CA1070731A - Technique for circumventing document coyping - Google Patents

Abstract

Abstract of the Disclosure A photocopy resistant article, useful, for example, in the preparation of share certificates, bond certificates, paper money, etc. is disclosed. Two adjacent areas of the surface of the photocopy resistant article appear visually continuous to the unaided normal human eye. However, one of the areas, known as the signalling area, comprises a plurality of spaced lines having an element frequency along a notional selected line which is within the resolution capability of the photocopy machine but substantially beyond the resolution capability of the normal unaided human eye. The second of the two adjacent areas, referred to as the comparison area, comprises a plurality of spaced lines having an element frequency along a selected notional line substantially beyond the resolution capability of the unaided normal human eye and of the photocopy machine. Both areas have approximately equal area occupancy ratios and the signalling area has a density above the sensitivity threshold of the photocopy machine.

Description

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BACKG~OUND OF THE INVENTION
This invention relates to an article which is resistant to accurate reproduction by a photocopying machine.
The advent of the photocopying machine and, in particular, the colour photocopier has caused concerns for those persons who are engaged in the art and science of printing and preparing security documents, such as paper money, bond certificates, share certificates, identi-fication documents, etc. The colour photocopying machine,for example the XEROX 6500 (trade mark), poses a threat to the continuing integrity oE security documents. The present inventors thus realized that a copy resistant security document was required. Experience with the general public has often shown a surprising degree of gullibility in respect of forged or counterfeit documents. Accordingly, it was also clear that a photocopy of a copy resistant article must carry an obvious written message which is apparent to even the most gullible. Typical messages are THIS IS NOT ORIGINAL, COPY, or VOID, etc.
SUMMARY OF THE INVENTION
me invention described herein concerns a patterned article (e.g., a printed document) resistant to accurate reproduction by photocopying which comprises a surface subdivided into two or more areas, at least two of which include a comparison area and a signalling area adiacent thereto or nearby. The comparison area is comprised of a plurality of spaced lines having an element frequency (e.g. number of lines per inch) along a selected notional line in a first range outside the resolution capabilities

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of the unaided normal human eye and the photocopy machine.
The signalling area is comprised of a plurality of spaced lines having an element frequency along a selected notional line in a second range within the resolution capability of the photocopying machine but substantially heyond the resolution capability of the unaided norm,al human eye.
soth the signalling area and the comparison area have substantially the same area occupancy ratio (viz. area occupied by the spacad lines to area not so occupied) and the apparent macroscopic visual pattern of the surface to the normal unaided human eye in the vicinity of the signalling and comparison areas is substantially independent of the pattern ormed by the border between the signalling and comparison areas. (This is another way of saying that the signalling and comparison areas form a macroscopic visual continuum). The density of the discrete elements - of both the comparison area and signalling area exceeds the sensitivity threshold of the photocopy machine.
The invention is a patterned article resistant to accurate reporduction by photocopying using a given photocopying machine comprising:
(a) a surface subdivided into two or more areas, at least two of which areas include a comparison area and a signalling area adjacent thereto;
(b) said comparison area comprising a plurality o~ spaced lines having an element fre~uency along a selected notional line in a first range substantially beyond the resolution capability of the normal unaided human eye and of the photocopy machine;
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(c) said signalling area comprising a plurality of spaced lines having an element frequency along a selected notional line in a second range within the resolution capability of the photocopy machine but substantially beyond the resolution capability of - the normal unaided human eye; :
(d) said comparison area and signalling area having approximately equal area occupancy ratios, the apparent macroscopic visual pattern of the surface to the normal unaided ~: human eye in the vicinity of the signalling ;
and comparison areas being substantially independent of the pattern formed by the border between the signalling and comparison areas; and (e) the signalling area having a density above the sensitiYity threshold of the photocopy machine.
In the article described above, the first range may be from 150 to 300 lines per linear inch, the second : ~ .
range being 70 to 140 lines per linear inch, and the area occupancy ratio lying in the range 1:20 (0~05) to 1:1 (1.0). :
Further, in the article described above, the element frequency in the signalling area may be of the order of.~
85 lines per linear inch, the element frequency of the: ~.
comparison area being of the order of 170 lines per linear ;~ ~
inch, and the area occupancy ratio being of the order of ~-1:3 (0.333), In addition, in the article described above, the lines of the comparison area and the signalling area ,'~

