<title>Security enhancement of holograms with interference coatings</title>

Abstract

Most people in developed nations are familiar with the hologram. It appears on VISA, Discover and MasterCard credit cards. It is doubtful however, that a great number of people could identify the respective images. The public tends to confirm the presence of such a security device rather than verifying the actual image. As noted by Steve McGrew, "It is very rare that security holograms are subjected to close inspection in actual use" .Thus,there is now the opportunity for the use of poor counterfeits or the substitution of commercial holograms for the genuine security hologram. In an effort to thwart counterfeiters, the hologram industry has resorted to more complex products with multiple images as the device is rotated. These enhanced images provide the observer with a high level of "flash" or aesthetic appeal. Unfortunately, this added complexity does not confer added security because this complex imagery is hard to communicate and recollection of such imagery is difficult, if not impossible, to remember2 Every type of first order diffraction structure, including conventional holograms and grating images, has a major shortcoming even if encapsulated in a rigid plastic. Under an overcast sky or other diffuse illumination all diffraction orders expand and overlap so that the diffraction colors are lost. Under such viewing conditions all such devices look silvery or pastel at best. An additional hurdle that security holograms must overcome, to be truly secure, is the ease of which holograms can be counterfeited. One step and two step optical copying, direct mechanical copying and even re-origination have been extensively discussed over the Internet.3 Ways to counteract all these methods have been discussed but the conclusion was that none of the countermeasures, taken alone, was an effective deterrent. It has been discovered that by combining diffraction effects with thin film interference effects, a security device can be made with new enhanced features that allow for ready identification by the average person while still preserving complex optical patterns. This new security device, using LightGateTM technology, also appears to overcome the cited disadvantages of holographic technology.