Holograms for security markings

DETAILED DESCRIPTION According to a first aspect of this invention there is provided a hologram for security markings comprising a laser transmission hologram known per se formed as a surface relief pattern embossed into an embossable material, said laser transmission hologram being formed by sequential layers of a carrier layer, an embossable thermo-plastic lacquer into which the hologram is formed, and a layer for adhesively bonding the hologram to a substrate, characterised in that a thin metallic layer is coated onto said thermo-plastic lacquer whereby the laser transmission characteristics of the hologram are transformed into laser reflection characteristics capable of reflecting light from a laser.
Preferably, the metallic layer is about 500 angstroms thick.
Advantageously, the hologram is mounted on a substrate of an item required to bear the security marking defined by the hologram.
In a feature of the invention there is provided an apparatus for reading light reflected from a hologram, in accordance with the first aspect of this invention, comprising a laser light source for projecting a laser light beam onto the hologram to be read, and means located on the same side as the laser light source for recognising a real image of the hologram.
Advantageously, means are provided for moving the hologram with respect to the laser light beam and for activating said laser.
In one embodiment, the hologram comprises an information image and a triggering image.
Advantageously, a plurality of triggering images are overlaid on respective information images, the spacing between the triggering images being at least equal to the diameter of the laser beam.
In said one embodiment, preferably the information image is a series of light reflective portions and light non-reflective portions, together forming a binary code, the triggering image being formed to indicate the presence of a light reflective portion and so as to reflect the laser light beam at a different angle to that reflected by the information image, there being provided a light detector for receiving the information image and a further light detector for receiving the triggering image, whereby the light detector is activated only when triggered by a signal from the further light detector indicating the presence of a light reflective portion