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may form parts of a recognlzable pattern independent of the pat-tern formed by the border between the signalling and comparison areas which includes at least a portion of adjacent ones of the signalling and comparison areas.
The surface of the article described above may either be a flat surface or a flattenable surface. In addition, the patterned article, described above, may further comprise a disguising pattern and the lines may be printed.
According to another aspect of the invention, the lines of the patterned article are parallel lines, the parallel lines of the comparison area being substantially parallel to the parallel lines of the signalling area. A
first range may be from lS0 to 300 parallel lines per linear inch of comparison area and the second range from 70 to 140 parallel lines per linear inch of signalling area and the area occupancy ratio varying from 1:20 (0.05) to 1:1 (1.0).
In addition, the patterned article, as described above, may comprise a disguising pattern.
The utility of this invention resides in the fact that articles produced in accordance with the invention, as described and as claimed herein, are resistant to accurate reproduction by photocopying.
Brief Description o~ the Drawings In the drawings, all depictions are greatly magnified, and are assumed to represent ink deposits or the like on a sheet of paper or other suitable substitute.
FIGURE 1 is a schematic diagram illustrating the fundamental concept of the invention, in a representative detail of an embodiment of the invention showing a comparison area adjacent to a signalling area, , .
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FIGURE 2 is a schematic diacJram of a representative field of curved lines, FIGURE 3 is a schematic diagram illustrating the choice of the "selected line" :Eor two sets of lines, - 3c -FIGURE 4 is a schematic diagram illustrating the concept of "area occupancy ratio", FIGIJRE 5 is a schematic diagram illustrating the case of two sets of spaced ].ines having the same element frequency but different area occupancy rat:ios, FIGURE 6 is a schematic diagram depicting the case in which two sets of spaced lines have differing element frequencies but the same area occupancy ratio, FIGURE 7 is a simplified diagrammatic illustration of a representative example of a comparison area, FIGURE 8 is a schematic diagrammatic illustration of a sample signalling area (complementing the comparison area of Figure 7), FIGURE 9 is a simplified diagrammatic illustration of a composite comparison area and signalling area, formed by combining the complementing areas of Figures 7 ~nd 8, FIGURE 10 is a simplified diagrammatic illustration of a preferred embodiment of the invention, comprising the composite signalling area and comparison area of Figure 9, over layed with a d:isguising pattern.
FIGURE 11 is a simp:Lified diagrammatic representation of the word NOT when prepared as the comparison area and not as the signalling area, :
FIGURE 12 is a simplified diagrammatic representation of the signalling area which is to surround the comparison area of Figure 11, FIGURE 13 is a simplified diagrammatic illustration formed by combining Figures 11 and 12, and -

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FIGURE 14 is a simplified diagrammatic representation of a comparison area and signalling area comprising lines of more than one colour.
Detailed Description with Reference to the Drawings Figure 1 illustrates the underlying principle of the invention, which rests upon the observation that the unaided normal human eye is unable to resolve discrete elements such as lines to the same degree as a photocopying machine. Thus, it is possible to generate a visually continuous or even uniform pattern comprising a field of spaced lines resoluble by a photocopying machine, but not by the unaided human eye, and an adjacent field of spaced lines not resoluble by the unaided human eye or by the photocopying machine. However, the photocopying machine, with its superior resolution capabilities, will distinguish the two areas and produce a photocopy which does not appear to be the same as the original document.
The term "adjacent" as used herein with reference to the location of the signalling area and comparison area means contiguous or that the two areas are sufficiently close to one another as to appear to the unaided normal human eye to be a visual continuum.
The term 'Iresolution'l is used herein in its ordinary meaning, viz. the ability of the human eye, a photocopier or some other optical device to separate visual ; information into its constituent components. For e~ample, the ordinary human eye cannot read, let alone distinguish, words from words or letters from letters of a typical printed book held ten or more feet away from the viewer.
At one foot or six inches distance, there is no difficulty '7~

