PAPER RESISTANT TO PHOTOCOPYING AND/OR TRANSMISSION BY TELEFACSIMILE

Abstract: Paper is provided with resistance to photocopying or tran smission by telefacsimile by spatial spectral modulation of the paper reflectance at a specific single or preferably multiple frequencies. Such paper has a coloured pattern of at least two colours repeating in at least one dimension of a face of a paper with at least one frequency in the range of from about 0.5 to about 50 times per cm. The colours contrast with black or similar dark colour to permit black or similar dark coloured information to be visibly readable when applied to the coloured pattern. The colours also cooperate with such information to provide a document resistant to photocopying and transmission by telefacsimile.

Description: This invention relates to copy-proof security paper, that is to say paper which, when carrying information in a conventional black or similar dark colour, cannot be readily photocopied or transmitted by telefacsimile in a visually readable manner. The present day availability of improved photocopiers has increased the problem of rendering documents or portions thereof resistant to photocopying in a readable manner. Anti-photocopying paper which is successful in preventing visually readable photocopying by most present day photocopiers is described in U.S. Patents 4,522,429 and 4,632,429. U.S. Patent 4,522,429 teaches the use of anti-photocopying paper having a colour with a reflection spectral response of less than about 10% for light with a wavelength below about 600 millimicrons and yet which is sufficiently visually contrasting with information, when such information is typed thereon or otherwise applied thereto, to enable such information to be read by the human eye when the paper is viewed under white light. U.S. Patent 4,632,429 teaches the use of anti-photocopying paper with a front face having a colour with a reflection spectral response which is effectively zero for light with a wavelength below about 625 millimicrons and less than about 1% up to about 1,000 millimicrons so as to render the paper substantially incapable of being photocopied in an information readable manner, after substantially non-translucent information has been typed or otherwise applied to the front face, the paper being capable of transmitting visible light from a rear face to the front face to cause sufficient contrast between the substantially non-translucent information and the transmitted light to enable the information to be read by a human eye viewing the front face of the paper when visible light is transmitted through the paper from the rear face to the front face thereof. Security paper of the type described in the above mentioned patents satisfactorily fulfils most present day needs, and represents a very significant improvement over prior proposals which were not successful in practice. Such paper is also resistant to transmission by telefacsimile. However, the increasing photocopying ability of new generation photocopiers presents a need for still further improved anti-photocopying paper. Some photocopiers which are now becoming available are capable of wider spectral response and improved resolution between the information and the information background compared to existing photocopiers. There is also a need for paper which is more resistant to transmission of information thereon by telefacsimile. Security paper of the type disclosed in the above patents consists of paper having a uniform single colour applied across the surface of the paper, or at least across that area of the paper containing the information to be protected, or to which that information is to be applied. An alternative, multi-colour approach has also been adopted. For example, in JP 60-17778(A), there is disclosed a security paper having a ground colour surface over which is printed in a different colour a continuous grid composed of vertical and horizontal lines and incorporating also a "tortoise" shell-shaped pattern, the combined effect of which is to prevent photocopiers discriminating between information printed thereon in black ink and the background. That information, although visible to the eye, is not reproducible by the photocopier. Also, in one of their Technical Disclosure Bulletins, Vol.17, No.12, May 1975, p 3786 IBM have disclosed a security paper preprinted with alphabetical characters and numbers, or with polka dots or random markings in each of the three primary colours: red, blue and green. That is alleged to protect information printed thereon in black ink from photocopying by making photocopies produced therefrom illegible, or at least difficult to read. The present invention seeks to improve upon the multicolour approach by providing a security paper preprinted over all or part of its surface with a coloured pattern of at least two colours repeating across the face of the paper in at least one dimension at a spatial frequency of from 0.5 to 50 times per cm, that pattern being made up of at least one pair of colours having substantially the same spectral response profile over a wavelength range of from about 400 to 700 nm, but with one of said pair of colours having a lower spectral response than the other over the whole of that range, that one colour having a spectral response over the wavelength range 480 to 580 nm that is substantially the same as that of the black or other dark colour in which the information to be protected is, or is to be displayed on that part of the sheet. When the paper is primarily intended for use with textual information, the coloured pattern may repeat with a spatial frequency in