Polarizing Filter
An optical device which transforms unpolarized light passing through it into polarized light. It is used as an analyzer for examination of light reflected from ink or thin-film coating. It blocks light rays of a specific polarization direction and at the same time it lets light rays of other polarization directions pass. A polarizer is placed between the examiner and examined object and the intensity of transmitted light is observed. If the examined light becomes dark, it means that the object reflects polarized light. If the intensity of the transmitted light does not change or changes slightly, it means that the examined light is not polarized.
A polarizer helps to determine if an ink or polymer coating contains liquid crystal pigments which polarize the light passing through them.
c
d
e
Operation principle of a polarizing filter. Scheme:
a — the light is not polarized: the polarizer lets the light rays pass; b — the light is polarized: the polarizer blocks linearly polarized light; c — circular polarizer for document authenticity verification (R. L. van Renesse. Optical Document Security. Boston | London. 2005. p.351), front side; d — the same image, reverse side; e — STEP element analysis: the light is polarized, the polarizer blocks the light reflected from the ink layer (dark window on the right). Conclusion: the ink contains optically active substances — liquid crystals which circularly polarize reflected light