FR3079167A1 - Inspection of micro-optical effect images in thin substrates - Google Patents

Abstract

A defect detection system in printed imagery used to generate optically variable images, the system comprising: a print feed path for a support substrate to be printed with the image; a printing station for printing the imaging and a test pattern on the support substrate, the test pattern having spaced apart image elements; and an image sensor for capturing images of the test pattern; so that brightness levels acquired by the image sensor are used to derive a contrast measurement for comparison at a reference contrast level.Figure for the abstract: Fig.4

Description

Title of the invention: Inspection of micro-optical effect images in thin substrates

Technical Field [0001] The invention relates to the production of optical devices of the type used for authenticating articles. The invention relates in particular to the control of faults in micro-optical devices.

Technical background [0002] To combat counterfeiting, security elements are often applied to articles of value such as security documents or security tokens. These security features often generate an "optically variable effect," which is an image that changes from different angles of view. These types of security features are often applied to valuable documents such as ID cards, passports, credit cards, banknotes, checks, vouchers and certificates to provide an apparently hard-to-reproduce item.

A category of these security elements is constituted by lens-based devices which use an array of microlenses to magnify an underlying array of micro-images in order to generate the optically variable effects. These devices require precision manufacturing since small defects have a significant impact on the optical effects generated. In this context, the optical characteristics of security elements are monitored during production, typically by an automated vision system (i.e., a camera and feedback control). However, checking the optical characteristics of a lens-based safety device is problematic. It is particularly difficult to measure the image contrast of magnified images produced by a microlens array applied to thin substrates such as banknotes. Likewise, it is difficult to detect defects in the network of micro-images, in particular a network of micro-images applied to thin substrates such as banknotes. In the production environment, these difficulties are exacerbated due to the need for testing at normal production speeds. An individual inspection of all micro-images produced, also known as 100% inspection rate, is exceptionally difficult to achieve at production speeds. Even if 100% inspection rates were achievable in the production environment, they would not guarantee maximum quality or reduced rejection due to image defects and / or low image contrast.

Microlenses on security documents such as banknotes have a focal length of lens limited by the thickness of the security document. The transparent substrate of the banknote is often used as an optical spacer between the lens array and the micro-images. Small microlenses are normally embedded in relief in a layer of transparent UV curable lacquer material which is then cured by UV radiation. The UV curable lacquer layer is relatively thin, which also limits the maximum depth available for the integration of microlenses. In practice, the maximum lens integration "lowering" (that is, the height of the lens above its base plane) is limited. The width of the lens in terms of the need to focus on the micro-images on the other side of the substrate is also correspondingly limited.

The limited width of the lenses on a thin substrate imposes a limit on the size of the micro-images on the other side of the substrate if optical effects such as moiré magnification, image swivels, animations, full images, etc. must be obtained. In summary, the geometry of the microlenses and the type of optical effect determine the size of the micro-images.

Among all the different optical effects, the so-called "contrast permutation" optical effect involves the largest size of the printed image element. This is because the optical permutation of contrast requires the smallest amount of image information that must be contained in the image layer under the microlens array. Figure 1 is a schematic representation of an optical effect of permutation of contrast. A single cell 10 (that is to say a single cylindrical lens 14 of the lens array and its corresponding picture element 18 on the opposite side 16 of the transparent substrate 12) is represented seen from three different viewing angles (20, 22 and 24). The distance between the image plane 16 and the top of the microlens 14 is less than the focal length of the lens 14. Each image element 18 is a printed line magnified by the corresponding lenticular lens 14 (from the network of lenticular lenses one-dimensional) intended to project a lenticular image pixel seen by the viewer.

The color intensity of the pixel seen by the viewer depends on the angle of view as well as the amount of offset of the printed line 18 with respect to the central vertical plane (that is to say the plane through the apex) of the lens 14. At the viewing angle 24, the color intensity is maximum while at the viewing angle 20, the color intensity is minimum. This effect is optimized on the printed element is sized to be approximately half the width of the microlens 14. A sufficient image contrast is also possible with a printed element 18 of more or less than half the width of the lens 14. However, the amount by which the printed line 18 is larger or smaller than the optimal width (typically half the microlens) determines the reduction in the maximum image contrast projected to the viewer. Therefore, a modest change in the size, or rather the width, of the display element 18 can cause the contrast in the manufactured safety device (in this case, the contrast swap device) to vary considerably. This creates an irregular perception of product quality for the end user. In addition, a change in contrast may reduce the security device's resistance to counterfeiting as relatively low quality replicas will reproduce the same optical effect.

Similarly, missing parts of the image elements 18 will generate defects in the enlarged image which will cause quality irregularity and therefore reduce the inherent resistance to counterfeiting.

If an optical device for permuting contrast is incorporated on a thin substrate (such as the transparent substrate of a banknote) with microlenses (in particular microlenses with a width of approximately 64 microns), it it will be necessary to adjust the size of the picture element 18 to a width of about 30 microns to 40 microns. Measuring such narrow lines is difficult to achieve at microscopic accuracy using an online camera inspection system. Typically, a camera system such as this uses line scan cameras that have a resolution of about 100 microns to 250 microns per pixel (although the actual resolution also depends on the speed of the substrate strip relative to camera), and the scanning resolution therefore decreases with increasing support delivery speed. This means that the smallest theoretically detectable change in size (at a typical production speed of at least 100 meters per minute) is approximately 100 microns to 250 microns. It is assumed that the repeated pattern of printed elements is presented to the camera in exactly the same relative position. Unfortunately, practice does not allow this. The continuous web of printed media in an engraving process can shift up to +/- 0.25 mm, which increases the smallest detectable change in size. As a result, even for a simple micro-optical effect such as contrast swapping, which uses the largest print element size of all the micro-optical effect devices, measuring the contrast of individual printed elements at using camera inspection systems can be subject to significant inaccuracies.

The offline test, using optical microscopes and the like, makes it possible to obtain more precise image width measurements without, however, preventing the production of printed material with contrast and off-set defects. of the specified allowable range. Offline inspection necessarily occurs after printing, while a 100% inspection technique performed online provides correction feedback during the printing process.

It is possible to use scanning or direct acquisition of the magnified images generated by the optical effect to measure the contrast. However, an inherent production tolerance varies the printed micro-imagery relative to the associated micro-lenses, creating a relative phase shift between the lenses and the imagery in each individual optical device. These variations cause corresponding variations in the magnified image projected onto the camera system, so it is impossible to validly compare the acquired image to a single reference image (called the "gold model"). In other words, even if the acquired image is printed with perfect contrast, it can differ significantly from the reference image and therefore be mistakenly identified as rejection.