the range of from about 2 to about 25 times per cm, preferably from about 4 to about 10 times per cm. When the paper is primarily intended for graphical or pictorial information, the coloured pattern may repeat with a spatial frequency in the range of from about 0.5 to about 10 times per cm, preferably from about 1 to about 5 times per cm. The coloured pattern may repeat with multiple spatial frequencies including a higher spatial frequency comparable to a higher Fourier spatial frequency of information of a predetermined kind and a lower spatial frequency comparable to a lower Fourier spatial frequency of such information. "Comparable" in this context means up to three times greater or smaller. When the information is textual, the higher spatial frequency may be in the range of from about 40 to about 50 times per cm, and the lower spatial frequency may be in the range of from about 2 to about 5 times per cm. When the information is graphical or pictorial, the higher spatial frequency may be in the range of from about 10 to about 25 times per cm, preferably from about 15 to about 25 times per cm, and the lower spatial frequency may be in the range of from about 0.5 to about 5 times per cm, preferably from about 0.5 to about 2 times per cm. One of the colours of said pair of colours may have a reflection spectral response with a minimum of about 5% at lower visible wavelengths of about 400 to 500 nm rising to about 10% at a wavelength of about 580 nanometers, and then rising to a maximum of about 20% at a wavelength of about 700 nanometers, whilst the other has a reflection spectral response with a minimum of about 4% at lower visible wavelengths of about 400 to 500 nm, rising to about 6% at a wavelength of about 580 nanometers, and then rising to a maximum of about 12% at a wavelength of about 700 nanometers. Advantageously, the reflection spectral responses of said colours fall back to their respective minima at wavelengths above about 700 nanometers. Alternatively or additionally, one of the colours of said pair of colours may have a reflection spectral response with a maximum of about 20% at lower visible wavelengths of about 400 nm, falling to about 10% at a wavelength of about 480 nanometers, and falling to a substantially constant minimum of about 8% over the range 500 to 700 nm, whilst the other has a reflection spectral response with a maximum of about 12% at lower visible wavelengths of about 400 nm, falling to about 6% at a wavelength of about 480 nanometers, and falling to a substantially constant minimum of about 5% over the range 500 to 700 nm. Advantageously, the reflection spectral responses of said colours fall back to their respective minima at wavelengths below about 400 nanometers. The coloured pattern may include an additional colour of relatively high reflectivity repeating in at least one dimension on a face of the paper with at least one spatial frequency in the range of from about 0.5 to about 50 times per cm to improve readability of information on the paper with the paper still being resistant to photocopying. Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings of which:- Figure 1 is a plan view of a sheet of paper with a front face having a first colour A. Figure 2 is a similar view having second colour B applied to form a coloured pattern of colours A and B in accordance with one embodiment of the invention. Figure 3 is a graph showing the reflection spectral response of the two colours A and B, and also showing the average spectral response of the human eye and a typical spectral response of a photocopier. Figure 4 is a graph similar to Figure 3 but showing reflection spectral responses of an alternative pair of colours C and D in accordance with another embodiment. Figure 5 is a graph similar to Figure 3 but also showing the reflection spectral responses of colours C and D. Figure 6 is a graph similar to Figure 3 but also showing the reflection spectral response of black information and a highly reflective colour W, and Figure 7 is a graph similar to Figure 5 but showing another embodiment. Referring to the accompanying drawings, Figure 1 shows a top face of a sheet of paper which has been coloured uniformly with a colour A during or after manufacture, the colour A having the spectral response indicated as line A in Figure 3. It will be noted that the reflection spectral response is a minimum (R min A) of about 5% at a wavelength of about 400 nanometers (millimicrons), rises gradually to about 10% at about 580 nanometers, such a wavelength being known as the cut off wavelength, and then rises to a maximum (R max A) of about 20% at a wavelength of about 700 nanometers. The sheet face coloured A is then overprinted with another colour B in a grid-like configuration, using an appropriately configured printing plate, to provide a coloured grid-like pattern in which the pair of colours A and B alternate in both dimensions of the face of the paper. Colour B is the result of overprinting colour A with another colour, the other colour being such as to provide colour B with a reflection spectral response indicated by line B in Figure 3. Colours A and B have substantially the same spectral response profile but the reflection spectral response colour B is less than that of colour A, with a minimum (R min B) of about 4% for a wavelength of about 400 nanometers, rising to about 6% at a wavelength of about 580 nanometers, and maximum (R max B) of about 12% at about 700 nanometers. The average spectral response of the human eye is shown by the line E, and the reflection spectral response of a typical photocopier is shown by the line PC. In this