encountered by the normal unaided human eye in seeing and recognizing the specific printed contents of the same book. Resolution by the eye is obviously a function of viewing distance. Since we are here conclerned with anti-counterfeiting measures, we shall be primarily concerned with normal human eye resolution at viewing distances of from roughly one foot to perhaps three feet. Inherent in the concept of resolution is the requirement that the lines to be resolved must be distinguishable from the substrate upon which they are deposited. Thus, black lines are resoluble against a white background but not against a black backyround. Similarly, white lines on a black background can be discriminated but cannot be discriminated if the white lines are viewed with a white background. To achieve contrastl the reflective density (discussed below~ of the individual lines must be different from that of the substrate or backyround on which the elements are deposited.
~n addition, the psychology of human vision 20 i5 significant. The fact is that people cursorily glancing at a security document do not take the trouble to distinguish everything that could possibly be seen by carefully scrutinizing the face of the document. Thus, the matter is not entirely a question of optical resolution by the unaided human eye but of ordinary casual perception. A photocopy of a copy resistant document should bear a clear notification of the fact it is a copy in order that it will be recognized as such, even at a glance.
Referring to Figure 1, a field 1 of spaced parallel lines 3 is placed adjacent to or near to field 2 of spaced .,. -.

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parallel lines ~ with a common border 5. The lines of field 1 and 2 are parallel to each other. The lines 3 of field 1 are selected so as to be substantially incapable of resolution by either the unaided human eye or b~ a photocopy machine. On the other hand, lines 4 of ~ield 2 are chosen to be within the resolution range of a photocopy machine but substantially outside the resolution range of the unaided normal human eye. Fields 1 and 2 have a further important characteristic, namely, that they create the same visual impression to the unaided human .
eye. That is, if suitably reduced in size, fields 1 and 2 present a uniform appearance to the unaided eye with no seeming distinction between field 1 and field 2. On the other hand, if field 1 were orange in colour and field 2 black it would be obvious that fields 1 and 2 are not identical. Accordingly, fields 1 and 2 must be printed in either the same colour or in colours which give rise to the same overall human visual response.
For the remainder of the discussion of the invention, until the subject of the use of more than one colour is raised again, it will be assumed that the spaced lines of both the comparison area and the signalling area are of the same colour.
If an article bearing the pattern of Figure 1, when prepared in accordance with the invention and suitably reduced in size, is photocopied, the copy will indicate that there are two separate fields present because field 2, whose individual lines are within the resolution range of the photocopy machine, will be copied. ~owever, field 1 will not be accurately copied as its constituent lines ~l~713~3~

are outside the resolution abilities of a photocopying machine. The result, then, is a photocopy which, when seen by a human eye, clearly reveals the presence of the non-uniformity as between fields 1 and 2 which the eye could not detect in the original article.
Henceforth, ~ield 1 of the original article will generally be referred to as a "comparison area".
Field 2 of the original article will be termed the l'signalling area" because it "signals" the photocopying machine but not the unaided human eye. That is, the key property of the signalling area is that it contains information to which the photocopy machine is sensitive but not the unaided normal human eye. In a photocopy, the comparison area provides a reference region for comparison with the signalling area. However, in the original article, the presence of a signalling area and comparison area cannot be readily perceived by the unaided normal eye, by virtue of the appropriate selection and spacing of ~he lines.
Having now e~amined the essence of the invention ~0 and some of the key concepts, the pertinent parameters are defined in some detail. After having defined the parameters of importance, their interrelationship and acceptable ranges of values in actual examples will be considered.
Definitions:
(a) Lines Individual lines are the fundamental components comprising the signalling and comparison areas.
If straight lines are employed, they should usually be parallel to one another. Otherwise intersections 7~73~

of the lines will occur which may render the signalling area and comparison area distinguishable by the human eye. The length of a line must be greater than its width.
Spaced curved lines, as seen in Figure 2 may also be used, providing that the element frequency, density, and area occupancy ratio as hereafter examined are appropriately chosen and providing that the signalling area and comparison area present a visual continuum to the unaided normal human eye.
; 10 A disguising pattern such as the one seen in Figure 10 may be utilized in conjunction with the invention.
This will have the effect of introducing a number of dis-continuities into the lines. Although the individual lines of the signalling area and comparison area become discontinuous by the application of a disguising pattern, the invention remains operative. The purpose of the dis-guising pattern is to render more difficult the perception of the comparison and signalling areas in the original document.
The colour of the lines must be one which contrasts with the colour of the substrate upon which the discrete element is deposited.
(b) ~L~IL_~L99Y~D-~
Referring to Figure 3, the element frequency, for the case of spaced parallel lines of field A is the number of lines per inch crossed by a selected notional line 6 which runs perpendicular to the parallel lines.
Selected notional line 6, as seen in Figure 3, encounters lines numbered 1 to 5. (Line 6 is notional in the sense that it is not physically reali~ed on the substrate).