Referring to Figures 2 and 3, an optical effect of permutation of contrast of the design presented in Figure 2 produces the selection of projected images presented in Figure 3. The image is printed with perfect contrast. The image projected on the inspection camera can be the positive image 18, created by being seen from a first angle of view and having a first measurement of contrast, or it can be the negative image 26, created by being seen from a second viewing angle and which presents a second contrast measurement (complementary to the first contrast measurement). Similarly, it can be any intermediate image 28 to 34 generated from other viewing angles which have a contrast which differs from that of the first or second contrast measurement. These inherent variations make it unnecessary to attempt to compare the acquired image to a single gold model for inspection purposes.

Similarly, the use of acquired camera image to detect defects in individual printing elements such as missing image parts (due to a lack of ink, for example due to the obstruction of cylindrical cells with dried ink) is also impossible due to the variation of the acquired image compared to the inherent variation of the lens-image phase associated with production tolerances.

In an effort to alleviate these problems, it may be possible to use multiple cameras that acquire the projected magnified images from a range of different viewing angles. All acquired images are compared to the gold model and if any of the acquired images match the gold model, the magnified image is classified as acceptable. However, this technique is based on capturing a sufficiently large number of images from different viewing angles to guarantee the capture of a viewing angle close to that of the gold model. Capturing multiple images of individual print elements from different viewing angles is complex, expensive and requires significant processing. An effective inspection system for individual print items using multiple cameras would only be practical for printing processes operating at low production speeds.

An alternative approach is to use a single camera view to acquire micro-imagery images which are then compared to several gold models. This type of technique was disclosed in document US 5,850,466, entitled “Golden Template Comparison for Rotated and / or Scaled Images”. While this technique reduces the number of cameras used, it still requires powerful image processing devices to compare the acquired image to each model in the set of gold models. Again, this technique is only suitable for processes operating at low production speed.

In another approach aimed at solving the problem of contrast measurement, a printed color sample is used as a control or reference for the contrast of the magnified image. For example, an optical device for producing a contrast permutation effect printed by etching on a banknote substrate can also simultaneously print a sample of the micro-image color on the substrate. The brightness / color density of the sample sample can then be measured using an online camera inspection system. The sample color density is correlated to the contrast of the large image to determine if the contrast is within the acceptable range. However, in practice, any change in the size of the microimaging that results in unwanted changes in the big image contrast is not correlated with a change in the color density of the color sample (which is detected at using the online inspection camera).

The control color sample has an ink density much higher than that of micro-imaging. Therefore, the ink composition greatly influences the brightness of the sample, but the contrast of the magnified image is influenced to a greater extent by factors that have no impact than the brightness of the color sample. For example, the depth of the fine engraving cells in the surface of the engraving cylinder used to print micro-imaging have a much greater influence than the contrast of the magnified image. For example, a 10% change in cell depth results in a negligible change in the brightness of the color sample (simply because the color sample cells are much larger), while this change produces a change significant contrast of the magnified image. Those skilled in the art will understand that a small change (+/- 10%) in the depth of the micro-imaging cells can significantly increase or decrease the contrast of the magnified image.

In view of the above, an image acquisition system capable of 100% inspection, online and at normal production speeds is not a realistic possibility in most installations. commercial printing. There is therefore still a need to develop other techniques for detecting micro-imaging defects and accurately measuring image contrast using online systems during the printing process.

Any reference made here to a patent document or any other subject defined as prior art should not be considered as acceptance of the fact that said document or subject is known or that the information it contains belongs to knowledge general conditions on the priority date of any one of the claims.

STATEMENT OF THE INVENTION Consequently, the invention provides a system for detecting faults in printed imagery used to generate optically variable images, the system comprising: a print supply path for a support substrate to print with imagery; a printing station for printing the imagery and a test pattern on the support substrate, the test pattern having picture elements spaced apart by spaces; and an image sensor for capturing images of the test pattern; so that, brightness levels acquired by the image sensor are used to derive a contrast measurement for comparison to a reference contrast level.

In another aspect, the invention provides a method for detecting faults in printed imagery used to generate optically variable images, the method comprising: providing a support substrate along a print supply path ; printing the imagery and a test pattern on the support substrate with a printing station, the test pattern having picture elements spaced apart by spaces; capturing images of the test pattern with a camera; and, recording brightness measurements of the test pattern for comparison to a reference brightness level in order to detect faults.

In yet another aspect, the invention provides a printing assembly configured to detect faults in printed imagery used to generate optically variable images, the printing assembly comprising: a printing station for printing the imagery and a test pattern on the support substrate, the test pattern having picture elements spaced apart by spaces; and an image sensor for capturing images of the test pattern; so that brightness levels are acquired from the test pattern to derive a contrast measurement for comparison to reference image contrast levels.

The invention is based on the acquisition of brightness levels from a test pattern to derive a contrast measurement which is indicative of the imaging contrast, instead of a direct contrast measurement of the imagery itself. It is a system well suited for measuring the contrast of a magnified image in a microlens security element and a technique which can be applied as an online test at production speeds . The test pattern can be printed inside or outside the "product area", that is, the printed micro-imagery or the printed item itself. The printing station can print the test pattern as well as the micro-imagery and / or other image element to be represented on the support substrate. The printing station is any equipment used for printing the substrate, taking into account the fact that the term "printing" encompasses more than the simple application of ink with a printing tool (see the section "Definitions" ci -Dessous). Typically, the image sensor is part of a camera system arranged to acquire the brightness levels of the test pattern and process this image data to provide a desired measurement of image contrast.

In one form, printed imagery is micro-imagery used in a security device with an array of lenses to generate optically variable magnifications; and the test pattern picture elements each have a microlens associated so that at least some of the picture elements are printed at different displacements relative to the respective microlenses to generate respective magnified images with levels different image brightness, seen from a predetermined viewing angle, the different image brightness levels defining a range for comparison to a predetermined acceptable brightness range to provide a measure indicative of contrast in optically variable magnifications of the safety device.

Preferably, the image elements and the spaces are of uniform size. Preferably, the image elements and spaces of the test pattern are sized to substantially match those of the printed imagery used to generate the optically variable image. Preferably, the associated microlenses intended to visualize the image elements of the test pattern and the microlenses in the microlens array intended to visualize the micro-imaging have the same geometry and the same orientation of lenses.

