US20020066543A1

US20020066543A1 - Fluorescent micro-particles embedded in a pigmented fluorescent coating for optical document security

Info

Publication number: US20020066543A1
Application number: US10/006,255
Authority: US
Inventors: William Lilly
Original assignee: Spectra Systems Corp
Current assignee: Spectra Systems Corp
Priority date: 2000-12-05
Filing date: 2001-12-04
Publication date: 2002-06-06
Also published as: WO2002046528A1; AU2002220249A1

Abstract

A security feature (1) that is used with a substrate (2). The security feature includes a plurality of fluorescent micro-particles (1A) that form a background component of the security feature and a plurality of fluorescent discrete particles (1B) that form a foreground component of the security feature. The discrete particles are larger in size than the micro-particles and are optically distinguishable from the micro-particles under at least one illumination condition. In the preferred embodiment the security feature is applied as a coating to the substrate, and the security feature further includes a coating binder (1C) in which the micro-particles and the discrete particles are contained. The coating binder can be made of, for example, at least one of an ink base, an adhesive, an epoxy, a varnish, a polymer solution, or a dry material having binding properties. The coating binder may be substantially transparent or substantially opaque. The discrete particles are optically distinguishable from the micro-particles under long wavelength ultraviolet light and/or under short wavelength ultraviolet light. Under no or low magnification conditions the security feature exhibits a generally uniform color to the naked eye, while in one embodiment under higher magnification conditions the discrete particles exhibit a first color while the micro-particles of the background component exhibit a second color, while in another embodiment the discrete particles exhibit a first color while the micro-particles of the background component exhibit a lack of color, while in a further embodiment the micro-particles of the background component exhibit a first color while the discrete particles exhibit a lack of color. The security feature exhibits a fluorescent signature having attributes given by at least one of particle count per unit area, specific particle size range which may or may not differ by fluorescent color, relative intensities of measured spectral emission peaks due to foreground component particle fluorescence versus background component micro-particle fluorescence and specific emission wavelength values of measured spectral emission peaks due to foreground component particle fluorescence versus background component micro-particle fluorescence.

Description

TECHNICAL FIELD

These teachings relate generally to the use of fluorescent pigments and materials and, in particular, to the use of fluorescent pigments in applications where it is desired to provide authentication and/or security function for documents, banknotes, financial instruments and the like.

BACKGROUND

Coatings containing fluorescent pigments are currently used to create threads, fibers, and other security structures that are incorporated into the manufacture of banknote, financial, and legal document papers. These structures may be though of as security features that serve as optical authentication devices, as well as to prevent document counterfeiting. Such coatings may also be applied to product packaging to identify authentic products and deter product counterfeiting. The fluorescence of such coatings is typically not apparent when illuminated with ordinary ambient light sources, but becomes visible when excited by an ultraviolet light source.

Reference can be had to the following U.S. Pat. Nos.: 5,903,340, “Optically-Based Methods and Apparatus for Performing Document Authentication”, by N. M. Lawandy and T. J. Driscoll; and 4,863,783, “Security Paper”, by N. A. Milton. Reference can also be had to European Patent Application 0 219 743 A1, “Security Paper Containing Vesiculated Beads, by R. H. Hamilton; and to French Patent No.: 2 478 695, “Antifalsification Paper with Luminescent Particles, its Method of Production and Method of Producing Said Particles” (Aussedat Rey).

For example, Milton describes a paper that contains 30-500 micron size granules of 3-5 micron size pigment particles that are chemically bound together by a cross-link binder. In order to provide contrast between the pigment and the background during inspection the granules are essentially free of finer particles.

In French Patent No.: 2 478 695 a paper is provided that includes luminescent particles that are a 30-50 micron size conglomerate of 3-5 micron size pigment particles. The particles may emit different wavelengths when exposed to UV radiation, and combinations of colors are said to enable the papers to be personalized. In addition to the use of the fluorescent particles, the paper may also contain colored fibers and other conventional means of identification.

