EP3401885A1

EP3401885A1 - Apparatus and methods for authenticating a security feature

Info

Publication number: EP3401885A1
Application number: EP17169978.8A
Authority: EP
Inventors: Jean Claude Fremy
Original assignee: European Central Bank
Current assignee: European Central Bank
Priority date: 2017-05-08
Filing date: 2017-05-08
Publication date: 2018-11-14

Abstract

The invention relates to an apparatus for authenticating luminescent security features, in particular luminescent security features designed for counterfeit protection of valuable items and documents such as banknotes. More specifically, the present invention relates to an apparatus for inspecting the colors of light emitted by the security feature at certain predefined ranges of emission angles. The apparatus can comprise an excitation device to excite the luminescent security feature, an inspection device to inspect the security features, and a positioning means to put the inspection device and the security feature into a defined spatial relation. The present invention also relates to a method for authenticating the aforementioned security features.

Description

FIELD OF THE INVENTION

The present invention relates to an apparatus for authenticating luminescent security features, in particular luminescent security features designed for counterfeit protection of valuable items and documents such as banknotes. More specifically, the present invention relates to an apparatus for inspecting the colors of light emitted by the security feature at certain predefined ranges of emission angles. The present invention also relates to a method for authenticating the aforementioned security features.

BACKGROUND

Generally, security features for valuable items and documents, such as banknotes, checks, securities, stocks, bonds, credit and debit cards, certificates, passports, identity cards, safety critical spare parts et cetera, may be categorized into three groups. The first group comprises overt security features that can be inspected and assessed by a human without the use of a tool, i.e. security features producing a certain look (e.g. watermarks or holograms) or feel (e.g. raised prints). The second group comprises concealed security features that can be inspected and assessed by a person using a tool, i.e. microprint that may be inspected using a magnifying glass or luminescent ink that can be excited by using an UV (ultra-violet) or IR (infra-red) lamp. The third group comprises concealed security features that may only be inspected and assessed by inspection devices adapted to examine - for example - material properties such as electrical or magnetic properties.
UV fluorescent features have proven to be effective in impeding at least primitive and hobbyist type counterfeiting in the past. However, as fluorescent inks become more easily available, there is a need for security features that are more robust against copying and counterfeiting.
One approach is to combine fluorescent inks which are sensitive to different wavelengths (e.g. UV-A and UV-C) and emit light of same or different colors. Thereby, a security feature may reveal first indicia when excited by light of a first wavelength and second/different indicia when excited by light of a second wavelength.
Another approach is to use security features providing a flip-flop effect when being tilted, i.e. reflecting light in different colors depending on the angle of incident of reflected light. WO 2007/140484 A2 discloses such a colored reflective security feature that can exhibit color shifting and inks and processes for making such features.
WO 2010/096914 A1 discloses a luminescent security feature comprising an optically variable structure that causes an angle-dependent shift of the perceived color of the emitted light.

