BACKGROUND OF THE INVENTION The present invention is directed to document security systems, and more particularly to a photocopy prevention system, i.e. method and product. Copying machines have become increasingly popular in recent years and can be found in most business offices and in public areas, such as shopping centers, hotel lobbies and the like. Because of the widespread availability of various types of copying machines, the ease with which they can be operated, and the low cost of each copy, virtually all documents are susceptible to being copied by anyone without the consent of the originator or owner. While the development of these copying machines certainly represents progress in the graphical communication field, they are often used to copy documents which may be abusive to the originator or owner. For example, the unauthorized and indiscriminate copying of materials such as news letters, poems, sheet music, drawings etc. may violate the legal rights of the publishers and/or authors denying to them the compensation to which they are entitled. Also, security can be flagrantly breached by copying material of a sensitive nature such as classified government information, industrial and commercial data, and personal correspondence of a confidential nature. The foregoing discussion of the prior art derives from Reinnagel U.S. Patent No. 4,025,673, in which a copy resistant document is described which comprises a sheet of material having an upper irregular surface comprised of a multiplicity of fibers extending upwardly from said sheet at angles less than 90° relative to the plane of said sheet. The fibers contain a coating of a colored material thereon applied by spraying at an angle corresponding to

the angle at which the document would normally be viewed for reading by the human eye. Graphical information is then imprinted on the irregular surface to produce a document easily readable at the normal human eye viewing angle but unreadable when viewed perpendicular to the plane of the document to create a useless image for reproduction by a copying machine. In U.S. Patent No. 4,522,429, confidential information is first printed, typed, or otherwise applied to paper with a color having a reflection spectral response of less than about 10% for light with a wavelength below about 600 NM. The color is sufficiently contrasting with the information to enable the information to be read by the human eye when the document is viewed under white light, but the document can not be successfully photocopied. In U.S. Patent No. 4,786,084, there is a report of a photocopy prevention film which is applied to a document to be protected that comprises either a volume holographic refraction grating or a surface holographic refraction grating. The refraction grating is bonded to a document and either causes normally scattered light to be focused towards the photoreceptors of a photocopy machine or causes light normally reflected off the document toward the receptors to be scattered away from the receptors. While the above patents disclose proposals to render documents resistant to photocopying, for one reason or another, none of these proposals provide a satisfactory solution to the problem of rendering documents resistant to photocopying , and none contemplate the advantages of the present invention. SUMMARY OF THE INVENTION Accordingly, it is a primary object of the present invention to provide copy resistant documents and methods for producing or treating original documents so as to inhibit, if not prevent, the reproduction thereof by conventional copying machines and processes.

It is another object of this invention to provide a method of producing obliterating patterns in the viewing path of the original document that are related to the angle of viewing. It is an additional object of the present invention to decrease the amount of document image light reaching the photo receptors of a photocopying machine so that no copy can be produced. It is a further object of the present invention to provide protection from photocopying and facsimile machines having an acceptance field angle as great as 70°. It is still a further object of the present invention to prevent unauthorized copying of a document while preserving high contrast and brightness for reading of the document. It is another object to provide protection from copying by photocopy machines using all visible wavelengths of light. The present invention is directed to an electrostatic copy prevention film applied to a document to be protected that comprises a micro lens array in combination with a semi-transparent mask located in the back focal plane of the array. The foregoing and other objects, advantages, and characterizing features of the present invention will become clearly apparent in the ensuing detailed description thereof, taken together with the accompanying drawings wherein like reference characters denote like parts through the various views. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagramatic view of the over laminate sheet which when applied to a document renders the document non-reproducible by an electrostatic copier operated in its normal mode.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS By specifying a miσrolens array in combination with a semi-transparent mask located in the back focal plane of the array, an overlaminate sheet was fabricated which when applied to a document rendered the document non- reproducible by an electrostatic copier operated in its normal mode. The first requirement of the microlens array is that it have a sufficiently small spatial period to adequately sample the document under treatment. In theory, the smallest the period would ever have to be for the average reader using the unaided eye would be 0.05mm or a fundamental array spatial frequency of 20 cycles/mm. This estimate of the upper limit is based on the Nyquist sampling theorem and the commonly held rule of thumb in visual optics that the highest spatial frequency discernible to the standard observer under normal viewing conditions is 10 cycles/mm. The sampling theorem holds that to preserve a signal with no loss of information, the sampling frequency must equal or exceed twice the maximum frequency of the signal. In practice, type pitch of 10 characters per inch (cpi) is adequately sampled with an array of 4 cycles/mm, i.e. with individual microlens diameters of 0.25mm (approximately 0.01") . The second requirement of the microlens array is that its back focal plane coincide with the back surface of the substrate in which it is formed. Thus, light sources, that subtend a small angle as viewed from the lens array, are brought to a sharp point image (or line image if the microlenses are cylindrical) on the back surface of the substrate. The third requirement of the microlens elements is that their focal length be extremely short, i.e. preferably IX to 1.5X of their diameter. The short focal length is necessary in order to keep the overlaminate