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For curved lines 7, 8, 9, 10 of Eield B of Figure 3, the appropriate notional selected line is line 11.
The element frequency is the number of lines crossed by line 11 for a given length of line 11. Selected line 11 is chosen so as to cross a representative number of the curved lines.
(c) Area Occu~ancy Ratio The area occupancy ratio (AOR) for a field of elements printed or otherwise present on a given substrate is defined by the following equation:

AOR = r ea of field which is covered by discrete el ments ~rea of field which is not covered by discrete elements~

The area occupancy ratio is a macroscopic, as opposed to microscopic parameter, representing the ratio of the area of a field ~the field being large enough to be readily visually perceptible) covered by lines to that not covered. The total area of the field is the numerator of the above expression when added to its denominator.
By way of example, Figure 4 illustrates the computation of the area occupancy ratio for a simple case easily susceptible of calculation. In Figure 4, field A (enclosed by broken lines) consists of N parallel lines 1, 2, 3, ...N of length L and width Wl, each line (e.gO line 1) being separated from the adjacent line (e.g. line 2) by a distance W2. The area of field A co~ered by the lines is N-L-Wl. The area of field A left uncovered is N~L~W2.
Hence, the area occupancy ratio is NLWl/NLW2, or Wl/W2.
If ~ield A is produced by a printing process, the area occupancy ratio is equal to the ink-bearing (say) area of the field of lines 1, 2, etc. to the area which is 73~.
, -uninked (or which has previously been inked overall insome other colour, or the like).
rrhe concept of area occupancy ratio is not restricted to parallel lines but includes individual elements of all shapes such as dots, asterisks etc. In fact, one could, if desired, compute the area occupancy ratio of a page of newsprint or of a newspaper photograph.
The physical significance of the area occupancy ratio is that, in conjunction with the element frequency, it is a measure of the physical dimensions (area, width) and number of the lines in a field.
Figure 5 depicts two fields, A and B, of parallel lines which have the same element frequency but quite different area occupancy ratios~ Field A has an area occupancy ratio which is smaller than that of field B.
To complete the discussion of this concept, reference should also be made to Figure 6 in which fields A and B each have an area of occupancy ratio of 1:1. However, the element frequency for field A (lines 3 and 4 along selected line 1) is half of the elemen~ frequency of field B
along selected linP 2. (Field B consists of lines 5, 6, 7 and 8). In conclusion, the purpose of Figure 6 is the illustration of two fields of parallel lines having the same area occupancy ratio but different element fre~uencies.
Hence, the area occupancy ratio is a physical parameter related to the area of a field actually covered by the lines.
; As a final general comment about Figure 6, it is worthy of note that the visual impression of fields A
and B would be the same if the individual lines of the fields were of a width too narrow to be distinguished by the unaided eye, and if the element frequencies employed were not quite so disparate (assuming also that the same colours are used for both fields A and B).
(d) Reflection Density As is well known~ reflection density or density, as it will be referred to hereafter, is defined by the following expression:

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where the incident light is the light falling onto, for example, a sheet of paper and the reflected light is that light which is reflected from the paper. Since the re-flected light is usually less than the incident light, the bracketed term is greater than or equal to unity .
` If one-half of the incident light is reflected, the reflection - density is 0.3 (log10 (1/(1/2)~ = 1Oglo(2)=0.3). For white paper, where substantially all of the incident light is reflected, the reflection density may approach zero and is often assumed to be 0Ø
All photocopying machines have a density sensitivity threshold, fo~ various colours (frequently about 0.3 for green and blue and 0.2 for red) and if the material to be photocopied has a density lower than the density sensitivity ~;~
threshold, accurate photocopying will not be obtained.
Also, metallic, gold and silver colours are not faithfully copied by present colour photocopying machines.
(e) Si ~
This term has been defined above but is repeated here for convenience. A signalling area is a field of lines whose element frequency and area occupancy ratio ~l~7~73~