SUMMARY

It is an object of the present invention to provide an apparatus for authenticating a luminescent security feature. It is also an object of the present invention to provide an improved method for authenticating a luminescent security feature.
According to an aspect of the invention, an apparatus for authenticating a substantially planar luminescent security feature extending in a first plane is provided. For example, the security feature can be disposed on a flat sheet material such as a banknote.
The luminescent security feature is configured to simultaneously emit light rays of different colors upon excitation, wherein the color of each of the emitted light rays depends on a respective emission angle.
Stated differently, the luminescent feature, which when stimulated, produces luminescent radiation of (at least) first and second colors, the second color being different from the first color. The emissivity of the security feature for luminescent radiation of the first and second colors changes with a change in emission angle of luminescent radiation. The "emissivity" of the security feature generally refers to the ability to emit radiation and more specifically refers to the intensity (or power) of radiation of the security feature. Thus, the higher the emissivity of the security feature for luminescent radiation of a particular color, the higher will be the intensity (or power) of luminescent radiation of the particular color emitted from the security feature. As the relative emissivity of the security feature for luminescent radiation of the first and second colors changes with emission angle, an angle dependent color shift in the luminescent emission can be observed. The emission angle at the color shift can be defined as color shift angle δ. However, dependent on the configuration of the security feature, it can be more suitable to define an angular range of color shift rather than a single color shift angle δ.
Dependent on the configuration of the security feature, the emission angle ε can be sufficiently defined by the elevation angle θ between the first plane and the initial direction of propagation of the emitted light rays in a range between 0° and 90°. The initial direction of propagation of a light ray is defined by the original and undeflected (without being deflected) direction of emission (of that particular light ray) just after leaving the security feature. Other embodiments of the security feature, however, may require defining the emission angle ε (or differently stated: the direction of emission) by a pair of angles (θ, ϕ), namely the elevation angle θ between the first plane and the initial direction of propagation of the emitted light rays between 0° and 90°, and the azimuthal angle ϕ between an azimuth reference direction and the projection of the initial direction of propagation of the emitted light ray onto the first plane between 0° and 360°.
The term "color" refers to either a single wavelength component in the electromagnetic spectrum or a combination of different wavelength components in the electromagnetic spectrum, each component having a particular intensity relative to the other components. Also, the term "color" applies to both the visible part of the electromagnetic spectrum and to parts outside the visible spectrum including ultraviolet (UV) and infrared (IR).
The term "luminescence" refers to a conversion of at least a part of incident energy into emitted radiation with a characteristic signature. For example, a luminescent material/feature may convert incident radiation of one wavelength into emitted radiation of a different wavelength. Non-limiting examples of luminescence exhibit fluorescence and/or phosphorescence.
According to an aspect of the invention, the apparatus for authenticating the security feature comprises an excitation device configured to excite the luminescent security feature so that the luminescent security feature emits light.
According to an advantageous aspect of the invention, the excitation device is suitable to excite the luminescent security feature by use of invisible light, advantageously invisible light in the range of ultraviolet wavelengths and/or infrared wavelengths. Advantageously, the excitation device can comprise a source of UV radiation, and advantageously a source of UV-A radiation (having a primary wavelength of, for example, approximately 365 nm) and/or a source of UV-C radiation (having a primary wavelength of, for example approximately 254 nm). Advantageously, the excitation device comprises an annular UV light source (UV ring light). Luminescent security features which can be excited by the above wavelengths are widely used for counterfeit protection. This allows easy integration of the present invention into apparatuses for counterfeit protection.
As an alternative to a source of UV radiation or in addition to a source of UV radiation, the excitation device can advantageously comprise a source of IR radiation, advantageously a source of IR radiation having a primary wavelength of approximately 980 nm. However, the excitation device can also be configured to excite the security feature using visible light or other kinds of radiation. This further strengthens the luminescent security feature as the respective inks are not as easily available as UV luminescent inks.
According to an advantageous aspect of the invention, the excitation device can comprise an automatic power on/off system. The automatic power on/off can be controlled by an electronic controller of the apparatus. This aspect can assist in reducing the power consumption of the excitation device.
According to an aspect of the invention, the apparatus for authenticating the security feature comprises an inspection device. The inspection device is configured to inspect a first color of light rays emitted within a first predefined range of emission angles and a second color of light rays emitted within a second predefined range of emission angles.
According to an aspect of the invention, the inspection device defines an inspection area for inspecting the color(s) of the emitted light. The inspection area is a two-dimensional or three-dimensional space having a spatial configuration comprising a spatial extension vector component in a direction perpendicular to the incidence direction of the inspected light rays into the inspection area. For example, the inspection area can be a planar (flat and/or prismatic) or curved area/plane that faces the luminescent security feature. The planar or curved area/plane can extend in directions that are substantially perpendicular or tilted with respect to the inspected luminescent security feature. The planar or curved area/plane can be arranged in a variable distance towards the luminescent security feature along the direction of incidence of the light, such that the two-dimensional space defined by the planar or curved area/plane in combination with the variable distance towards the luminescent security feature define a three-dimensional inspection space. The emitted light rays which are emitted within the first predefined range of emission angles and the emitted light rays which are emitted within the second predefined range of emission angles are directed to the inspection area.