embodiment, the spatial frequency of the pattern repeats is approximately the same in both directions of the coloured face of the paper and is approximately 10 per cm. Figures 4 and 5 show the reflection spectral responses of another pair of colours C and D, with colours C and D having substantially the same spectral response profile, but with the spectral response of D being less than C. The spectral response of colour C is a maximum (R max C) of about 20% at low visible wavelengths of about 400 nm, falling to about 10% at a wavelength of about 480 nanometers and a minimum (R min C) of about 8% at higher visible wavelengths. Colour D has a reflection spectral response with a maximum (R max D) of about 12% at lower visible wavelengths of about 400 nm, falling to about 6% at about 480 nanometers and a minimum (R min D) of about 5% at higher visible wavelengths. The colour pattern may comprise changes from colour C to colour D, but may also include changes from colour A to colour B to colour C and to colour D in each pattern, with such a pattern being produced for example by overprinting with successive plates, with each plate being appropriately displaced to provide the required different positioning of different colours in the pattern. The coloured pattern may in fact change from one colour to another in any desired manner. Also, if desired, each colour may be built up by the application of more than one layer of the same colour. The production of the coloured pattern can thus be carried out in a multi-colour printing facility. It will be appreciated that this is essentially a multi-layer optical filtering technique with each layer providing a different spectral and spatial characteristic. The superposition of the required number of layers thus results in the overall spectral characteristics shown in Figure 5. Figure 6 shows the reflection spectral response ( R Black) of typical black information I printed or otherwise applied to paper, R Black being about 6% across the entire spectral range. When an attempt is made to photocopy such a document with a photocopier having a typical response PC, the photocopier will perceive enough contrast in those portions of information I which fall on background of colour A but will fail to "see" any contrast where portions of information I fall on background of colour B and will therefore fail to reproduce such portions of information I. The photocopy thus obtained will show at least traces of information I in the form of a scrambled and unreadable version of information I. The scrambling of the photocopy will be effective over a large range of photocopiers which may have upper cut off wavelengths somewhat beyond 600 nanometers ( λ C2 ). However, for photocopiers with upper cut off wavelengths substantially beyond 600 nanometers, for example up to 700 nanometers or beyond in the infrared range, paper with a colour pattern of colours C and D is preferable, such photocopiers typically having colour cut off wavelengths around 400 nanometers ( λ C1 ). Thus, anti-photocopying paper with a colour pattern comprising permutations of colour A, B, C and D is preferable because it provides anti-photocopying resistance to a wide range of photocopiers. The black information I is visible to the human eye because of the contrast between the colour of information I and colours A, B, C and D within the range of the eye sensitivity curve E at either the long wavelength or short wavelength ends of curve E. It has been observed that the visibility to the human eye, i.e. the readability, of information I on the original document can be dramatically improved by superimposing on any anti-photocopying background a spectral colour modulation or pattern, at frequencies similar to those previously mentioned, with a highly reflective colour W such as light green, yellow or even white with a reflectance R W of the order of 90% (see Figure 6). Although those portions of information I which fall upon background of colour W will be easily reproduced by a photocopier, the spectral modulation of colour W will also be reproduced with a resultant further scrambling effect. However, the presence of the highly reflective pattern of colour W will increase the average reflectivity of the paper and this will make the paper appear lighter or whiter. This is thus a very important step in achieving the desirable goal of producing an anti-photocopying paper which is as light coloured as possible. According to a further embodiment as shown in Figure 7, colours A and B are modified so that their reflectance falls to the R min A and R min B level at a wavelength of about 700 nanometers and beyond. Colours C and D are modified so that their reflectance falls to the R min C and R min D level at a wavelength of about 400 nanometers and lower. Resistance to photocopying in accordance with the invention is accordingly widened even further to cover photocopiers which operate in the infrared or ultraviolet regions of the spectrum. In other words, λ C2 is shifted in the direction of 700 nanometers and beyond, and λ C1 is shifted in the direction of 400 nanometers and lower. The coloured pattern may of course only be applied to a portion of a paper document if it is desired to render resistant to photocopying only information appearing or intended to appear on that portion. The comments which have been made above with respect to resistance to photocopying also apply to resistance to transmission by telefacsimile. It will be apparent that other embodiments of the invention will be within the skill of the person skilled in the art, without departing from the scope of the invention as claimed.