A problem that has not been adequately addressed by the prior art lies in complicating the task of the counterfeiter or forger by increasing the complexity of the optically-based security feature.

SUMMARY OF THE PREFERRED EMBODIMENTS

The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings.

In accordance with the teachings of this invention there are provided non-apparent fluorescent microscopic particles that are present within a coating for the purposes of increasing a maximum number of unique fluorescent emission combinations, also referred to herein as unique fluorescent signatures, for secreting these signatures from ready discovery, and for increasing the difficulty of creating counterfeit documents and product packaging.

These teachings are an advancement over the prior art since their use complicates the task of the counterfeiter or forger by increasing the complexity of an optically-based security feature. These teachings provide a security paper and substrate with an efficient multi-level encoding mechanism using particles containing pigment that emit light with different wavelengths and/or that emit or do not emit light depending on the characteristics of the stimulus. The use of light emitting particles within at least two different size regimes increases the complexity of the security feature and furthermore increases the amount of information that can be encoded by the light emitting particles.

Disclosed herein is a security feature that is used with a substrate. The security feature includes a plurality of fluorescent micro-particles that form a background component of the security feature and a plurality of fluorescent particles that form a foreground component of the security feature. The particles are larger in size than the micro-particles and are optically distinguishable from the micro-particles under at least one illumination condition. For example, the particles may have a size in the range of about 10 microns to about 20 microns in diameter and the micro-particles may have a size in the range of about 0.2 microns to about 2 microns in diameter. In general, and further by example, an average size of the particles is greater by a factor of at least five than an average size of the micro-particles, or the average size of the particles is greater by a factor of at least one order of magnitude than the average size of the micro-particles.

In the preferred embodiment the security feature is applied as a coating to the substrate, and the security feature further includes a coating binder in which the micro-particles and the particles are contained. The coating binder can be made of, for example, at least one of an ink base, an adhesive, an epoxy, a varnish, a polymer solution, or a dry material having binding properties.

The coating binder may be substantially transparent or substantially opaque. The particles are optically distinguishable from the micro-particles under long wavelength ultraviolet light (320-380 nm range, peaking at ˜365 nm) and/or under short wavelength ultraviolet light (180-280 nm range, peaking at ˜254 nm). Under no or low magnification conditions the security feature exhibits a generally uniform color to the naked eye, while in one embodiment under higher magnification conditions the discrete particles exhibit a first color while the micro-particles of the background component exhibit a second color, while in another embodiment the discrete particles exhibit a first color while the micro-particles of the background component exhibit a lack of color, while in a further embodiment the micro-particles of the background component exhibit a first color while the discrete particles exhibit a lack of color. In the presently preferred embodiment the security feature exhibits one of many possible unique fluorescent signatures having attributes given by at least one of particle count per unit area, specific particle size range which may or may not differ by fluorescent color, relative intensities of measured spectral emission peaks due to foreground component particle fluorescence versus background component micro-particle fluorescence and specific emission wavelength values of measured spectral emission peaks due to foreground component particle fluorescence versus background component micro-particle fluorescence. In the preferred, but not limiting embodiment, the discrete particles are invisible or nearly invisible to the naked eye.

Also described is an automatic reader system for resolving and identifying the various spectral and spatial properties of the security feature.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other aspects of these teachings are made more evident in the following Detailed Description of the Preferred Embodiments, when read in conjunction with the attached Drawing Figures, wherein: [0014]
FIG. 1 is graph showing an exemplary emission spectrum for a yellow-red system film; [0015]
FIG. 2A shows the film system of FIG. 1 as seen by the naked eye ([0016] magnification 1×), while FIG. 2B shows a microscopic view of the film system (magnification 100×);
FIG. 3 is a system-level block diagram of a reading system for stimulating the particles contained within a substrate and for reading the resulting emission(s) of light.[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention provides a novel approach for enhancing the security of documents, packaging and other substrates by including non-apparent fluorescent microscopic particles into a coating and/or directly into the substrate, for the purposes of increasing the number of unique fluorescent signatures possible, for secreting these signatures from ready discovery, and for increasing the difficulty factor in creating counterfeit documents and product packaging. While described herein in the general context of paper-based documents, such as legal documents (e.g., deeds and contracts) and financial documents (e.g., currency, stock certificates, bonds and letters of credit), these teachings may be employed with any of a number of different types of documents. These teachings may also be employed with other than paper-based substrate materials, where the term “substrate” should be broadly construed to include any type of substrate material that is suitable for being printed on, including ceramic, polymeric, plastic and plastic-containing substrate materials. [0018]
There are a number of possible embodiments of these teachings. These embodiments include, but are not limited to, the following embodiments shown by way of Examples A through I. [0019]