Preferably, the microlens array is an array of lenticular lenses. Preferably, the micro-imaging and the image elements are spaced from the respective vertices of the associated lenticular lenses by a distance less than the focal length.

Preferably, micro-imaging is a network of micro-images which interacts optically with the network of lenses to create an optical effect of permutation of contrast.

Preferably, the printing station is an etching cylinder and the support substrate is a continuous strip extending from a supply roller to a collection roller following the print supply path.

Preferably, the image elements are each a single line printed on a surface of the continuous strip opposite the surface supporting the array of lenticular lenses.

Preferably, the micro-imagery and the image elements are printed in more than one color.

Preferably, the single line has a width between 30% and 70% of the width of each lenticular lens. More preferably, the single line width is approximately half the width of one of the lenses.

Preferably, the test pattern has a number of regions in which the single lines are offset in increasing increment relative to the vertices of the associated lenticular lenses, respectively.

Preferably, the test pattern and the associated lenticular lenses generate a moire magnification so that a maximum brightness level and a minimum brightness level are measured from the images captured by the camera for comparison with reference brightness levels indicative of contrast.

Definitions [0034] Security document or token

As used herein, the designation security document includes all types of documents and tokens of value and identification documents including, but not limited to: currency effects such as bank notes and coins, credit cards credit, checks, passports, identity cards, securities and share certificates, driving licenses, property titles, travel documents such as plane or train tickets, entry cards and tickets, birth certificates, of death and marriage and transcripts. The invention applies in particular, but not exclusively, to security documents such as banknotes or identification documents such as identity cards or passport documents formed from a substrate on which at least a printing layer is applied. The diffraction gratings and optically variable devices described here may also find application in other products, such as packaging.

Substrate

As used here, the term substrate refers to the base material from which the security document or token is formed. The base material can be paper or another fibrous material such as cellulose; a plastic or polymeric material including, but not limited to, polypropylene (PP), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), polyethylene terephthalate (PET); or a composite material of at least two materials, such as a paper laminate and at least one plastic, or at least two polymeric materials. The use of plastics and polymers in the manufacture of security documents, started in Australia, has met with frank success because polymer banknotes are more durable than their paper counterparts and can also incorporate new devices and security features. A particularly popular feature of polymer banknotes produced for Australia and other countries is a transparent area or "window".

Transparent windows and half-windows

As used herein, the term window refers to a transparent or translucent area in the security document in comparison to the substantially opaque region to which printing is applied. The window may be fully transparent so as to allow light transmission without substantially alteration, or it may be partially transparent or translucent and allow transmission, but without allowing clear viewing of objects through the window. A window zone can be formed in a polymer security document which has at least one layer of transparent polymer material and at least one more opacifying layer applied to at least one side of a transparent polymer substrate, omitting at least an opacifying layer in the region forming the window area. If opaque layers are applied to both sides of a transparent substrate, a fully transparent window can be formed by omitting the layers on both sides of the transparent substrate in the window area. A partially transparent or translucent area, hereinafter referred to as "half window", can be formed in a polymer security document which has opacifying layers on both sides by omitting opacifying layers on only one side of the document security in the window area so that the "half window" is not fully transparent, but allows some of the light to pass through without allowing clear viewing of objects through the window.

Alternatively, it is possible to form the substrates from a substantially opaque material, such as paper or a fibrous material, with an insertion of transparent plastic material inserted in a cutout, or a recess in the paper or fibrous substrate to form a transparent window or a translucent half-window area.

Printing

As used herein, the terms printed or printed refer to a surface treatment or technique for forming images or markings by modifying the characteristics of the incident radiation reflected or transmitted. The incident radiation need not be in the visible spectrum, and the wavelength of the transmitted or reflected radiation may be different from the incident radiation or may be a subset of it. The surface treatment or technique may involve the deposition of a material such as ink, a metal or a polymer, in particular a radiation-curable resin. Similarly, the surface treatment or technique may involve modifications such as embossing, etching or laser ablation. The surface structure can be embossed or otherwise restructured to create structures, including diffractive or holographic structures, and thereby form the images or markings. Of course, the term printing also refers to more conventional and well known printing techniques such as intaglio, engraving, offset printing, inkjet printing, laser printing, screen printing, flexographic printing, etc.

Opacifying layers

It is possible to apply at least one opacifying layer on a transparent substrate to increase the opacity of the security document. An opacifying layer is such that L _T <L ₀ , L _o being the amount of light incident on the document and L _T being the amount of light transmitted through the document. An opacifying layer may include at least one of a variety of opacifying coatings. For example, opacifying coatings may include a pigment, such as titanium dioxide, dispersed in a binder or carrier of crosslinkable heat activated polymeric material. Alternatively, a transparent plastic substrate could be interposed between opacifying layers of paper or other partially or substantially opaque material on which markings could be subsequently printed or otherwise applied.

Security device or element

As used herein, the term security device or element includes any one of a large number of security devices or elements intended to protect the security document or token from counterfeiting, copying, alteration or break-in. The security devices or elements can be provided in or on the substrate of the security document or in or on at least one layer applied to the base substrate, and can take a wide variety of forms, such as security wires embedded in layers of the security document; security inks such as fluorescent, luminescent and phosphorescent inks, metallic inks, iridescent inks, photochromic, thermochromic, hydrochromic or piezochromic inks; printed and embossed elements, including relief structures; interference layers; liquid crystal devices; lenses and lenticular structures; optically variable devices (OVD) such as diffractive devices, including diffractive gratings, holograms and diffractive optical elements (DOE).

Embossable radiation curable ink

The term embossable radiation curable ink used herein refers to an ink, lacquer or other coating which can be applied to the substrate in a printing process, and which can be embossed before curing to form a relief structure and cured by radiation to fix the embossed relief structure. The curing process does not take place before the embossing of the radiation curable ink, but it is possible for the curing process to be carried out either after the embossing or at substantially the same time as the embossing step. The radiation curable ink is preferably curable by ultraviolet (UV) radiation. Alternatively, the radiation curable ink can be cured by other forms of radiation, such as electron beams or X-rays.

The radiation curable ink is preferably a transparent or translucent ink formed from a colorless resin material. Such a transparent or translucent ink is particularly suitable for printing transmissive security elements such as subwavelength gratings, transmissive diffractive gratings and lens structures.