The apparatus for authenticating the luminescent security feature can eliminate visual uncertainty and increase the counterfeit resilience of the feature.
Advantageously, the inspection device can be configured to inspect a distribution of color(s) of the emitted light in the inspection area. The inspection area allows a proper inspection of the first and second colors of the emitted light rays. Advantageously, the inspection area allows a direct comparison of the first color of the light rays emitted within the first predefined range of emission angles and the second color of the light rays emitted within the second predefined range of emission angles. This further eliminates uncertainty relating to the interpretation of the emitted colors.
According to an aspect of the invention, the apparatus for inspecting the security feature comprises a positioning means that is configured to put the luminescent security feature into a defined spatial relation with the inspection device. In particular, the positioning means is configured to put the luminescent security feature into a defined spatial relation with the inspection area. This allows proper inspection of the security feature.
According to an advantageous aspect of the invention, the light rays emitted within the first predefined range of emission angles can occupy a first portion of the inspection area, and the light rays emitted within the second predefined range of emission angles can occupy a second portion of the inspection area. The first portion of the inspection area can be different from the second portion of the inspection area. This aspect further simplifies the concurrent inspection of light emitted within a predefined first range of emission angles and light emitted within a predefined second range of emission angles.
According to an advantageous aspect of the invention, the inspection device can comprise a first refractive, diffractive or reflective optical element that is configured to deflect the light rays emitted within the first predefined range of emission angles towards the inspection area. This aspect allows the concurrent inspection of the light rays emitted within the predefined first range of emission angles and the light rays emitted within the predefined second range of emission angles.
According to an advantageous aspect of the invention, the initial directions of propagation of the light rays emitted within the second predefined range of emission angles can be directed towards the inspection area. Alternatively, the inspection device can comprise a second refractive, diffractive or reflective optical element that is configured to deflect the light rays emitted within the second predefined range of emission angles towards the inspection area. This aspect allows the concurrent inspection of the light rays emitted within the predefined first range of emission angles and the light rays emitted within the predefined second range of emission angles.
Advantageously, the optical element or the optical elements can be one or more out of a mirror, in particular a flat mirror, a prismatic mirror, a concave mirror, a parabolic mirror, a mirror trough, a convergent lens or a convergent cylindrical lens configured and arranged (with respect to the positioning means and/or the inspection area) to reflect/deflect the emitted light rays of the first range of emission angles towards the inspection area. The concave or parabolic mirror or the mirror trough can comprise an aperture at the vertex of the concave or parabolic mirror or the mirror trough, such that the luminescent security feature can be positioned in or adjacent to the aperture. This aspect simplifies the inspection as the inspected security feature is optically enlarged. Further, the excitation radiation is focused towards the luminescent security feature.
Advantageously, the aperture at the vertex of the concave or parabolic mirror or the mirror trough can define a plane extending through the focal point or focal line of the mirror. The mirror through can comprise a concave or parabolic profile. Accordingly, the aperture can have an elongated shape. This aspect enhances the visibility/detectability of the first and the second color.
According to an advantageous aspect of the invention, the apparatus can comprise an optical group. The optical group can comprise a convergent lens and a mirror. The mirror can be a flat mirror, a concave mirror or a parabolic mirror. Alternatively, the optical group can comprise a convergent cylindrical lens and a mirror trough. The mirror trough can comprise a concave or parabolic profile. The luminescent security feature can be arranged in or adjacent to an aperture at a vertex of the mirror or the mirror trough. The aperture can be of elongated shape. The optical group can further enhance the visual appearance and/or the analysis of the luminescent security feature.
According to an advantageous aspect of the invention, the inspection means can comprise at least one color sensor. The color sensor can be, for example, a spectroradiometer or a spectrophotometer. However, those color sensors are quite expensive and may only be considered, for example at a large counterfeit deterrence laboratory. Alternatively, the color sensor can be a simple and cheap RGB color sensor usually comprising three UV cut off filters and/or color filters (i.e. bandpass filters for the primary colors) and respective photodiodes (or another photoelectric cell). An additional evaluation unit can process the sensor output.
Advantageously, the inspection means can comprise a matrix of color sensors. The matrix of color sensors can comprise at least a first and a second color sensor. The first color sensor can be configured and arranged to detect the color of the light rays emitted within the first predefined range of emission angles. The second color sensor can be configured and arranged to detect the color or the light rays emitted within the second predefined range of emission angles. This aspect provides the machine readability of the luminescent security feature.
Advantageously, the first and the second color sensors can be arranged to detect the color of emitted light rays at an emission angle that is within a predefined maximal angular distance (e.g. +/- 1°, +/- 3°, +/- 5°, +/- 10°, +/- 15°) to the color shift angle δ or the angular range of color shift, respectively. This aspect enhances the counterfeit deterrence.
According to an advantageous aspect of the invention, the matrix of color sensors can be in form of an image sensor. Image sensors can include (but are not limited to) semiconductor charge-coupled devices (CCD) or active pixel sensors in complementary metal-oxide-semiconductor (CMOS) or N-type metal-oxide-semiconductor (NMOS, Live MOS) technologies. More and more detailed (image) data can provide a more detailed "visual fingerprint" of the luminescent security feature. This aspect enhances the automated authentication process.
According to an advantageous aspect of the invention, the inspection device can comprise one or more filter means. The filter means can be arranged between the luminescent security feature and the inspection area. Advantageously, the excitation device can be arranged between the luminescent security feature and the filter means. The filter means enhance the quality of inspection.