EXAMPLE A

Particles excited by Long Wavelength Ultraviolet (Long UV) light emit a fluorescent color X (for example, yellow, where the particles contain a 2-5 percent mixture of a fluorescent pigment of the benzothiazolyl family embedded in a clear, hard plastic matrix, or of similar construct using fluorescent pigments of other colors) and have a specific size range (for example, about 10 microns to about 20 microns in diameter). These particles are also referred to herein as discrete particles. A coating binder may be comprised of, for example, an ink base, an adhesive, an epoxy, a varnish, a polymer solution, or a dry powder material having binding properties. The coating binder contains a Long UV fluorescent pigment having an emission color Z (for example, a red fluorescent pigment such as a Europium chelate), with the pigment being milled into the coating to a much smaller particle size range (for example, 0.5 micron to 2.0 microns, also referred to herein as micro-particles), or being fully dissolved into the coating. This background field pigment loading in the coating may range from 2-20% by weight, with a preferred loading at about 10% (wet basis), so to result a dry basis loading of about 10-30%. For example, the average size of the discrete particles can be greater by a factor of at least five than the average size of the micro-particles. Further by example, the average size of the discrete particles can be greater by a factor of at least one order of magnitude than the average size of the micro-particles. The preferred loading of the discrete particles in a given film may vary between about 0.2-10% (wet basis) so to result in a dry basis loading of about 0.2-30%. Such wide variation of this attribute contributes to the large number of possible unique signatures within any single color combination or other single embodiment of the security feature, where such attribute is measurable as discrete particle count per unit area of coated or co-manufactured substrate. [0020]
The 10-20 micron sized discrete particles are combined and mixed with the coating binder and its micro-particles, or the discrete particles and the micro-particles could be added together into the coating binder, or they can be added in either order into the coating binder, and then mixed. In any case, the resulting liquid or solid phase coating material containing the discrete particles and the micro-particles is applied as a thin film to a substrate of interest. The coating material can be applied by painting on, rolling on, spraying on, through commercial printing techniques, or by any suitable application process, such as by writing the coating material onto the substrate using a pen or a marker (e.g., a felt-tip type marker) that contains the coating material. [0021]
The net visual effect to the naked eye, or under low magnification, is a single emission color Y (orange), which is the visually averaged combination of the yellow and red fluorescent emissions. However, inspection of the coating film under higher magnification reveals the presence of the discrete particles having the fluorescent color X (yellow in this example) disposed within a field comprised of micro-particles having fluorescent color Z (red in this example). If the coating binder is itself transparent, a thin film of this coating is translucent to transparent (substantially transparent) when viewed under ordinary ambient light by the naked eye. [0022]
An emission spectrum (as seen by the naked eye) of such a yellow-red system film is shown in FIG. 1. A drawing representing the appearance of the same film (as viewed by the naked eye vs. through a microscope) is shown in FIGS. 2A and 2B. FIG. 2B shows the micro-particles [0023] 1A, discrete particles 1B, and the coating binder 1C.