In a particularly preferred embodiment, the transparent or translucent ink preferably comprises a lacquer or a colorless embossable UV curable coating based on acrylic.

Such UV curable lacquers can be obtained from various manufacturers, including Kingfisher Ink Limited, product UVE-203 of ultraviolet type, or the like. Alternatively, the radiation curable waffle coatings may be based on other compounds, for example nitrocellulose.

The radiation curable inks and lacquers used here have been found to be particularly suitable for embossing microstructures, in particular diffractive structures such as diffractive gratings and holograms, and microlenses and lens gratings. However, they can also be embossed with larger relief structures, such as optically variable non-diffractive devices.

The ink is preferably embossed and cured by ultraviolet (UV) radiation substantially simultaneously. In a particularly preferred embodiment, the radiation curable ink is applied and embossed substantially simultaneously in an etching printing process.

Preferably, in order to be suitable for engraving printing, the radiation curable ink has a viscosity located substantially in the range of about 20 to about 175 centipoise, and more preferably about 30 at around 150 centipoises. The viscosity can be determined by measuring the time necessary for the lacquer to empty itself from a Zahn n ° 2 cup. A sample which empties in 20 seconds has a viscosity of 30 centipoise, and a sample which empties in 63 seconds has a viscosity of 150 centipoise.

With certain polymeric substrates, it may be necessary to apply an intermediate layer on the substrate before applying the radiation curable ink in order to improve the adhesion of the embossed structure formed on the substrate. The intermediate layer preferably comprises a primer layer, and more preferably, the primer layer comprises a polyethyleneimine. The primer may also include a crosslinking agent, for example a multifunctional isocyanate. Examples of other suitable finishes for use in the invention include: hydroxyl-terminated polymers; hydroxyl-terminated polyester-based copolymers; crosslinked or uncrosslinked hydroxylated acrylates; polyurethanes; and UV curable anionic or cationic acrylates. Examples of suitable crosslinking agents include: isocyanates; polyaziridines; zirconium complexes; aluminum acetylacetone; melamines; and carbodümides.

Brief Description of the Drawings Preferred embodiments of the present invention will now be described by way of examples, only with reference to the accompanying drawings.

[Fig-1] is a schematic illustration of an optically variable device of the prior art, intended to generate a permutation effect of contrast.

[Fig.2] shows the positive image generated by the contrast permutation effect device shown in Figure 1.

[Fig. 3] shows six different images generated by the contrast permutation effect device of FIG. 1 in transition from the positive image to the negative image from six different viewing angles.

[Fig.4] is a schematic representation of a test pattern according to the present invention having four regions at different image element offsets covered by an array of microlenses.

[Fig.5] is a schematic illustration of a variation of the test pattern shown in Figure 4 in which the image element offsets are positioned so that no region generates maximum brightness or minimum for the detection camera.

[Fig. 6] is a schematic illustration of a test pattern according to the present invention in which the period of the image element is 25% greater than the period of the array of lenticular lenses.

[Fig.7] shows a lower row of images representing moiré magnifications of a measurement test pattern at six different offset locations while the upper row shows the corresponding moiré magnification image generated through the product area of the device, seen from the detection camera.

[Fig.8] shows a contrast measurement system in which the test pattern is viewed by the detection camera in the transmitted light.

[Fig.9] shows a contrast measurement system in which the test pattern is viewed by the detection camera in the light reflected against a background surface with high contrast.

[Fig.10] shows a contrast measurement system in which the test pattern is viewed indirectly through a printing substrate in the transmitted light.

[Fig.l 1] shows a contrast measurement system similar to that of Figure 9 in which a white test pattern is viewed indirectly by the detection camera through the printing substrate 12 in the light reflected against a dark background with high contrast.

[Fig. 12] shows a contrast measurement system in which the test pattern is viewed by the detection camera in the reflected light with the array of opposing lenses against a background surface of high contrast.

[Fig. 13] shows four banknotes in a micro-imaging area intended to generate an optically variable effect and a test pattern printed outside the micro-imaging areas, but in the periphery of the banknotes;

[Fig. 14] shows a variant of FIG. 13, in which the test pattern is printed in the micro-imaging areas in each of the banknotes;

[Fig. 15] shows another possibility in which the test pattern for each of the banknotes is not only printed outside the microimaging areas, but also beyond the periphery of the associated banknote; and [fig. 69 16] shows a sheet of banknotes with the test pattern printed on the peripheral margin of the sheet surrounding the bill matrix, so that each column of notes is associated with a single test pattern, respectively.

Detailed description With reference to FIG. 4, an image contrast measurement system is illustrated diagrammatically in the form of the test pattern 72 having four regions 36, 38, 40 and 42. The four regions of the pattern d Test 72 all have picture elements 46, 48, 50 and 52 respectively printed on one side of the printing substrate 12, optionally at a position spaced from the product area. However, the lenses 14 have the same geometry and pitch as the lenses in the product area, and the test pattern picture elements are printed with the same printing station used to apply the micro images in the product area.

The picture elements in each region have different offsets relative to the vertices 62 of their associated lenses 14. The amount of offset increases in fixed increments for each successive region, each increment being the lens pitch divided by the number of regions. In region I, the picture elements 46 are offset by an amount 64 relative to the apex 62 of the associated lens 14. The geometry of the lenses 14 is such that the focal length is longer than the distance between the apex of the lens and the picture elements. From the angle of view of the camera 54, each lens 14 focuses on bands 44, 56, 58, 60, etc. respective extending to the lenses along the opposite side of the printing substrate 12. In practice, the lens geometry and / or the thickness of the substrate 12 is chosen so that the width of these focal strips 44, 56, 58 and 60 roughly matches the width of the picture elements 46, 48, 50 and 52 shown in the test pattern. It is thus possible to provide the greatest difference in color brightness detected in all regions.

In the example shown in Figure 4, the picture elements are approximately half the width of the lenses 14. In region I, the test pattern 36 has picture elements 46 at an offset 64 of the lens vertices 62. At this offset, the focal strip 44 of the lenses 14 sees half of a printing element 46. In region II, the test pattern 38 has picture elements 48 at one offset 66 smaller with respect to the vertices 62 of the lenses 14. In this region, the focal bands 56 see all of the picture elements 48. Consequently, the color brightness seen in region II must be greater than the region I and therefore at minimum brightness in all regions (assuming that the picture elements are darker on the spaces and the background).

