Ok, everybody , they detect by invisi ble fibers on the paper , under UV light it must show the colours yellow, green and red

SecureGuard™ DOCUMENT SECURITY FEATURES ..... and How They Work

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UV FIBER SECURE^TM Security Feature

SECURITY FIBER TECHNOLOGY 

Invisible Fluorescent Fibers
Invisible fibers are made visible when viewed under "black light."
[covert security feature]

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Invisible Fibers Under Normal Light

Invisible Fibers Under Black Light

http://www.highsecuritypaper.com/features--fiber.html