is such that the lines can be substantially resolved by a copying machine but not by the unaided human eye.
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The comparison area is a field of lines whose element frequency and area occupancy ratio are 50 selected that the individual lines cannot be substantially resolved by either a photocopying machine or the unaided human eye. The comparison area will usually be located beside or near to the signalling area. Where the signalling area comprises parallel straight lines, for ease of achieving a uniform visual impression of the comparison and signalling areas, the lines of the comparison area should also be parallel to the lines of the signalling area.
Lastly, the use of the expressions "comparison area" and "signalling area" is really a matter of nomenclature.
The imporkant point is that there are two areas which look the same to the unaided normal human eye but which can be discriminated by a photocopy machine.
This concludes the consideration of the governing definitions by which the present invention is most readily explained and defined. The explanation of one case, the simplest, is presented in some detail to draw together into a unified whole the significance of the various definitions.
Referring to Figure 7, a comparison area CA
is prepared with a series of equidistant parallel lines of equal width with an element frequency of 175 lines per inch and an area occupancy ratio of 3:4 or 0.75.
As depicted by Figure 7, a part of the comparison area is left blank and this comprises the words THIS IS NOT
ORIGINAL.

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Figure 8 illustrates the signalling area SA, when examined separately and apart from the comparison area CA of Figure 7. The word NOT is comprised of parallel lines which have an element frequency of 87 lines per inch and an area occupancy ratio of 4:5 or 0.80. The lines of the signalling area are parallel to those of the comparison area in Figures 7 and 8. As discussed above, the area occupancy ratio of the comparison area and signalling area must be approximately the same and the unaided human eye should be substantially incapable of distinguishing the two respective fields of discrete elements. In this example, the area occupancy ratios are 0.75 and 0.80 respectively~
In Figure 9, the composite of the comparison area and signalling area is seen. The words THIS IS
ORIGINAL appear in a solid colour so as to be readily visible. The signalling area (the word NOT) has been inserted into the originally empty space formed by the word NOT in Figure 7.
It has been found useful to place an overlaying disguising pattern on the comparison area and signalling area. Figure 10 depicts the comparison area and signalling area of Figure 9 when overlaid with such a pattern. The effect of the disguising pattern, seen in Figure 10~ is to interrupt the lines of the signalling area and comparison area of Figure 9 so as to form a multiplicity of shorter non-continuous lines. The shape and dimensions of the signalling area, the element frequency, density and area occupancy ratio thereof r and the visual characteristics of the disguising pattern are selected so that the human - ~7~3~ :

eye is not readily capable of distinguishing the comparison area from the signalling area on the original printed document (or as the case may be).
Having described Figures 7-10 on a step-by step basis to clarify the details of Figure 10, it is important to note that, in practice, if printing technology is employed, successive printing steps are, of course, unnecessary.
In fact, an intaglio plate designed to print the pattern of Figure 10 can be prepared and printing may then proceed.
Note that there is no necessity that intaglio printing be employed in preparing patterned articles in accordance with the present invention. Any type of printing, or other means whatsoever, capable of producing material of the necessary quality may be employed.
If, when appropriately reduced in size, and printed, the pattern of Figure 10 is photocopied, it is readily seen that the word NOT will be apparent to the viewer whose attention will immediately be drawn to the warning message THIS IS NOT ORIGINAL. However the word NOT, hidden in the original article, is not easily seen by the unaided eye.
In Figures 11 and 12, the comparison area and signalling area have been reversed when compared to their us~ge in Figures 7 and 8. In Figure 11, the comparison area which comprises the letters of the word NOT consists of a plurality of parallel lines whose element frequency is substantially beyond the resolution capability of the photocopy machine and of the unaided normal human eye.
In Figure 12l the signalling area, here consisting of the area surrounding the letters of the word NOT, consists of a plurality of parallel lines having an element frequency beyond the resolution range of the unaided normal human "
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eye but resoluble by the photocopy machine. In Figure 13, the lines of the comparison area and signalling area are parallel to one another. Assuming that the area occupancy ratios of Figures 11 and 12 are approximately the same and that the macroscopic visual effect of the composite of Figures 11 and 12, seen in Figure 13, is one of a continuum, then the unaided normal human eye would be unable readily to distinguish the word NOT from the background material of Figure 13 if the figure were appropriately prepared and reduced in size. In conclusion, as shown in Figures 11, 12 and 13, there is no conceptual difficulty in reversing the signalling area and comparison area.
With respect to the colours in which the pattern of Figure 10 or 13 may be prepared, assuming that a single colour is used throughout, the inventors have found that any colour may be used if the reflective density of the printed pattern exceeds about 0.3. Black is useful, in the case of a colour photocopier because of the difficulty ;encountered by the XEROX 6500 (trade mark) colour photocopier in accurately reproducing that colour.
As previously discussed, in all cases, it is important that the area occupancy ratio of both the comparison area and signalling area be roughly the same and that both areas give the same overall visual impression. The impression given by both areas to the human eye is a function of colour, element frequency and area occupancy ratio of the respective areas. For a given background comparison area (e.g. a photograph of clouds), some trial and error experimentation with the element frequency and area occupancy ratio of the signalling area may be necessary to achieve a good visual match of the two areas.