In region III, the focal bands 58 see half of the picture elements 50 in the test pattern 40 because the offset 68 is set to zero. The brightness detected in regions I and III must be equal since the lenses 14 in each region see the same proportion of the image elements in the test patterns.

In region IV, the picture elements 52 are at an offset 70 relative to the vertices 62, so that the focal bands 60 see none of the picture elements 52 in the test pattern 42. The Color brightness seen by the camera in region IV is therefore the highest of the four regions and theoretically constitutes the highest possible level. Conversely, region II should have the lowest possible light level, the difference between these light levels therefore providing an accurate measure of the color contrast in the magnified image seen in the product area. If the color contrast detected in the four regions of the full test pattern 72 reaches or exceeds a predetermined threshold, the optically variable device is suitable.

Although the test pattern 72 shown in Figure 4 has four regions with picture elements at fixed offsets of 0%, 25%, 50% and 75% of the width it can be seen that the test pattern requires only a minimum of two regions in which the picture elements are at two offsets, preferably equal to half the width of the lenses. However, the benefits of having additional regions and a wider range of shifts are discussed in more detail below.

Again with reference to FIG. 4, each region effectively implements a contrast permutation effect in which the stages of contrast permutation are seen from a single angle of view 54 (i.e. that of the camera ). The regions of minimum and maximum brightness (regions II and IV respectively) represent the contrast permutation effect generated by the optically variable device in the product area, seen from two complementary viewing angles. The detection camera displays the test pattern 72 along the perpendicular 54 to the lenses 14 to simultaneously acquire the projected image. The brightness level in each region is determined and the highest and lowest brightness levels are used for the calculation of the image contrast.

A common measure of contrast is the Michelson contrast, which is calculated as follows:

Michelson contrast = τ ^max , ί · ¹¹ ™ ° ^u 'hnax “Γ • min

I _max is the maximum brightness level detected; and

I _min is the minimum brightness level detected.

Those skilled in the art will understand that other definitions of contrast can be used, such as:

- the luminance contrast;

- Weber contrast;

- RMS contrast (mean square);

- any defined function of the brightness levels, color intensities or gray levels detected in the magnified image of the test pattern 72 acquired by the camera.

As mentioned above, a contrast measurement theoretically requires only two regions of different brightness levels, preferably at maximum brightness and minimum brightness. However, in an actual production environment, the offset of the lines of print elements in the test pattern, relative to their associated microlenses, varies from product to product due to alignment tolerance between lens and support of the printing press, substrate deformation, etc. As a result, in some products (i.e. optically variable devices), and therefore their corresponding test patterns, no region will have picture elements at offsets that project images at maximum and minimum brightness levels. according to the fixed viewing angle of the detection camera. In other words, none of the regions will have the precise shifts shown in regions II and IV shown in Figure 4. The projected maximum and minimum brightness levels will rather be lower or higher than the actual maximum and minimum values, respectively.

Figure 5 is a schematic representation of this scenario. Here, the picture elements 46 to 52 in regions I to IV are respectively at effective shifts of -12.5%, 12.5%, 37.5% and 62.5%. In other words, they differ by a theoretical or optimal lag of 12.5%. In this case, regions I and IV of test pattern 74 have the highest brightness levels because their focal bands 44 and 60 see the least amount of picture elements 46 and 52, respectively. Likewise, regions II and III of test pattern 74 project the lowest brightness levels because their focal bands 56 and 58 see more picture elements 48 and 50, respectively. However, the brightness of regions I and IV is lower than the maximum brightness level (which would be seen from a different angle of view) and conversely, the brightness level in regions II and III is higher than the minimum brightness levels seen from the complementary viewing angle.

Assuming that the brightness is proportional to the unprinted area of the focal strips 44 to 60, then the highest and lowest brightness levels detected by the camera in the test pattern 74 will differ from the levels 12.5% real maximum and minimum brightness. More generally, the percentage of maximum difference between the measured luminosity and the maximum or minimum luminosity level (number of fixed offsets) ^{2 × 100} This illustrates the possibility of alleviating the problem of measuring the maximum luminosity levels and real minimums with a camera at a fixed viewing angle, while taking into account production tolerances causing a variation in the offset between the microlens and the printing element using a higher number of pattern regions d , each with a different print element offset. The test patterns 72 and 74 shown in Figures 4 and 5 each use four different offsets. However, it will be seen that more precise contrast measurements can be obtained if eight uniformly distributed offsets are used (i.e. at 0%, 12.5%, 25%, 37.5%, 50%, 62 , 5%, 75% and 87.5%, expressed as a percentage of lens width). With eight different fixed offsets in eight different regions, the maximum possible irregularity from the maximum and minimum brightness intensity levels is equal to

4- 8 4- 2 X 100 = 6.25%. In other words, the maximum irregularity [0083] [0085] [0086] [0087] [0088] [0089] [0090] is halved compared to that shown in Figure 5, in which four offsets for four regions are used.

With reference to FIG. 6, another embodiment is illustrated, in which the number of fixed offsets (and therefore the number of corresponding regions) is considerably increased.

The technique used in Figures 4 and 5 has test patterns 72 and 74, in which four regions have four different offsets distributed uniformly over a lens width. In test pattern 76 shown in Figure 6, each region consists of a single microlens and the regions are adjacent to each other in a contiguous manner. The picture elements 46, 48, 50 and 52 are spaced uniformly at a step given by the following equation:

no printing element = no lens + (no lens number of shifts distributed uniformly)

In the diagrammatic illustration of Figure 6, the print element pitch is: lens width + (lens width + 4) = 1.25 x 1 lens width

This formula applies if the picture elements are shifted to the right with each next lens 14. If the picture elements are shifted to the left, each subsequent lens moving from left to right, then the formula for the picture element step is given by the following equation:

no print element = no lens - (no lens 4- number of shifts evenly distributed)

The test pattern shown in Figure 6 provides picture elements 46, 48, 50 and 52 in the form of parallel lines having a pitch different from that of the lens pitch. This lack of correspondence between the lens pitch and the image element pitch generates a moiré magnification seen by the detection camera (not shown). From the captured moiré magnification image, the highest and lowest brightness intensity levels are measured by detecting the light and dark bands (see Figure 7). Using the highest and lowest brightness levels, it is possible to determine an accurate measure of image contrast.

