Why a line you can feel is worth more than a line you can only see

Almost every argument about document security eventually reduces to one question: what can a copy not carry? Colour copies. Fine detail copies, given enough resolution. Layout copies trivially. Relief does not, because relief is not information about a surface — it is the surface.

This is why intaglio has outlived every technology that was supposed to replace it. When a plate is inked, wiped and pressed into damp paper under several tonnes, the paper is driven into the engraved lines and comes out carrying ink on a ridge. The ridge is mechanical. A scanner records its appearance and reproduces a flat image of a shadow.

The examiner's thumb

What matters practically is that nobody has to be taught this. Hand someone an engraved certificate and they will run a thumb across it without being asked, because texture is one of the things hands do. The check happens whether or not the person knows they are checking.

That is a rare property. Most security features require either equipment or instruction. A feature that recruits a reflex is worth several that require attention.