Method of authenticating watermarked paper

Details Method of authenticating watermarked paper Method of authenticating watermarked paper

1997-06-03

1998-12-29

Rosenberger, Richard A.

Optics: measuring and testing

Document pattern analysis or verification

356 51, 2503581, 250556, G06K 974

Patent

active

058546731

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a method of authenticating watermarked paper. Watermarks are have been used as a security feature for several centuries. They provide an easily identifiable image to the unaided eye of a viewer. Their value as a security feature derives from the fact that a watermark is difficult to either fabricate or simulate. A true watermark must be formed during the manufacture of the paper and requires large and expensive facilities not available to the small-scale counterfeiter Simulation by some other means that looks convincing under close scrutiny is also difficult.
Watermark inspection is generally performed by a human operator, often in a rushed and haphazard manner with no equipment to assist viewing or authentication of the image. Watermarks are often partially obscured by printing on one or both sides of the document making authentication more difficult. The anti-counterfeiting potential of a watermark is therefore not fully realised in practice.
A watermark generally comprises areas of high and low density paper of uniform thickness which are introduced during the paper making process. The density variations may be substantial typically spanning a factor of two. The watermark may incorporate grey-scale and the best examples have significant artistic merit. The spatial resolution is such that feature sizes smaller that 1 mm can be readily resolved.
Watermarks were originally developed to be viewed by the unaided human eye. They are generally not easily seen in reflected light but are readily visible in transmitted light. The most obvious route to machine reading watermarks is therefore conventional optical imaging. For example the watermark may be illuminated with white light or light of a specific wavelength and imaged using a silicon charge coupled device (CCD) camera. A low cost authentication system can readily be designed on the basis of such optical imaging but, if the aim is to obtain a high contrast image of the watermark unobscured by printed matter, then its usefulness would be restricted. Furthermore, any authentication process based on the use of transmitted light must contend with ease with which the counterfeiter can use simple printing technology to simulate a watermark and thereby defeat an authentication system.
Optical imaging may be extended to imaging with wavelengths outside the visible spectrum in the UV or the IR. Although good image contrast may be obtained using a broad-band UV source, any printed matter on the item is included in the image and complicates its analysis. In the near-IR, dye-based printing inks are transparent and an image of the watermark without any printed features may be obtained. However carbon-containing inks do absorb in this waveband and confuse the analysis.
Banknote authentication systems are known which incorporate infra-red sources to measure the optical transmission coefficient of the notes in the infra-red part of the spectrum. Swedish Patent Number SE 451 041 describes a system in which infra-red radiation of wavelength 900-1600 nm is transmitted through a banknote and the intensity of the transmitted radiation is measured by infra-red detectors. Norwegian Patent No. 922387 describes a system for obtaining a two-dimensional image of a watermark based on infra-red transmission measurements of radiation of wavelength between 2 and 12 .mu.m.
One standard method of revealing watermarks in a research environment is to take a radiograph using an X-ray plate and a beta radiation source. This method forms an image based on the local variations in density and hence radiation absortion power. The layers of ink being relatively thin, have little or no visible effect on the image. The imaging process has very high resolution and yields an image with impressive clarity. This approach is not viable for wide-spread use due to radiation hazards and due to the time taken to process the plate or due to the high cost of an alternative solid state large area X-ray detector.
International Patent Application Number PCT/NO86/00052 having an Internation

REFERENCES:
patent: 4296326 (1981-10-01), Haslop et al.
patent: 4481418 (1984-11-01), Vanzetti et al.
patent: 4608598 (1986-08-01), Murakami et al.

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