The period of the magnified moiré bands can be calculated using the following known moiré magnification formula:

Moire period = (no lens x no picture element): Abs (no picture element - no lens).

In the examples shown in Figure 6, the moiré period is equal to five times a lens pitch. Since the microlenses 14 are very small, this example is a very short moiré period and would be difficult to detect the highest and lowest brightness levels. In the same way, the problems mentioned above concerning the insufficient fixed offsets used, the detected maximum and minimum brightness levels can differ considerably from the actual maximum and minimum values. In this context, the test pattern 76 used in an actual production environment generates a much longer moiré period to allow the camera to detect the actual maximum and minimum brightness levels. For example, a test pattern 76 using picture elements 46 to 52 with a pitch that is 1% greater than that of the lens pitch (which is equivalent to using 100 different fixed offsets) will produce a moiré period equal to 101 times the lens width. If, for example, the lens width is 64 microns and the print element pitch is 64.64 microns, the dark and light bands generated in the moiré magnification will have a period of 6.464 mm. Detecting maximum and minimum brightness levels in part of the 6.464 mm wide moiré magnification of test pattern 76 is simple for the image processing device receiving images captured from the detection camera. The maximum error or irregularity between the highest and lowest brightness levels and the maximum and minimum brightness levels will be 0.5%:

° - ^s = (10o) = ^{2 x 100} The moiré magnification of the test pattern 76 consists of a repeated pattern of light and dark bands with a single dark band and a single light band in each moiré period. From these image brightness data, it is possible to determine any of the different contrast measurements mentioned above (Michelson contrast, RMS contrast, Weber contrast, etc.).

The bands need not be created by a mismatch between the pitch of the image elements and that of the lenses. Optionally, the moiré magnification of the test pattern 76 can be generated using a relative oblique between the microlenses 14 and the image elements 46 to 52. Those skilled in the art will also understand that a magnification of moiré can also be generated by a variation of the pitch of the picture element (relative to the pitch of the lens) and the introduction of a relative oblique between the array of lenses 14 and the picture elements 46 to 52. A using known moiré strip configurations created in moiré magnifications generated by mismatches of steps and / or obliques, a period and a moiré strip orientation defined with respect to the detection camera can be regulated and established according to a predetermined design. However, the contrast measurement test pattern and the associated microlens array must be large enough to cover an area of at least one period of magnified moiré band, which is appropriately detected by the level detection camera. highest and lowest brightness in moiré magnification.

Figure 7 shows a moiré magnification of a test pattern and the magnified image generated in the product area of the associated optically variable device. The upper row shows the magnified images 78A to 78F generated by the optically variable device in the product area, while the associated moiré magnifications 80A to 80F are shown directly below in the lower row. Each of the moiré magnifications 80A to 80F indicates a magnified moiré period. That is, the test pattern size is equal to a moire period, which allows the detection camera to capture the highest 82 and the lowest brightness intensity levels 84 for the magnified image of moiré.

The moiré magnifications 80A to 80F show the particular moiré magnifications seen by the detection camera for six different image shift distances between the lens tops and the printed images (as mentioned above in relation to the figures 4 and 5). It is easy to predict the occurrence of these different offset distances in a production environment due to the various tolerances involved, as discussed above. However, despite the different offset positions, the same highest 82 and lowest 84 brightness levels are captured. Only the position of the light bands 82 and the dark bands 84 changes according to changes in the offset position between the lenses and the picture elements of the test pattern. The measured contrast will therefore be regular and independent of variations between the lens and the impression inherent to some extent in the production environment. The measured contrast will be different from the actual contrast by a negligible percentage which can be calculated using the following equations:

(Number of fixed offsets) ' ^x Ιθθ in which:

number of fixed offsets = no lens 4- Abs (no picture element - no lens).

The techniques described above in connection with FIGS. 4 to 7 provide a test pattern for contrast measurement which is printed using the same printing station (eg an engraving cylinder) which prints the micro-imaging of the optically variable device product. As a result, the contrast of the magnified image of the optically variable device product will be accurately represented in the contrast of the magnified test pattern image. These techniques described above overcome the problem of the variation of phase shift between the lens and the printing element which complicates the direct measurement of the contrast in the magnified image generated by the optically variable device product. Providing a test pattern projected consistently and accurately representative of an accurate contrast measurement allows a single detection camera viewing from a single angle of view to provide an accurate contrast measurement while operating in line and at normal production speeds. As mentioned above, the contrast test pattern can be placed inside or outside the product area.

It should be noted that it is not necessary for the test pattern image elements to have the same size as micro-imaging. However, test pattern imagery and micro-imagery (as well as interspersed spaces) of similar size will provide a measure of contrast more representative of the contrast in the magnified image. While it is preferable that the test pattern elements and micro-imaging are broadly equivalent, it is therefore not essential. If the test pattern item sizes and spaces are smaller (e.g. 50% smaller) or larger (e.g. 100% larger), the techniques will still work. Likewise, the test pattern can be made up of several sizes of image elements and of several sizes of spaces, for example sizes which are in a range or a distribution. Indeed, it is possible to use a variety of test pattern geometries in a single test pattern (such as geometries with element sizes and random space sizes), but the sizes of Test pattern picture elements and spaces corresponding to those of product imaging are preferable in most situations.

Figures 8 to 12 show other techniques for measuring the contrast in accordance with the present invention. However, these techniques differ from the methods described above in that:

- the detection camera visualizes the image elements of the test pattern directly; or

- the detection camera visualizes the image elements of the test pattern through the substrate, but not through the lens array; or

- the detection camera visualizes the image elements of the test pattern through at least one layer of ink applied to the test pattern.

In the measurement technique represented in FIGS. 8 to 12, the picture elements of the test pattern and the spaces between the picture elements are preferably constant and of similar size to the picture elements and spaces. interposed in the micro-imaging network of the product area (although this is not essential, as mentioned above). However, in these techniques the offset between the microlens and the printing element is not a relevant parameter. Again, the test pattern can be printed outside the product area encompassing the optically variable device or it can be incorporated into the product area and be part of the micro-imaging used by the optically variable device .

[0101] Direct viewing by the camera of the test pattern image elements

As shown in FIG. 8, if no microlens is opposed to the test pattern, it will be preferable to visualize the test pattern 36 in the light transmitted from a white light source 88 positioned opposite of the field of vision 92 of the detection camera 90. The camera directly displays the picture elements 18 and the interspersed spaces 16, which are sized to correspond overall to the sizes of the micro-imaging elements and spaces interspersed in the product area in order to guarantee a measurement of a realistic representation of the contrast.

