV lamp, it can appear bright against the paper.
The glow is not the ink producing light from nowhere. It is responding to a kind of light that is usually invisible to us. Different fluorescent inks can produce different colors or patterns, depending on their ingredients and how they are printed.
A second detail for inspection
Under UV light, an inspector can examine features that are difficult to see in normal lighting. A stamp’s fluorescent response gives another point to compare with what is expected, alongside visible details such as its shape and placement. An unexpected glow—or a missing one—may prompt a closer look, though the UV check is one clue, not a verdict on its own.
That extra layer matters because a mark that looks convincing in daylight may not behave the same way under ultraviolet light. The ordinary-looking stamp can hold a detail that only appears when an inspector changes the light.