EXAMPLE B

Discrete particles excited by either Long or Short UV light emit a fluorescent color X (for example, blue) and have a specific size range (for example, 10 microns to 20 microns). The coating binder can be any one described above in Example A. The coating contains a Long UV/Short UV color-shifting background pigment blend having Long UV emission color X (for example, a blue emitting pigment of the stilbene family) and Short UV emission color Z (for example, an inorganic red emitting pigment such as Uveda™ YO obtained from United Mineral Corporation). Such a blend of background pigments may contain a ratio of between 1:5 and 1:10 Long UV: Short UV pigments. The blend is milled into the coating to a final size range (micro-particles of, for example, 0.5 micron to 2.0 microns) smaller than the discrete particles, then the discrete particles and micro-particles are added and thoroughly mixed into the coating (solution or dry powder). When a thin film of this coating is excited under Long UV light and viewed either by the naked eye or under high magnification, the net appearance is a flat field having emission color X (blue). Outlines of the discrete particles are visible under magnification. When the film is excited under Short UV light and viewed by the naked eye the net appearance is a flat field having emission color Y (violet). When viewed under high magnification, however, the presence of discrete particles having fluorescent color X (blue) standing in a field of fluorescent color Z (red) is revealed. If the coating binder used is transparent, this coating is translucent to transparent when viewed under ordinary ambient light by the naked eye. [0024]

EXAMPLE C

Discrete particles excited by Long UV light emit fluorescent color X (for example, blue), and when excited by Short UV light emit a different fluorescent color Y (for example, red) and have a specific size range (for example, 10 microns to 20 microns). The coating binder can be any one described in above in Example A. The coating contains a Long UV background pigment having emission color Z (for example, yellow), but having very little or no emission when excited by a Short UV light source. Again, the background pigment is milled into the coating and has a smaller size range (micro-particles of, for example, 0.5 micron to 2.0 microns) than the particles. The discrete particles and micro-particles are added and thoroughly mixed into the coating (solution or dry powder). When a thin film of this coating is excited under Long UV light and viewed by the naked eye, the net appearance is a flat field having emission color W (green, in this case). When viewed under high magnification the presence of the discrete particles having fluorescent color X (blue) standing in a field of fluorescent color Z (yellow) is revealed, similar to Example A above. However, when this film is excited under Short UV light and viewed by the naked eye the net appearance is a disperse field having emission color Y (red). When viewed under high magnification the presence of discrete particles having fluorescent color Y (red) standing in a non-fluorescing background field is revealed. If the coating binder used is transparent, this coating is translucent to transparent when viewed under ordinary ambient light by the naked eye. [0025]

EXAMPLE D

In this example the discrete particles emit Long UV color X (for example, yellow), but do not fluoresce under Short UV excitation. The coating binder can be any one described in above in Example A. The coating contains a Long UV/Short UV color-shifting background pigment having Long UV emission color Y (for example, blue) and Short UV emission color Z (for example, red). Again, the background pigment is milled into the coating and has a smaller size range (micro-particles of, for example, 0.5 micron to 2.0 microns) than the particles. The particles and micro-particles are added and thoroughly mixed into the coating (solution or dry powder). When a thin film of this coating is excited under Long UV light and viewed by the naked eye, the net appearance is a flat field having emission color W (green, in this case). When viewed under high magnification the presence of discrete particles having fluorescent color X (yellow) standing in a field of fluorescent color Y (blue) is revealed, similar to Example A above. However, when this film is excited under Short UV light and viewed by the naked eye the net appearance is a disperse field having emission color Z (red). When viewed under high magnification the presence of discrete particles having no fluorescence (dark spots) standing in a background field of fluorescent color Z (red) is revealed. If the coating binder used is transparent, this coating is translucent to transparent when viewed under ordinary ambient light by the naked eye. [0026]

EXAMPLE E

In this example non-fluorescing discrete particles were selected that may be opaque, translucent, or transparent, and have a specific size range (for example, 10 microns to 20 microns). The coating binder can be any one described in above in Example A, and the background pigment may be as described above in either Example A or Example B. When a film of this coating is excited under Long UV light and viewed by the naked eye the net appearance is a flat field having Long UV emission color X (for example, green). When viewed under high magnification the presence of discrete particles having no fluorescence (dark spots) standing in a background field of fluorescent color X (green) is revealed. If the background pigment is as described in Example B, and the film is excited under Short UV light, when viewed by the naked eye the net appearance is a flat field having Short UV emission color Y (for example, orange). When viewed under high magnification the presence of discrete particles having no fluorescence (dark spots) standing in a background field of fluorescent color Y (orange) is revealed. If the coating binder used is transparent, this coating is translucent to transparent when viewed under ordinary ambient light by the naked eye. [0027]