[0102] Figure 9 shows microlenses 14 opposite the test pattern 36. In this situation, transmission lighting is not appropriate because it produces variable magnified images for the camera 90. Consequently, the pattern of test 36 is viewed in reflection from a white light source 88 on the camera side of the substrate

12. For a more precise contrast measurement, the lens side of the substrate 12 is held against a high-contrast bottom surface 96, such as a white surface, which can be formed by one of the rollers in the pressure of impression. The color of the background surface 96 is of a highly contrasted color with respect to that of the test pattern picture elements 18. In the light reflected with the lenses 14 in direct contact with the contrasting background surface 96, magnified images are not generated or viewed by the camera 90. Similarly, direct contact between the lenses 14 and the contrasting support surface 96 prevents the formation of magnified optical images of the image elements 18 by the reflected light from the internal lens to the aerial interface focused on the image elements 18.

It should be noted that the use of lighting by reflection as presented in FIG. 9 also allows the camera 90 to acquire an appropriate image of the test pattern 36, if the lenses 14 are not present.

[0104] Visualization by the camera of the test pattern picture elements through the substrate but not through the lens array [0105] With reference to FIG. 10, the camera 90 visualizes the test pattern 36 through through the substrate 12 (formed with a suitable transparent or translucent material), but not through an array of lenses formed as the opposite side of the substrate. In the lighting arrangement 98 shown in Figure 10, the white light source 88 is positioned on the opposite side of the substrate 12, so that the camera views the picture elements 18 of the test pattern in the light transmitted, however placing the light source 88 on the same side of the substrate 12 as the camera 90, so that the test pattern 36 is viewed in reflected light, would also be appropriate, if the test pattern is placed in contact with a surface having a contrasting color. An example of this configuration is illustrated as Figure 11.

In FIG. 11, the test pattern 102 is made up of a series of white image elements 18. Consequently, the test pattern 102 is placed against the contrasting dark surface 104 (here again possibly the surface of a roll) so that the camera 90 views the test pattern 102 through the substrate 12 in the reflected light provided by the white light source 88.

The decision to use transmission or reflection lighting may depend on whether at least one layer of contrasting color is or is not overprinted on the test pattern 36. If a layer of contrasting ink overprinted on the elements image 18 viewing the test pattern 36 through the substrate 12 in the reflected light is preferred, the overprinted layer will preferably be placed in contact with a surface of similar color in order to enhance the contrast of the acquired image on the test pattern.

Visualization by the camera of the test pattern picture elements through at least one layer of overprinted ink [0109] The arrangement 106 presented in FIG. 12 shows the picture elements 18 of the pattern test 36 visualized by the camera 92 through a layer of contrasting ink 108 applied to the test pattern 36. As mentioned above, the presence of the lenses 14 on the opposite side of the substrate 12 means that the transmission lighting does not must not be used in order to avoid the generation of magnified images in the field of view of camera 92. The image elements 18 must rather be viewed from the side known as “reverse lens side” in a reflection lighting provided by the white light source 88. Similarly to the arrangements 94 and 100 shown in FIGS. 9 and 11 respectively, the lenses 14 are in direct contact with a support surface 96 of color for contrasted compared to that of the picture elements 18. This configuration of reflection lighting and of a support surface of contrasting color 96 placed against the lens side effectively eliminates all magnified optical effect images liable to appear. in other lighting arrangements. As mentioned above, the magnified images of the image elements 18, generated on the reverse side of the lens in transmission lighting, are undesirable optical effects which cause the detection camera to identify false refusals.

If the lenses are not present on the arrangement 106 presented in FIG. 12, the test pattern 36 can be viewed in transmission or reflection lighting. If reflection lighting is used, the substrate 12 is placed against a contrasting support surface 96, which has the effect of enhancing the contrast between the test pattern 36 and its background.

To measure the contrast using the techniques presented in Figures 8 to 12, a brightness measurement is determined from the image of the test pattern 36 acquired by the camera 92. As an example , a possible measure of brightness is the average gray level of a gray scale image acquired from the test pattern. In other words, each pixel in the image captured by the camera has a gray level that can be used as a measure of brightness for that pixel. The average gray level is simply the sum of the gray levels of each pixel, divided by the number of pixels. This luminosity measurement is compared with a predetermined luminosity threshold value to verify whether the contrast of the optically variable device in the product area is acceptable.

The brightness measurement calculated from the acquired test pattern image is representative of the image contrast in the optically variable device because the size of the picture elements 18 in the test pattern 36 roughly corresponds the size of the picture elements in the product area. A variation in the size of the image elements in the product area results in a corresponding variation in the contrast in the magnified images generated by the optically variable device. Since the test pattern is printed with the same printing station (such as an engraving roller) as that which prints the micro-imagery in the product area, the variations in the micro-imagery correspond to the variations in the picture elements 18 in the test pattern. An increase or decrease in the size of the picture elements 18 and the test pattern 36 results in a corresponding increase or decrease in the size of the unprinted areas of the test pattern 36. As a result, the brightness measurement of the test pattern 36 derived from image data acquired by camera 90 will be representative of the contrast in the magnified image of the optically variable device in the product area. The use of an averaging technique or similar image data manipulation to derive a single brightness measurement is necessary because the actual dimensions of the picture elements 18 and interspersed spaces 16 of the pattern. trial 36 are under the spatial resolution of camera 92 and a small change in size (having a negative impact on product contrast) cannot be measured from the acquired camera image.

Method for detecting online faults

The lighting configurations and arrangements shown in Figures 8 to 12 can also be applied for the purpose of detecting defects in the microimaging printed in the product area. In this case, missing parts of the micro-imagery used by the optically variable device are detected directly using the image acquired by the camera instead of the calculation of a brightness measurement generated by the picture elements. 18 of test pattern 36. The missing parts of the microimaging are detected in the image acquired by the camera through a process of comparison to a gold model. The missing parts detected have a minimum size corresponding to the image acquisition resolution of the camera system.

The production of optically variable devices typically involves supplying the lenses to the substrate before the application of micro-imaging on the reverse side, the inspection of the missing parts of product micro-imaging therefore being carried out with the lenses. 14 present as the substrate 12. As mentioned above, the camera must acquire the images from the micro-imaging in a lighting by reflection with the lenses in direct contact with a support surface of a color strongly contrasted with that of the micro-imaging. . In addition, the camera must acquire these images before applying any other layer on the micro-imagery. This guarantees the acquisition of images with maximum contrast and therefore maximum defect detection sensitivity.