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The range of suitable values of the element fre~uency of the comparison area is from about 150 to 300 lines per inch, while that of the signalling area is about 70 to 140 lines per inch. The area occupancy ratios for both the signalling area and comparisorl area may range from about 1:20 (0.05) to lol (1.0). Both areas should have about the same Area Occupancy Ratio. An area occupancy ratio of 1:3 (0.33) and an element frequency of the order of 85 lines per inch for the signalling area and of the order of 170 lines per inch for the comparison area have proven particularly satisfactory.
Although preferred embodiments of this invention, have been disclosed in detail, it is to be understood that modifications thereof can be made without departing from the broad spirit and scope of the invention.
There is, of course, no conceptual difficulty in preparing the comparison area and signalling area in different colours so long as the overall visual impression ; to the human eye is one of uniformity. For example, this invention will be inoperable if the comparison area is printed in black and the signalling area in orange. The unaided eye would rapidly note the presence of a comparison area and distinctive signalling area. However, one of the areas could be printed in suitably selected red and blue lines (say) and the other area in magenta lines (say) to give substantially the same macroscopic visual effect.
Assuming that the comparison area and signalling area are ~repared in two or more passes through a press using different colours, and that a uniform visual impression is to be created, there is still the practical problem of achieving good re~istration between the two areas.
This does not pose a difficulty when a single pass is all that is necessary, as when a single intaglio printing plate of the type conventionally used by banknote printers inked in only one colour is employed in the preparation of the patterned article. In summary, this invention is not restricted to the case where the comparison area and sigjnalling area are the same colour. However, where different colours are employed, care must be taken to ensure that the comparison area and signalling area project the same overall visual impression to the unaided human eye and that appropriate registering oE the two areas is attained.
When the lines of the comparison area or signalling area are printed or prepared in any other manner in more than one colour, complications in describing this invention arise. As is now apparent, the colour contrast between the lines and the substrate or background upon which the lines are deposited is vital to the operability of this invention~ I~ adequate colour contrast between the lines and the substrate is not present, the invention will not function. For example, if orange coloured lines are deposited on a substrate of the same colour, both colours having approximately the same reflective density, the desired result of this invention will not be attained.
On the other hand, if the contrast between the overall macroscopic colour response of the comparison area and signalling area is too great, then, once again, the invention will be inoperable because the unaided human eye will quickly spot the distinction between the comparison area 73;~

and signalling area. Thls has been previously discussed in an example which mentioned the use of a comparison area giving the impression of the colour black and a signalling area giving the impression of the colour orange.
In summary, when different colours are used for the signalling area and comparison area or when varying colours are employed for the lines of the signalling area or for the comparison area, special precautions must be taken when using this invention. Firstly, there must be an adequate colour contrast between the substrate of the comparison area or signalling area and the lines deposited thereon and, secondly, the comparison area and signalling area must present a substantially uniform macroscopic visual impression to the unaided normal human eye.
In Figure 14, the comparison area CA and adjacent signalling area SA comprise alternating parallel red and blue lines on a white background (i.e.-substrate). The element frequency of the comparison area is the number of red and blue lines per inch of white background along line 1. This definition of element frequency was selected because the red and blue lines individually and separately have a lower reflective density than that of the white background and so contrast sharply with it. The macroscopic visual impression of the comparison area and signalling area will be that of the colour purple (or violet, mauve, etc.) to the human eye if the area occupancy ratios of both areas are approximately the same. In the case of Figure 14, the area occupancy ratio is defined as the area of the comparison area or signalling area, as the -- lg --~7~7~

