Anti-Photocopying Paper and/or Anti-Telefacsimile Paper

This invention relates to anti-photocopying and anti-telefacsimile 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. Pat. No. 4,522,429 (Gardner et al) issued June 11, 1985 and U.S. Pat. No. 4,632,429 (Gardner et al) issued Dec. 30, 1986.

U.S. Pat. No. 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. Pat. No. 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.

Anti-photocopying paper of the type described in the above mentioned patents satisfactorily fulfills 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.

It is therefore an object of the present invention to provide improved anti-photocopying and anti-telefacsimile paper.

According to the present invention, an improved anti-photocopying and anti-telefacsimile effect is achieved by spatial spectral modulation of the paper reflectance at a specific single or preferably multiple spatial frequencies.

The present invention provides anti-photocopying and anti-telefacsimile paper having a coloured pattern of at least one pair of colours repeating in at least one dimension of a face of a paper with at least one spatial frequency in the range of from about 0.5 to about 50 times per cm., the colours of each pair having substantially the same spectral profile but with one colour having a lower spectral response than the other colour over substantially all wavelengths, said colours contrasting with black or similar dark colour to permit black or similar dark coloured information to be visibly readable when applied to the coloured pattern, said colours also cooperating with such information to provide a document resistant to photocopying.

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 time 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 the higher fourier spatial frequency of information of a predetermined kind and a lower spatial frequency comparable to the 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 a pair of said colours may have a reflection spectral response with a minimum of about 5% at lower visible wavelengths of about 400 nanometers, 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, with the other colour of the pair having a reflection spectral response with a minimum of about 4% at lower visible wavelengths of about 400 nanometers, 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 response of said colours falls to said minima at wavelengths above about 700 nanometers.

Alternatively or additionally, one of the colours of a pair of said colours may have a reflection spectral response with a maximum of about 20% at lower visible wavelengths of about 400 nanometers, falling to about 10% at a wavelength of about 480 nanometers, and falling to a minimum of about 8% at higher wavelengths, with the other colour of the pair having a reflection spectral response with a maximum of about 12% at lower visible wavelengths, of about 400 nanometers falling to about 6% at a wavelength of about 480 nanometers, and falling to a minimum of about 5% at higher wavelengths. Advantageously, the reflection spectral response of said colours falls to said minimum at wavelengths below about 400 nanometers.

The coloured pattern may include an additional colour of relatively high reflectivity repeating in at least one dimension of 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:

FIG. 1 is a plan view of a sheet of paper with a front face having a first colour A.

FIG. 2 is a similar view having a second colour B applied to form a coloured pattern of the part of colours A and B in accordance with one embodiment of the invention,

FIG. 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,

FIG. 4 is a graph similar to FIG. 3 but showing reflection spectral responses of an alternative pair of colours C and D in accordance with another embodiment,

FIG. 5 is a graph similar to FIG. 3 but also showing the reflection spectral responses of colours C and D,

FIG. 6 is a graph similar to FIG. 3 but also showing the reflection spectral response of black information and a highly reflective colour W, and

FIG. 7 is a graph similar to FIG. 5 but showing another embodiment.

Referring to the accompanying drawings, FIG. 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 FIG. 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 two colours A and B forming a pair 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 FIG. 3.

Colour A and colour B have substantially the same spectral 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 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.

FIGS. 4 and 5 show the reflection spectral responses of another pair of colours C and D, with the colours C and D having substantially the same spectral profile but with the spectral response of colour D being less than that of colour C. The spectral response of colour C is a maximum (R max C) of about 20% at low visible wavelengths of about 400 nanometers, falling to about 10% at a cut off wavelengths 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 nanometers, 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 difference 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 FIG. 5.

FIG. 6 shows the reflection spectral response (.sup.R Black) of typical black information I printed or otherwise applied to paper, .sup.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 (.lambda..sub.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 (.lambda..sub.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.sub.w of the order of 90% (see FIG. 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 FIG. 7, colours A and B are modified so that their reflectance falls to the R min .sub.A and R min .sub.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 in .sub.C and R in .sub.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, .lambda..sub.C2 is shifted in the direction of 700 nanometers and beyond, and .lambda..sub.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.

Other embodiments of the invention will be readily apparent to a person skilled in the art, the scope of the invention being defined in the appended claims.