[0115] Location of test pattern

Figures 13 to 16 show various possibilities for positioning the test pattern with respect to the printed article and / or the product area which generates the optically variable effect. For purposes of illustration, the printed articles shown in Figures 13 to 16 are bank notes 110, each of which has a product area 112 containing micro-imagery used to generate an optically variable effect. The product area 112 is typically covered by a network of microlenses to create an optically variable synthetic magnification of the underlying microimaging. These optically variable devices based on lenses constitute an effective element in the fight against counterfeiting which is extremely difficult to reproduce. Individual banknotes can be printed on a continuous strip extending between a supply roll and a reception roll, or they can be printed on individual sheets which are then guillotined into individual 110 banknotes. The support substrate sheet may have the banknotes arranged in a rectangular array. An engraving printing process can be used with the dimensioned engraving printing cylinder to print a single sheet of banknotes (arranged in a rectangular array) per cylinder rotation. As discussed above, the same cylinder prints both microimaging 112 and test pattern imagery 114.

As shown in FIG. 13, the test pattern 114 can occupy an area in the external periphery of each of the banknotes 110, but external to the microimaging in the product area 112.

In Figure 14, the test pattern 114 is printed within the product area 112 of each of the banknotes 110. This may require careful design to incorporate or merge the pattern imagery d test 114 and the other micro-imaging of the product area 112, in order to generate the desired optically variable effect. This arrangement has the advantage of not requiring a separate second microlens array for the test pattern (as may be the case in Figure 13).

In FIG. 15, the test patterns 114 for each of the banknotes 110 are printed externally to the banknotes. Thus, the test patterns 114 are not part of the final printed articles 110. The test patterns 114 are rather used to assess the contrast in the product area 112, then eliminated when the sheet or strip is guillotined in banknotes 110 individual bank accounts. This option does not require an additional (or extended) microlens array for each of the test patterns 114 and a greater proportion of the support substrate is "wasted" to accommodate the test patterns 114.

A more profitable use of the support substrate 116 is presented in FIG. 16. The sheet 116 of support substrate has peripheral zones 118, 120 and 122 surrounding the rectangular network of banknotes 110. The test patterns 114 can be positioned anywhere in these peripheral margins, however for convenience the test patterns can be located in the peripheral margin zone 122 directly aligned with each column of the product zones 112. Thus, the network of microlenses can be applied continuously along the entire sheet 116. In addition, Figure 16 indicates that it is not necessary to provide a test pattern 114 for each of the product areas 112.

The present invention has been described here by way of example only. Those skilled in the art will readily recognize that the many variations and modifications do not depart from the spirit and scope of the general inventive concept.

Claims (1)

claims [Claim 1] A system for detecting defects in printed imagery used to generate optically variable images, the system comprising: a print supply path for a support substrate to be printed with the imagery; a printing station for printing the imagery and a test pattern on the support substrate, the test pattern having picture elements spaced apart by spaces; and an image sensor for capturing images of the test pattern; so that, brightness levels acquired by the image sensor are used to derive a contrast measurement for comparison with a reference contrast level. [Claim 2] The system of claim 1, wherein the picture elements and spaces of the test pattern are of uniform size. [Claim 3] The system of claim 1 or 2, wherein the image elements and spaces of the test pattern are sized to substantially match those of the printed imagery used to generate the optically variable image. [Claim 4] The system of any of claims 1 to 3, wherein the printed imagery is micro-imagery used in a security device with an array of microlenses to generate optically variable magnifications; and the test pattern picture elements each have a microlens associated so that at least some of the picture elements are printed at different displacements relative to the respective microlenses to generate respective magnified images with different levels of image brightness, viewed from a predetermined angle of view, the different levels of image brightness defining a range for comparison to a predetermined acceptable brightness range to provide a measure indicative of a contrast in the optically variable magnifications of the device of security. [Claim 5] A method of detecting defects in printed imagery used to generate optically variable images, the method comprising: providing a support substrate along a print supply path; printing the imagery and a test pattern on the substrate of support with a printing station, the test pattern having picture elements spaced apart by spaces; capturing images of the test pattern with a camera; and, recording brightness measurements of the test pattern for comparison to a reference brightness level in order to detect faults. [Claim 6] The method of claim 5, wherein the picture elements and spaces of the test pattern are printed with a uniform size. [Claim 7] The method of claim 5 or 6, wherein the image elements and spaces of the test pattern are sized to substantially match those of the printed imagery used to generate the optically variable image. [Claim 8] The method of any of claims 5 to 7, wherein the printed imagery is micro-imagery used in a security device with an array of microlenses to generate optically variable magnifications; and the test pattern picture elements each have a microlens associated so that at least some of the picture elements are printed at different displacements relative to the respective microlenses to generate respective magnified images with different levels of image brightness, viewed from a predetermined angle of view, the different levels of image brightness defining a range for comparison to a predetermined acceptable brightness range to provide a measure indicative of a contrast in the optically variable magnifications of the device of security. [Claim 9] A printing assembly configured to detect defects in printed imagery used to generate optically variable images, the printing assembly comprising: a printing station for printing the imagery and a test pattern on the substrate of support, the test pattern with picture elements spaced apart by spaces; and an image sensor for capturing images of the test pattern; so that brightness levels are acquired from the test pattern to derive a contrast measurement for comparison to reference image contrast levels. [Claim 10] The printing assembly according to claim 9, wherein the picture elements and spaces of the test pattern are of uniform size. [Claim 11] Printing assembly according to claim 9 or 10, in which the image elements and the spaces of the test pattern are dimensioned to substantially match those of the printed imagery used to generate the optically variable image. [Claim 12] A print assembly according to any of claims 9 to 11, wherein the printed imagery is micro-imagery used in a security device with an array of microlenses to generate optically variable magnifications; and the test pattern picture elements each have a microlens associated so that at least some of the picture elements are printed at different displacements relative to the respective microlenses to generate respective magnified images with different levels of image brightness, viewed from a predetermined angle of view, the different levels of image brightness defining a range for comparison to a predetermined acceptable brightness range to provide a measure indicative of a contrast in the optically variable magnifications of the device of security.