EXAMPLE F

Fluorescent discrete particles were selected having emission properties as described in any of Examples A, B, C or D, and have a specific size range (for example, <10 microns). A transparent coating binder is used, such as one of those described in Example A, but the coating contains no background pigment. When a film of this coating is excited under appropriate (Long and/or Short, per particle properties) UV light and is viewed by the naked eye the net appearance is a disperse field having emission color X (for example, green). When viewed under high magnification the presence of discrete particles having fluorescent color X (green) standing in transparent background field is revealed. When viewed under ordinary ambient light by the naked eye, this coating is transparent (invisible). [0028]

EXAMPLE G

Fluorescent discrete particles were selected having emission properties as described in any of Examples A, B, C, or D, and have a specific size range (for example, <10 microns). A transparent coating binder is used, such as described in Example A, and the coating contains a non-fluorescent, substantially opaque background pigment (for example, black). When a film of this coating is excited under appropriate (Long and/or Short, per particle properties) UV light and is viewed by the naked eye the net appearance is a disperse field having emission color X (for example, green). When viewed under high magnification the presence of discrete particles having fluorescent color X (green) standing in an opaque (black) background field is revealed. When viewed under ordinary ambient light by the naked eye, this coating is opaque (black). [0029]

EXAMPLE H

A mixture of two or more different fluorescent colored discrete particles is used having emission properties as described in any of Examples A, B, C, D or E, and having a predetermined size range (which may or may not differ according to particle fluorescent color). The coating binder can be as described in Example A, and the coating may contain any fluorescent or other background field pigment having properties as described in Examples A, B, C, D, E, F, or G. When a thin film of this coating is excited under appropriate (Long and/or Short, per particles and background micro-particle properties) UV light and viewed by the naked eye, the net appearance is a flat field having a single color (being the sum of emission wavelengths and intensities as integrated by the human eye). When viewed under high magnification, the presence of discrete particles having various fluorescent colors is revealed. The background pigment field color may or may not be of the same as any one of the particle colors. [0030]

