Common reveal tools include a built-in UV or decoder light; some sets also include decoder markers. A few novelty sets add play features such as a magnifying glass, a hidden compartment, or a small stamp.
Listed lengths include about 6 inches, approximately 5 inches, and approximately 5.5 inches across various items.
Descriptions warn that thermo-sensitive ink can become colorless from erasing friction or heat (noting color loss above 140°F) and that color can be restored by cooling (to about 14°F). These pens are not recommended for legal or official documents.
Several pens combine regular visible ink with a separate invisible-ink mode on the same barrel; hidden writing is typically revealed with a built-in UV or decoder light. Some spy pens also pair visible multi-color switching with an invisible-ink reveal.
Themed spy pens often feature character or decorative designs, a decoder/UV light to reveal messages, multiple ink colors or color-switching, built-in magnifiers, hidden compartments, and novelty stamps; several list compact lengths around 5–5.5 inches.
Erasable pens use thermo-sensitive gel ink that becomes clear with erasing friction or heat and can be restored by cooling. UV-reveal pens use a separate invisible ink that remains unseen until exposed to a built-in UV or decoder light.
Package quantities shown include 4, 5, 6, 8, 12, and 25.
