Optics: measuring and testing – Document pattern analysis or verification
Patent
1997-07-02
2000-08-08
Rosenberger, Richard A.
Optics: measuring and testing
Document pattern analysis or verification
356364, B42D 1500
Patent
active
061009679
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a method of optically monitoring a body having a reflective surface and a covert optical discontinuity produced on the surface.
It is often desirable to apply a covert mark to an object, for example, in order to identify goods or the like, which can be useful in anti-counterfeiting measures or which can allow for the application of a code specific to a container, thus facilitating product tracking.
Many types of covert marking systems have been proposed. It is known for example to mark objects using laser radiation. U.S. Pat. No. 4,758,703 describes a method of covertly encoding a microscopically visible pattern on a surface of an object in which a beam of unfocussed laser radiation is passed through a mesh to produce the desired pattern, the intensity of the laser beam being carefully controlled so that the pattern is barely etched onto the surface and remains visible to the eye. U.S. Pat. No. 4,769,310 describes a method of marking ceramic materials, glazes, glass ceramics and glasses that contain at least one radiation-sensitive additive in which a laser beam is focussed on to the surface of the material to be marked so as to induce a colour change within the irradiated area.
GB2247677 discloses a method and apparatus for providing a body of material with a sub-surface mark in the form of an area of increased opacity to electromagnetic radiation. The method comprises directing a high energy density beam to which the material is transparent and bringing the beam to focus at a location spaced from the surface and within the body so as to cause localized ionisation of the material. The apparatus includes a laser and provides means to move the focus of the beam relative to the body so as to enable the mark to be of a predetermined shape.
Such covert marking systems are generally only effective for glass or ceramic based products.
OBJECT OF THE INVENTION
Therefore, it is the purpose of the present invention to alleviate such difficulties and to provide a method and apparatus for optically monitoring a body having a reflective surface, and an optical discontinuity produced thereon.
SUMMARY OF THE INVENTION
Thus, according to a first aspect of the invention there is provided a method of optically monitoring a body having a reflective surface and a coating layer of material transparent to visible light present on the surface, which method comprises: producing the layer with a covert optical discontinuity in an imagewise distribution; illuminating the imagewise distribution with polarized light; and viewing the imagewise distribution through a polarized filter.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In a first embodiment of the present invention, the optical discontinuity is produced by directing a high energy laser beam at a predetermined area of the layer such that the irradiated area is detectable by the viewing. The high energy laser beam is typically such that it causes photoablation of the transparent material, but no substantial change to the reflective surface. Power densities suitable for causing photoablation are known in the art (typically 10.sup.3 W/cm.sup.3 to 10.sup.11 W/cm.sup.3).
In one preferred mode of operation of the first embodiment of the invention, the beam has a focus which is movable relative to the transparent material. In an alternative mode of operation the beam may be pulsed through a mask such that the entire optical discontinuity is formed simultaneously.
In a second embodiment of the present invention the optical discontinuity is produced by printing the transparent material in the imagewise distribution on the surface such that the coating layer is itself discontinuous.
Thus, advantageously, a covert mark may be produced on a reflective surface, which surface itself can comprise the product to be identified or alternatively which can be on a product to be identified. The mark can be easily viewed through a filter upon illumination with polarized light.
The term "transparent" as used herein with reference to
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patent: 4519064 (1985-05-01), Takagi et al.
patent: 4544836 (1985-10-01), Galvin et al.
patent: 5284364 (1994-02-01), Jain
patent: 5543608 (1996-08-01), Rantalainer
Farrer Richard Mark
Hood Barry Alan
Gallagher Thomas A.
Gordon David P.
Jacobson David S.
Rosenberger Richard A.
SLS Biophile Limited
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