Magnetic detection of security articles
US5992741A · kind A · utility
Inventors
Key dates
| Filing date | Dec 10, 1997 |
| Grant date | Nov 30, 1999 |
| Priority date | — |
| Expiry date | Dec 10, 2017 |
Classification
- Technology area (CPC G)Physics
- CPC primaryG07F7/086
- WIPO fieldControl
- WIPO sectorInstruments
Abstract
A method for detecting the presence of magnetic elongated particles in a substrate the base material of which has magnetic properties substantially differing from the corresponding magnetic properties of the elongated particles. The elongated particles have such a long and thin form that their demagnetisation factor N is smaller than 1/250, they have a diameter smaller than 30 micrometer and a magnetic saturation field greater than 100 A/m. The method comprising the following steps: PA1 (a) emitting an electromagnetic source signal of one or more particular base frequencies to the substrate so that any present magnetic elongated particles go into a non-linear part of their B-H curve for at least part of a cycle of the source signal; PA1 (b) detecting an electromagnetic detection signal emanating from the substrate; PA1 (c) testing the detection signal for the presence of particular higher harmonics of the base frequencies or of any linear combination of the base frequencies or of the harmonics, the particular higher harmonics being indicative of the presence of the magnetic elongated particles.
Source: USPTO / EPO open patent data. Objective bibliographic and citation counts.