case may be, covered by discrete elements to the area not so covered. No distinction is drawn between the red lines and the blue lines where the overall visual impression of both areas is one of substantial uniformity.
There i5 another method for preventing accurate colour photocopying, which is now disclosed at greater length. This method is included for the sake of completeness but no claim to an exclusive property or privilege in respect of the following method is claimed.
The visual impression of a pastel colour may be achieved in one or both of the manners described below:
(1) The appearance of a pastel colour may be achieved through the application of a pastel colourant, of density less than 0.3 onto the article.
(2) In addition to method 1, the visual response of a pastel is achieved through half-tone printing wherein, individual lines of, for example, ink having a density in excess of 0.3 (and large enough to be resolved by the photocopier) are distributed on the material to be printed.
Although each individual line has a density exceeaing 0.3, the overall visual impression of the half-tone area will be that of a pastel colour. If the composite of a solid block of pastel colour, located adjacent to a visually similar half-tone block, is photocopied the solid pastel area will not copy because its density is less than the threshold density o~ the photocopying machine.
On the other hand, the half-tone block will photocopy because its individual discrete elements possess a density in excess of the photocopier machine threshold. Thus, while the solid pastel block and half-tone block would .

appear to be identical in the original document, a photocopy would clearly indicate the presence oE two differing areas.
Reverting to the language used earlier, the solid pastel area may be referred to as the comparison area and the half-tone printed block as the signalling area. Thus, the signalling area, perhaps readiny VOID or FRAUD could be hidden in a surrounding solid pastel block. A photocopy of the original document, would reveal the presence of the signalling area, which reads VOID or FRAUDI against a poorly copied version of the comparison area.

Claims (11)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A patterned article resistant to accurate reproduction by photocopying using a given photocopying machine comprising:
(a) a surface subdivided into two or more areas, at least two of which areas include a comparison area and a signalling area adjacent thereto;
(b) said comparison area comprising a plurality of spaced lines having an element frequency along a selected notional line in a first range substantially beyond the resolution capability of the normal unaided human eye and of the photocopy machine;
(c) said signalling area comprising a plurality of spaced lines having an element frequency along a selected notional line in a second range within the resolution capability of the photocopy machine but substantially beyond the resolution capability of the normal unaided human eye;
(d) said comparison area and signalling area having approximately equal area occupancy ratios, the apparent macroscopic visual pattern of the surface to the normal unaided human eye in the vicinity of the signalling and comparison areas being substantially independent of the pattern formed by the border between the signalling and comparison areas;
(e) said signalling area having a density above the sensitivity threshold of said photocopy machine.

2. An article as in claim 1 wherein said first range is 150 to 300 lines per linear inch, said second range is 70 to 140 lines per linear inch, and said area occupancy ratio lies in the range 1:20 (0.05) to 1:1 (1.0).

3. An article as in claim 1 wherein the element frequency in the signalling area is of the order of 85 lines per linear inch, the element frequency of the comparison area is of the order of 170 lines per linear inch, and the area occupancy ratio is of the order of 1:3 (0.333).

4. An article, as recited in claim 1, wherein the lines of said comparison area and said signalling area form parts of a recognizable pattern independent of the pattern formed by the border between the signalling and comparison areas which includes at least a portion of adjacent ones of said signalling and comparison areas.

5. An article as recited in claim 1 or claim 2, wherein said surface is a flat surface.

6. An article as recited in claim 1 or claim 2, wherein said surface is a flattenable surface.

7. An article as recited in claim 1, wherein said lines are printed.

8. An article as recited in claim 1, wherein said lines are parallel lines, the parallel lines of said comparison area being substantially parallel to the parallel lines of said signalling area.

9. An article as recited in claim 8, wherein said first range is 150 to 300 parallel lines per linear inch of comparison area and said second range is from 70 to 140 parallel lines per linear inch of signalling area and said area occupancy ratio varies from 1:20 (0.05) to 1:1 (1.0).

10. An article as recited in claim 1 wherein said patterned article further comprises a disguising pattern.

11. An article as recited in claim 8 wherein said patterned article further comprises a disguising pattern.