The filter means can comprise a UV cut off filter and/or color filter. The UV cut off filter and/or color filter can be adapted to block the exciting radiation irradiating towards the inspection area. Differently stated, the UV cut off filter and/or color filter can be a band-pass filter. The band-pass filter can allow light rays of the first and the second color to pass through the band-pass filter towards the inspection area. The UV cut off filter and/or color filter avoids parasitic lighting by the excitation device. This aspect enhances the "readability" of the security feature and/or protects the sensor and/or the eyes of an observer.
Alternatively or additionally, the filter means can comprise an anti-scatter filter, such as a honey comb filter or an anti-scatter grid (also known as scatter removal grid). Although the term anti-scatter grid often refers to a filter device used in diagnostic radiology, it is to be understood that the underlying physical principle also applies here. The anti-scatter filter can block scattered light rays resulting from the spatial extent of the luminescent security feature. Thereby, the anti-scatter filter can improve the "readability" of the luminescent security feature. Stated differently, the anti-scatter filter can enhance the distinction between light rays emitted within the first predefined range of emission angles and light rays emitted within the second predefined range of emission angles. The anti-scatter filter can further enhance the visual/optical quality of the inspected luminescent security feature.
According to an advantageous aspect of the invention, the inspection device can be configured to equalize a hidden anamorphous pattern in the luminescent security feature. In other words, the inspection device can make an otherwise concealed pattern or symbol visible to the naked eye or an inspection means. Optical elements such as concave or parabolic mirrors naturally create anamorphous reflected images related to its profile. The luminescent security feature can be adapted to comprise an altered (transformed) symbol/indicia/image. The transformed (and thereby hidden) symbol is reversely transformed by the anamorphous reflection. The hidden anamorphous pattern / image can only be seen with the appropriate mirror. This aspect enhances the robustness of the authentication.
According to an advantageous aspect of the invention, the positioning means can be adapted to transport the luminescent security feature (which, for example, can be disposed on a banknote) along a direction traversing the inspection area. This aspect supports the automation of the inspection. It can further provide for a more simplified configuration of the inspection device.
According to an advantageous aspect of the invention, an apparatus for authenticating luminescent security features can comprise an excitation device, an inspection device and a positioning means. The inspection device can comprise at least one color sensor in an inspection area. The positioning means can be adapted to transport the luminescent security feature (which, for example, may be disposed on a banknote) along a direction traversing the inspection area at a predetermined speed. A control unit can be adapted to authenticate the luminescent security feature using a sequence of colors emitted by the luminescent security feature and detected by the at least one color sensor. Alternatively or additionally, a recorded timing of the sequence can be used to calculate respective emission angles associated with the detected colors. Stated differently, the control unit can calculate (taking into account the predetermined speed and the recorded sequence of colors) the color shift angle δ of the security feature. Accordingly, the control unit can authenticate the security feature by comparing the sequence of detected colors and/or the respective timing of the sequence and/or a calculated color shift angle and/or a calculated angular range of color shift with respective nominal data. This aspect provides a simplified mechanical configuration of a highly automated apparatus for authenticating a large number of documents/banknotes comprising respective luminescent security features.
According to an advantageous aspect of the invention, the parabolic/concave mirror can be smaller than 50 mm in diameter and 30 mm in height. The corresponding diameter of the aperture can be smaller than 20 mm. Advantageously, the parabolic/concave mirror can be smaller than 35 mm in diameter and 25 mm in height. Accordingly the aperture can have an aperture smaller than 18 mm. Advantageously, the parabolic/concave shape can be larger than 18 mm in diameter and 8 mm in height. The corresponding aperture has a diameter of at least 8 mm. Alternatively, moulded PMMA lenses or prisms can be utilized in exchange for the reflective optical element.
According to an aspect of the invention, a method for authenticating a luminescent security feature is provided. The luminescent security feature is configured to simultaneously emit light rays of a first and a second color upon excitation. The color of each of the emitted light rays depends on a respective emission angle.
An inspection device can be positioned in a predefined spatial relation to the luminescent security feature. Alternatively, the luminescent security feature can be positioned in a predefined spatial relation to the inspection device.
The luminescent security feature can be excited, for example by irradiation from an excitation device, to emit light of the first and the second color.
The emitted light rays which are emitted within a first predefined range of emission angles can be deflected towards a first portion of an inspection area. An optical element such as a flat or concave mirror, a lens or a prism can be used to deflect the respective light rays.
A first color of light rays emitted within the first predefined range of emission angles and a second color of light rays emitted within a second predefined range of emission angles can be inspected in the inspection area, either with the naked eye or alternatively using inspection means such as, for example, optical (color) sensors.
A distinction between authentic and counterfeit luminescent security features can be made by comparing the first color of light rays emitted within the first predefined range of emission angles with a first predefined color and comparing the second color of light rays emitted within the second predefined range of emission angles with a second predefined color. The luminescent security feature can be classified based on the result of the comparison.
The method provides for a more secure and robust authentication of the luminescent security device.
According to an advantageous aspect of the invention, light rays emitted within a second predefined range of emission angles can also be deflected towards the inspection area, such that the light rays emitted within the second predefined range of emission angles occupy a second portion of the inspection area that is different from the first portion. This aspect enhances the inspection procedure.
According to an advantageous aspect of the invention, the distinction between authentic and counterfeit luminescent security features can be made by analyzing a color balance matrix corresponding to the color data of respective color or image sensors.