Claims: Copy-proof security paper preprinted over all or part of its surface with a coloured pattern of at least two colours repeating across the face of the paper in at least one dimension, those two colours providing said paper with a sufficient spectral response at one or both ends of the visible spectrum that information displayed over that coloured pattern in black or other dark colour is visible to the naked eye but with insufficient response at wavelengths in the range of about 400 to 700 nm that that information is non-reproduceable by photocopying or telefacsimile machines operating in that range of wavelengths, characterised in that said pattern has a spatial frequency across the sheet in the said at least one dimension in the range 0.5 to 50 times per cm and is made up of at least one pair of colours each having substantially the same spectral response profile over said range of 400 to 700 nm, but with one of the colours making up said pair having a lower spectral response than the other over the whole of that range, that one colour having a spectral response that is substantially the same as said black or other dark colour over the range 480 to 580 nm thereby providing substantially no contrast therewith over that range. Copy-proof security paper according to claim 1, characterised in that said pattern repeats in two dimensions across the face of the paper. Copy-proof security paper according to claim 1 or 2, characterised in that the pattern has a repeat spatial frequency across the face of the paper of from 2 to 25 times per cm, preferably 4 to 10 times per cm. Copy-proof security paper according to claim 1 or 2, characterised in that the pattern has a repeat spatial frequency across the face of the paper of from 0.5 to 10 times per cm, preferably 1 to 5 times per cm. Copy-proof security paper according to any one of claims 1 to 4, characterised in that said pattern repeats with multiple spatial frequencies across the face of the paper, said multiple spatial frequencies comprising i) a high spatial frequency that is up to 3 times greater or smaller than a higher Fourier spatial frequency of information displayed or to be displayed on said paper in said black or other dark colour, and ii) a low spatial frequency that is up to 3 times greater or smaller than a lower Fourier spatial frequency of that information displayed or to be displayed on said paper. Copy-proof security paper according to claim 5, characterised in that said higher Fourier spatial frequency is the highest Fourier spatial frequency of the information displayed or to be displayed on the paper, and in that the said lower Fourier spatial frequency is the lowest. Copy-proof security paper according to claim 5 or 6, characterised in that said high spatial frequency is in the range 40 to 50 times per cm and said low spatial frequency is in the range 2 to 5 times per cm. Copy-proof security paper according to claim 5 or 6, characterised in that said high spatial frequency is in the range 10 to 25 times per cm, preferably 15 to 25 times, and said low spatial frequency is in the range 0.5 to 5 times per cm, preferably 0.5 to 2 times. Copy-proof security paper according to any one of claims 1 to 8, characterised in that said one colour has a minimum reflection spectral response of about 4% in the range 400 to 500 nm rising to about 6% at about 580 nm and to a maximum of about 12% at about 700 nm, and in that the other one of said pair has a minimum reflection spectral response of about 5% in the range 400 to 500 nm rising to about 10% at about 580 nm and to a maximum of about 20% at about 700 nm. Copy-proof security paper according to claim 9, characterised in that the reflection spectral responses of the colours making up said pair drop back to said minima of about 4% and 5%, respectively, at wavelengths above said about 700 nm. Copy-proof security paper according to any one of claims 1 to 8, characterised in that said one colour has a maximum reflection spectral response of about 12% at a wavelength of about 400 nm falling to about 6% at about 480 nm and to a minimum of about 5% over the range 500 to 700 nm, and in that the other one of said pair has a maximum reflection spectral response of about 20% at wavelengths of about 400 nm falling to about 10% at about 480 nm and to a minimum of about 8% over the range 500 to 700 nm. Copy-proof security paper according to claim 11, characterised in that the reflection spectral responses of the colours making up said pair drop back to said minima of about 5% and 8%, respectively, at wavelengths below said about 400 nm. Copy-proof security paper according to any one of claims 1 to 8, characterised in that the coloured pattern is created by two pairs of colours repeating across the face of the paper, the first pair consisting of two colours having respective reflection spectral responses as defined in claim 9 and said second pair consisting of two colours having respective reflection spectral responses as defined in claim 11. Copy-proof security paper according to claim 13, characterised in that said first pair of colours consists of two colours having respective reflection spectral responses as defined in claim 10, and said second pair consists of two colours having respective reflection spectral responses as defined in claim 12. Copy-proof security paper according to any one of claims 1 to 14, characterised in that the coloured pattern includes an additional colour of relatively high reflectivity as compared with that of said pair of colours, or said pairs of colours, if more than one pair, that additional colour appearing in said pattern at a repeat frequency of from 0.5 to 50 times per cm. Copy-proof security paper according to claim 15, characterised in that said additional colour has a substantially constant reflection spectral response of about 90% over a wavelength range of from about 400 to 700 nm.

Inventors: GUNDJIAN, ASHAVIR
Applicants: NOCOPI INTERNATIONAL INC.
Application year: 1988
Main classification: G03G21
IPC codes: B42D15, B41M3, G03G21, G03C5