| 摘要 |
Optical filters (1) designed in pairs, which are essentially colourless in their individual general appearance designed to take on a strong coloration when viewed together in transmission, said filters being particularly, useful for identification of counterfeit articles. The optical filters (1) utilize resonantly-coupled Fabry-Perot etalons of, an asymmetric design. The filters (1) are configured such that one of the Fabry-Perot cavities within the filter incorporates an additional multiple quarter wave thickness layer (20), said additional layer having a contrasting refractive index to that of the cavity spacer.
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