BRIEF DESCRIPTION OF DRAWINGS

Further aspects, characteristics and advantages of the invention will ensue from the following description of the embodiments with reference to the accompanying drawings, wherein

FIG. 1 is a simplified cross sectional view of an embodiment of the luminescent security feature,
FIG. 2a is a simplified cross sectional view of a first embodiment of the invention,
FIG. 2b is a simplified top view of the first embodiment of the invention,
FIG. 3a is a simplified cross sectional view of a second embodiment of the invention,
FIG. 3b is a simplified top view of the second embodiment of the invention,
FIG. 4a is a simplified cross sectional view of a third embodiment of the invention,
FIG. 4b is a simplified top view of the third embodiment of the invention,
FIG. 5a is a simplified cross sectional view of a fourth embodiment of the invention,
FIG. 5b is a simplified top view of the fourth embodiment of the invention,
FIG. 6a is a simplified cross sectional view of a fifth embodiment of the invention,
FIG. 6b is a simplified top view of the fifth embodiment of the invention,
FIG. 7 is a simplified cross sectional view of a sixth embodiment of the invention,
FIG. 8a is a simplified cross sectional view of a seventh embodiment of the invention,
FIG. 8b is a simplified top view of the seventh embodiment of the invention,
FIG. 9a is a simplified cross sectional view of an eighth embodiment of the invention,
FIG. 9b is a simplified top view of the eights and an additional ninth embodiment of the invention,
FIG. 10 is a simplified cross sectional view of a tenth embodiment of the invention,
FIGS. 11a though 11e depict 5 sequential steps in a simplified side view of an eleventh embodiment of the invention,
FIGS. 12a to 12h show 8 sequential steps in a simplified side view of a twelfth embodiment.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a simplified cross-sectional view of an embodiment of the luminescent security feature 1.The luminescent security feature 1 simultaneously emits light rays of (at least) a first color 11 and a second color 12 upon excitation. The color of the light depends on the emission angle ε1, ε2. A light ray having a first emission angle ε1 that is smaller than an angle of color shift δ is of first color 11, e.g. green. A light ray having a second emission angle ε2 that is larger than the angle of color shift δ is of second color 12, e.g. red. In the black-and-white sketches, the first color 11 is generally represented by horizontal hatching; the second color 12 is generally represented by vertical hatching. The luminescent security feature can be disposed on a valuable document 2 such as a banknote.
For reference, x,y,z-axes, the elevation angle θ and the azimuthal angle ϕ are also depicted. The luminescent security feature extends in the x,y-plane and is substantially flat. The elevation angle θ is 0° parallel to the x,y-plane and 90° perpendicular to the x,y-plane. The azimuthal angle ϕ is defined between the x-axis as azimuthal reference direction and the projection of the direction in question (e.g. the direction of emission or the direction of observation) onto the x,y-plane.
The color shift angle δ of the depicted luminescent security feature is solely defined by the elevation angle θ and independent of the azimuthal angle ϕ. In the present case, a color shift angle δ of 45° is depicted. In other words, a light ray emitted at an emission angle of 45° or less is of the first color 11, a light ray emitted at an emission angle greater than 45° is of the second color 12.
Accordingly, the spatial distribution of the second color light can be described as cone-shaped, extending along the z-axis and broadening with increasing distance to the luminescent security feature. The spatial distribution of the first color light 11 (in an x,y-plane, e.g. an inspection area) can then be described as ringshaped (or circular or annular) and extending radially around the cone.
Due to the fact that the luminescent security feature is a flat light source rather than a point light source, light of a third (mixed) color 13 can extend between the first color light rays and the second color light rays. The spatial distribution of the third (mixed) color 13 can therefore have the shape of a cone-mantle.
It should be noted, however, that several other and non-depicted luminescent security features can also be authenticated in accordance with aspects and embodiments of the present invention. For example, the color shift angle δ can be solely defined by the azimuthal angle ϕ (i.e. it can be independent of the elevation angle θ). Accordingly, first color light rays can be emitted in a first range of emission angles defined by the azimuthal angle ϕ from 0° to 180°, and second color light rays can be emitted in a second range of the azimuthal angle ϕ from 180° to 360°.
As long as the emitted light rays are not deflected, the emission angles ε1, ε2 directly correspond to observation angles σ1, σ2. A deflection of the light ray, however, renders the observation angle σ1, σ2 independent from the emission angle ε1, ε2.
Without deflecting any light rays, an observer (for example a person observing the security feature with the naked eye, or alternatively a sensor) will perceive a first color 11 at the first observation angle σ1 (corresponding to the first emission angle ε1) and a second color 12 at the second observation angle σ2 (corresponding to the second emission angle ε2). In the direction of the color shift angle δ, the observer can perceive a third (mixed) color 13.