EXAMPLE I

In accordance with a further aspect of these teachings a coating containing a binder as described in Example A, and containing any fluorescent or other background field pigment having properties as described in Examples A, B, C, D, E, F, or G, is first applied as a base film and allowed to dry or cure. A second coating, as described in Example F above, except that this coating may instead contain discrete particles as described in Example H, is applied as a thin film to the top surface of the base film. In this case, the discrete particles are not embedded in the pigmented base coating, but are surface mounted above the base coat. To the naked eye, when illuminated under appropriate UV light source from directly above, such binary coating system film appears as a flat field having a single color (being the sum of emission wavelengths and intensities as integrated by the human eye), but assumes a speckled or disperse appearance as the incident angle of UV illumination is changed. When viewed under high magnification, the layer of discrete particles in the top coat will be at a shorter focal length than the background pigment field color in the base coat. [0031]
The creation of a specific and unique fluorescence signature in any of the above Examples A-I is accomplished through a balance of pigment concentrations contained in the particles of the foreground component of the security feature coating and the micro-particles of the background component of the security feature coating, the overall concentrations of particles and of the background micro-particles, the excitation range(s) selected, and the fluorescent color(s) selected, all within an appropriate application thickness of the coating film. [0032]
Attributes of such fluorescent signature may include particle count per unit area, specific particle size range (which may or may not differ by fluorescent color for the case of Example H), net spectral characteristic relative intensities of measured UV emission peaks due to particle fluorescence vs. background pigment fluorescence, specific emission wavelength values due to particle fluorescence vs. background pigment fluorescence, and other characteristics such as particle density(s) and background field pigment concentration. For a given coating formulation, the values of certain of these attributes will vary as a function of applied film thickness. [0033]
Referring to FIG. 3, a [0034] security feature 1 as described above in the Examples A-I is provided on a substrate 2 and may be coupled with an automated reading device 10 to authenticate the presence of the security feature 1 and/or to verify its specific fluorescent signature. The reading device 10 can include an image collection element 12, which may include a digital or video camera image capture device 14, magnification optics 16, a physical standoff 16A to readily establish a correct focal distance between the optics 16 and the target coating film of the security feature 1 on the substrate 2, and appropriate UV target illumination source(s) 18 (Long UV) and 20 (Short UV). The reading device 10 also includes a measurement element 22 that can include a spectrometer 24 and image and spectral data processing elements 26, such as a real-time or captured image display 28, a data link 30 to a computer 32 having a memory 34, and image processing software 36 for evaluating the target image and spectra, and for comparing their values with signature-specific allowed value ranges. The memory 34 can store a lookup table or other type of data structure 34A wherein various authentic combinations of background and foreground colors, particle sizes, particle densities and the like are stored.
As an example, a particular document may be provided with an optical signature given by a thin film coating security feature containing background micro-particles that fluoresce with a yellow color when excited by Long UV, and that do not have any appreciable fluorescent emission when excited by Short UV, and with discrete particles of size about 20 microns that fluoresce with a red color when excited by Long UV and with a yellow color when excited by Short UV, where the particles have a density of about 10/mm[0035] ².
In this case the reading device [0036] 10 may be automatically operated so as to first illuminate the coating with the Long UV light source 18, and to detect the discrete yellow and red emissions, then to illuminate the coating with the Short UV light source 20, and to detect the discrete yellow emission (but of smaller magnitude than the magnitude of the yellow emission due to the background micro-particles when exited by the Long UV). The output of the image capture device 14 may also be processed to confirm that the red and yellow emissions originate from particles having a size of about 20 microns and a density of about 10/mm². If all of these criteria are satisfied then the substrate maybe declared to be genuine or authentic, as the security feature is validated against data stored in the data structure 34A. A further test, or an initial test, may be one performed by a human operator who verifies that the expected color(s) are present under different Long and Short UV illumination conditions. It can be appreciated that these teachings provide a multi-dimensional security feature (e.g., excitation source wavelength(s), emission wavelength(s), particle size(s), particle density(s), and emission amplitude(s)).
While described in the context of the use of the two UV sources (Long and Short) [0037] 18 and 20, a single tuneable output UV source could be used. Also, these teachings can be employed with a single UV source, e.g., either Long or Short, or with more than two UV sources, for example one that covers at least the 280 nm to 320 mn between the Short UV and the Long UV ranges. Also, these teachings are not restricted for use with only the described pigments, particle and micro-particle sizes, ranges of particle and micro-particle sizes and/or types of coating binders. Furthermore, while described in the context of a coating that is applied to form a thin film on the substrate 2, it is also within the scope of these teachings to incorporate at least the micro-particles and the particles into the substrate, such as by adding them during the substrate manufacturing process.

Claims

What is claimed is:

1. A security feature that is used with a substrate, comprising:

a plurality of fluorescent micro-particles that form a background component of said security feature; and

a plurality of fluorescent discrete particles that form a foreground component of said security feature, said discrete particles being larger in size than said micro-particles and being optically distinguishable from said micro-particles under at least one illumination condition.

2. A security feature as in claim 1, wherein said security feature is applied as a coating to said substrate, and further comprising a coating binder in which said micro-particles and said discrete particles are contained.

3. A security feature as in claim 2, wherein said coating binder is comprised of at least one of an ink base, an adhesive, an epoxy, a varnish, a polymer solution, or a dry material having binding properties.

4. A security feature as in claim 1, wherein said discrete particles are optically distinguishable from said micro-particles under long wavelength ultraviolet light.

5. A security feature as in claim 1, wherein said discrete particles are optically distinguishable from said micro-particles under short wavelength ultraviolet light.