sheet as thin and flexible as possible. Specifically, to produce an overlaminating film of 1 mil to 1.5 mil thickness, requires that the microlenses also have diameters of 1 mil to 1.5 mil which results in spatial frequencies of 30 cycles/mm to 40 cycles/mm, an order of magnitude greater than demanded by character sampling considerations. The short focal length is also necessary in order to achieve wide acceptance angle operation. The need for wide acceptance angle operation will be clear when the function of the semi-transparent mask is described. Finally, the short focal length implies a large numerical aperture which in turn permits efficient optical coupling. One function of the mask layer, which is applied to the back surface of the microlens array, is to present a controlled image to any optical system (including a human observer) whose optical axis is set normal to the array surface. Within the mask layer the controlled image may either be continuous or sampled and may consist of a random noise pattern, a decorative pattern, a warning notice that the treated document is not to be copied, or virtually any preselected image a user might choose. If the controlled image is sampled, then each sample element must be registered on the optical axis of its corresponding microlens element. A more detailed description of the sampled image requirements will be given later. Since the optical axis of commercial electrostatic copy machines and facsimile machines are set normal to the surface of the document being copied, such machines will only copy the controlled image of the overlaminate sheet and not the protected document. The second function of the semi-transparent mask layer is to permit reading of the protected document at specific view angles. This is accomplished by establishing clear windows in the mask pattern; one window for each corresponding microlens element. In the case of a

lenticular array (i.e. cylindrical microlenses), the windows take the form of slits whose width is preferably between about 20% and about 30% of the microlens width and whose length is limited only by the size of the overlaminate sheet _* In the case of a two dimensional array (e.g. hexagonal), the windows take the form approximately of an annulus whose outer diameter approximates the microlens diameter and whose inner diameter preferably is between about 84% and about 90% of the microlens diameter. The window does not have to occupy the entire annulus area but must be restricted to the annulus region. The view angle is controlled by the distance the windows are displaced from the optical axis in the back focal plane of the microlenses. In addition to the microlens array on the top surface and the mask pattern with windows on the bottom surface, the overlaminate sheet must also have an adhesive layer on the very bottom surface. The adhesive can be either pressure sensitive or thermally activated. The latter is the preferred approach because it will lead to a thinner finished product and one that is most like existing commercial office laminating systems. In fact, by packaging the overlaminate material described in this disclosure in roll form of appropriate widths, General Binding Corporation (GBC) laminators could be used to prepare protected documents. The only laminate modification that might be required would be in the thermostatic control setting. Since the microlens structure is replicated in a thermoplastic film under controlled heat and pressure, care must be taken not to exceed the thermoplastic flow temperature of the array, otherwise the microlens structure could be erased. The strategy is to use a high flow temperature thermoplastic for the array and a low temperature thermally activated adhesive.

Turning now to the Figures, Figure 1 illustrates a cross-section of a laminated document in accordance with the present invention. The microlens array with the windowed (10) mask pattern is permanently applied over the printed surface (12) of the document. Accordingly, the mask pattern with the windows in combination with the microlens array (11) restricts the document viewing angle to the region as defined at (13). The mask pattern, which is applied to the back surface of the microlens array, can be seen at (14), which appears at or near normal incidence to the document. From the foregoing, it is apparent that the present invention fully accomplishes its intended objects and provides methods for both treating existing documents and producing original documents in such a manner as to at least inhibit production of intelligible copies therefrom by conventional copying machines. Preferred forms of this invention having been disclosed in detail, it is to be understood that modifications thereof can be made without departing from the broad spirit and scope of this invention.