FIG. 2a is a simplified cross sectional view of a first embodiment of the invention. Also shown is the luminescent security feature 1, disposed on the surface of a banknote 2. The apparatus 3 for authenticating the luminescent security feature 1 comprises an excitation device 4 that is configured to excite the luminescent security feature 1. For example, the excitation device 4 can comprise one or more UV- emitter 41, 42 emitting ultraviolet light. The UV-emitter can be an UV point light source or any other type of UV illumination emitting a narrow or broad band of wavelengths as long as it includes the wavelengths that excite the fluorescent inks. For example, the UV-emitter 4 can comprise one or more UV LEDs (light emitting diodes) 41, 42. Advantageously, the one or more UV- emitters 41, 42 can be configured/arranged in a ring shape, advantageously in the x,y-plane. The present embodiment of the luminous security feature is not dependent on the angle of the UV excitation beam.
The apparatus 3 further comprises an inspection device 5 and a positioning means 6. The inspection device 5 is configured to inspect a first color 11 of light rays emitted within a first predefined range of emission angles and a second color 12 of light rays emitted within a second predefined range of emission angles. In the present embodiment, due to the fact that the luminescent security feature is a flat light source rather than a point light source, light of a third (mixed) color 13 comprising the light of the first color 11 is inspected instead of solely light of the first color 11. The light rays emitted within the first predefined range of emission angles and the light rays emitted within the second predefined range of emission angles are directed to an inspection area 56. Therefore, a spatial distribution of colors can be inspected in the inspection area 56. For example, the inspection area can be defined by a plane that is substantially parallel to the x,y-plane. The inspection area can be substantially smaller than the area that is defined by the natural beam angle of the luminescent security feature and the distance between the inspection area and the luminescent security feature. A positioning means 6 is configured to put the luminescent security feature 1 into a defined spatial relation with the inspection device 5. In the present embodiment, the positioning means 6 is part of the inspection device 5, and more precisely part of a reflector 51.
More specifically, the reflector 51 comprises a parabolic mirror. The parabolic mirror 51 deflects/reflects light rays of the first color 11 towards the inspection area 56.
The parabolic mirror 51 comprises an aperture 57 at the bottom. Differently stated, the aperture 57 is arranged at the vertex of the parabolic mirror, such that the luminescent security feature 1 can be positioned in or adjacent to the aperture 57. Thereby, the parabolic mirror itself acts as the positioning means 6.
In the present embodiment, the parabolic mirror 51 does not deflect the light rays emitted within a second range of emission angles. In other words, the parabolic mirror 51 does not deflect light rays of the second color 12. The emitted light rays of the second color 12 are directly emitted towards the inspection area 56.
Utilizing the inspection device, an observer 7 can observe/inspect the spatial distribution of colors 11, 12, 13 in the inspection area 56 from above (in other words: in a vertical direction). This allows a direct comparison of the first color 11 of the light rays emitted within the first predefined range of emission angles and deflected/reflected by the mirror (by actually inspecting a mixed color 13 comprising the first color 11 and the second color 12) and the second color 12 of the light rays emitted within the second predefined range of emission angles.
The spatial distribution of colors 11, 12, 13 in the inspection area 56 is depicted in corresponding FIG. 2b. The light rays emitted within the first predefined range of emission angles occupy a first (horizontally hatched, annular) portion 561 of the inspection area 56, and the light rays emitted within the second predefined range of emission angles occupy a second (vertically hatched, circular) portion 562 of the inspection area (partly overlapping the first, annular portion 561).
It should be noted that, although no further optical elements are depicted in FIG.2a, the inspection device 5 can still optionally comprise further filters and/or lenses as described later with regard to other embodiments of the invention.
FIG. 3a is a simplified cross sectional view of a second embodiment of the invention. The inspection device 5 is positioned on a banknote 2 comprising the luminescent security feature 1. Again, the apparatus 3 comprises an excitation device 4, an inspection device 5 and positioning means 6. The general mechanical configuration is comparable to that of the first embodiment.