6. A security feature as in claim 1, wherein under no or low magnification conditions the security feature exhibits a generally uniform color to the naked eye, while under higher magnification conditions the discrete particles exhibit a first color while the micro-particles of the background component exhibit a second color.

7. A security feature as in claim 1, wherein under no or low magnification conditions the security feature exhibits a generally uniform color to the naked eye, while under higher magnification conditions the discrete particles exhibit a first color while the micro-particles of the background component exhibit a lack of color.

8. A security feature as in claim 1, wherein under no or low magnification conditions the security feature exhibits a generally uniform color to the naked eye, while under higher magnification conditions the micro-particles of the background component exhibit a first color while the discrete particles exhibit a lack of color.

9. A security feature as in claim 1, wherein said security feature exhibits a fluorescent signature having attributes given by at least one of particle count per unit area, specific particle size range which may or may not differ by fluorescent color, relative intensities of measured spectral emission peaks due to foreground component particle fluorescence versus background component micro-particle fluorescence and specific emission wavelength values of measured spectral emission peaks due to foreground component particle fluorescence versus background component micro-particle fluorescence.

10. A security feature as in claim 1, wherein said discrete particles have a size in the range of about 10 microns to about 20 microns in diameter.

11. A security feature as in claim 1, wherein said micro-particles have a size in the range of about 0.2 microns to about 2 microns in diameter.

12. A security feature as in claim 1, wherein an average size of said discrete particles is greater by a factor of at least five than an average size of said micro-particles.

13. A security feature as in claim 1, wherein an average size of said discrete particles is greater by a factor of at least one order of magnitude than an average size of said micro-particles.

14. A security feature as in claim 2, wherein said coating binder is substantially transparent.

15. A security feature as in claim 2, wherein said coating binder is substantially opaque.

16. A security feature as in claim 1, wherein said discrete particles are optically distinguishable in a first way from said micro-particles when viewed under short wavelength ultraviolet light, and are optically distinguishable in a second way from said micro-particles when viewed under long wavelength ultraviolet light.

17. A reader system comprising a plurality of light sources and a magnification optics coupled to an imaging system, said reader system comprising a controller for illuminating a security feature using said plurality of light sources, said security feature comprising a plurality of fluorescent micro-particles that form a background component of said security feature and a plurality of fluorescent discrete particles that form a foreground component of said security feature, said discrete particles being larger in size than said micro-particles and being optically distinguishable from said micro-particles when illuminated by at least one of said plurality of light sources, said controller cooperating with said imaging system for resolving a fluorescent signature of said security feature, said fluorescent signature having attributes given by at least one of particle count per unit area, specific particle size range which may or may not differ by fluorescent color, relative intensities of measured spectral emission peaks due to foreground component particle fluorescence versus background component micro-particle fluorescence and specific emission wavelength values of measured spectral emission peaks due to foreground component particle fluorescence versus background component micro-particle fluorescence.

18. A reader system as in claim 17, wherein said security feature is applied as a coating to a substrate using a coating binder in which said micro-particles and said discrete particles are contained.

19. A method for reading a security feature, comprising steps of:

operating a reader system having a plurality of light sources and magnification optics coupled to an imaging system for illuminating the security feature using said plurality of light sources, said security feature comprising a plurality of fluorescent micro-particles that form a background component of said security feature and a plurality of fluorescent discrete particles that form a foreground component of said security feature, said discrete particles being larger in size than said micro-particles and being optically distinguishable from said micro-particles when illuminated by at least one of said plurality of light sources; and

responsive to the illuminating, resolving a fluorescent signature of said security feature, said fluorescent signature having attributes given by at least one of particle count per unit area, specific particle size range which may or may not differ by fluorescent color, relative intensities of measured spectral emission peaks due to foreground component particle fluorescence versus background component micro-particle fluorescence and specific emission wavelength values of measured spectral emission peaks due to foreground component particle fluorescence versus background component micro-particle fluorescence.

20. A method as in claim 19, and comprising a preliminary step of applying said security feature as a coating to a substrate using a coating binder in which said micro-particles and said discrete particles are contained.