The inspection device 5 further comprises a filter means 58. The filter means 58 of the current embodiment comprises an optional anti-scatter filter 54 and an optional UV cutoff filter and/or color filter 53. The filter means 58 is arranged between the luminescent security feature 1 and the inspection area 56. The inspection area 56 can optionally comprise a screen (such as a frosted glass element). In the present embodiment, the screen 56 is disposed on one side of the filter means 58 that is facing away from the luminescent security feature 1. The excitation device 4 is arranged between the luminescent security feature 1 and the filter means 58, and more precisely between the luminescent security feature 1 and the color filter 53. The UV cutoff filter and/or color filter 53 blocks the excitation irradiation from irradiating towards the inspection area 56. The anti-scatter 54 filter is arranged directly on top of the reflector 51 and between the reflector 51 and the inspection area 56. The anti-scatter filter 54 prohibits secondary irradiation which is reflected multiple times within the reflector 51 and/or scattered irradiation due to the flat light source of the luminescent security feature 1 from irradiating towards the inspection area 56.
The optional anti-scatter filter 54 can be composed of a series of parallel strips from an opaque substance such as a black plastic forming a grid. The anti-scatter filter 54 is placed between the reflector 51 and the inspection area 56. Primary beam radiation passes through the grid as it travels roughly parallel to the parallel strips, but scattered radiation which has, almost by definition, deviated from a parallel beam, cannot easily pass through the grid as it encounters the opaque strips at an angle, and is attenuated (or lost) from the beam. The excitation device 4 is arranged between the anti-scatter filter 54 (as well as the UV cutoff filter and/or color filter 53) since the loss of intensity of the excitation radiation would be significant, if the excitation device 4 is arranged on a side of the anti-scatter filter 54 that is facing away from the luminescent security feature 1.
The spatial distribution of the first and second colors 11, 12 can be inspected in the inspection area 56, advantageously from a vertical direction (top view).
FIG. 3b is a simplified top view of the second embodiment of the invention. In contrast to the first embodiment, the distinction between the first color 11 and the second color 12 is enhanced, as the anti-scatter filter 54 prohibits a scattered portion of the second color 12 from reaching the inspection area 56. The light rays emitted within the first predefined range of emission angles occupy a first (horizontally hatched, annular) portion 561 of the inspection area 56, and the light rays emitted within the second predefined range of emission angles occupy a second (vertically hatched, circular) portion 562 of the inspection area in the center of the annular first portion 561.
FIG. 4a is a simplified cross sectional view of a third embodiment of the invention. The inspection device 5 is positioned on a banknote 2 comprising the luminescent security feature 1. In this embodiment, the luminescent security feature 1 is disposed in the surface of the banknote 2.
The apparatus 3 comprises an excitation device 4, an inspection device 5 and a positioning means 6. The general mechanical configuration is comparable to that of the first and/or the second embodiment.
The positioning means 6 comprises an aperture 57 that is smaller than the luminescent security feature 1. This aspect provides a defined size and position of the luminous (excited and inspected) part of the luminescent security feature 1.
An anti-scatter filter 54 is not depicted but can be arranged in the inspection device 5. The filter means 58 comprises a UV cut off filter and / or color filter 53. Accordingly, the excitation device 4 is arranged inside a volume limited by the reflector 51, the UV cut off filter and/or color filter 53 and the luminescent security feature 1. The inspection device 5 can comprise one or more optional converging and/or diverging lens(es), advantageously a first lens 591 adjacent to the filter means 58 and/or a second lens 592 adjacent the aperture 57. Advantageously, the first lens 591 is a converging lens, i.e. be a biconvex lens, planoconvex lens or positive meniscus lens. The second lens 592 can be a converging or a diverging lens, depending on the color shift angle δ and the shape of reflector 51, i.e. be a biconvex lens, planoconvex lens or a positive meniscus lens and alternatively a negative meniscus lens, a planoconcave lens or a biconcave lens. Advantageously, the first lens 591 can be a planoconvex lens and the second lens 592 can be one out of a planoconvex, positive meniscus, negative meniscus and planoconcave lens. The additional lenses 591, 592 improve the inspectability of the luminescent security feature 1.
The visual appearance of the spatial color distribution in the inspection area, as it is shown in FIG. 4b, is comparable to that of the first embodiment shown in FIG. 2b. A redundant recitation of features is therefore omitted.
FIG. 5a is a simplified cross sectional view of a fourth embodiment of the invention. The inspection device 5 is positioned on the luminescent security feature 1.
The apparatus 3 comprises an excitation device 4, an inspection device 5 and a positioning means 6. The positioning means further comprises a control unit and an energy source (such as a battery) 43 for driving the excitation device 4.
The inspection device 5 comprises an anti-scatter filter 54 which enhances the visual/optical appearance of the luminescent security feature 1. A planoconcave lens 592 is disposed in the aperture 57. The reflector 51 is rotationally symmetric and comprises a first annular reflective area 511 having a first profile of substantially parabolic shape reflecting emitted light rays of a predefined first range of emission angles towards the inspection area. Adjacent and on a distant side (with regard to the luminescent security feature), the reflector 51 comprises a second annular reflective area 512 having a second and more inclined profile of substantially parabolic shape reflecting emitted light rays of a predefined second range of emission angles ε towards the inspection area. The apparatus can comprise further lenses and/or filters that are not depicted for clarity.
The spatial distribution of colors 11 and 12 is shown in FIG. 5b. Light rays emitted within a first range of emission angles (having a first color 11) are reflected by the first reflective area 511 of the reflector 51 towards the inspection area 56 and occupy a first annular area 561. Light rays emitted within a second range of emission angles (having a second color 12) are (in part) directly emitted towards the inspection area 56 and occupy a second (circular part of an) area 562a in the center of the first annular area 561, and (in part) reflected by the second reflective area 512 of the reflector 51 towards the inspection area 56, occupying a second, outer annular area 562b.
FIG. 6a is a simplified cross sectional view of a fifth embodiment of the invention. The inspection device 5 is positioned on the luminescent security feature 1.
The apparatus 3 comprises an excitation device 4, an inspection device 5 and a positioning means 6. The inspection apparatus 3 can comprise optional lenses and/or an optional a UV cut off filter and/or color filter. The optional features are omitted for clarity.
The inspection device 5 can comprise an anti-scatter filter 54 which enhances the visual/optical appearance of the luminescent security feature 1. The reflector 51 is (in part) rotationally symmetric and comprises a first reflective area 511 having a first profile of substantially parabolic shape reflecting emitted light rays of a predefined first range of emission angles towards the inspection area 56. Additionally, the reflector 51 comprises a second (party rotationally symmetric) reflective area 512 having a second and more inclined profile of substantially parabolic shape reflecting emitted light rays of a predefined second range of emission angles towards the inspection area. A mask 581 can be arranged between the luminescent security feature and the inspection area 56 to block direct view onto the security feature 1. The excitation device 4 is arranged in the center of the inspection device.
The spatial distribution of colors 11 and 12 is shown in FIG. 6b. Light rays emitted within a first range of emission angles (having a first color 11) are reflected by the first reflective area 511 of the reflector 51 towards the inspection area 56 and occupy a first portion 561 of the inspection area 56. Light rays emitted within a second range of emission angles (having a second color 12) are reflected by the second reflective area 512 of the reflector 51 towards the inspection area 56, occupying a second portion 562 of the inspection area 56. A central area 560 is masked by the opaque mask 581 and is therefore dark.
FIG. 7 is a simplified cross sectional view of a sixth embodiment of the invention. Again, the apparatus 3 comprises an excitation device 4, an inspection device 5 and a positioning means 6. The general mechanical configuration is comparable to the third embodiment in FIG. 4a.
Comparing the sixth and the third embodiment, the order of the first lens 591 and the UV cut off filter and/or color filter 53 is reversed. The excitation device 4 comprising the first and the second UV- emitter 41, 42 is arranged outside the reflector 51. More precisely, the UV- emitters 41, 42 are arranged adjacent to (or in) a gap between the first lens 591 and the UV cut off filter and/or color filter 53. That way, the UV cut off filter and/or color filter 53 still blocks the excitation radiation from reaching the inspection area.
The visual appearance of the spatial color distribution in the inspection area is the same as it is shown in FIG. 4b. A redundant recitation of features is therefore omitted.
FIG. 8a is a simplified cross sectional view of a seventh embodiment of the invention. Again, the apparatus 3 comprises an excitation device 4, 41, 42, an inspection device 5 and a positioning means 61, 62. The general mechanical configuration is comparable to the sixth embodiment in FIG. 7.
Instead of a human observer 7, however, the inspection device 5 comprises color sensors 71 to 75. The color sensors 71 to 75 are arranged in the inspection area 56. The spatial distribution of colors 11, 12, 13 in the inspection area 56 is depicted in corresponding FIG. 8b. The light rays emitted within the first predefined range of emission angles occupy a first (horizontally hatched, annular) portion 561 of the inspection area 56, and the light rays emitted within the second predefined range of emission angles occupy a second (vertically hatched, circular) portion 562 of the inspection area (partly overlapping the first, annular portion 561). Color sensors 71, 73, 74 and 75 are arranged in a circular manner within the first portion 561 of the inspection area 56. Color sensor 72 is arranged in the center of the second portion 562 of the inspection area 56. It is advantageous to arrange four or more color sensors in the first portion of the inspection area 56 and one or more color sensors in the second portion of the inspection area 56. The control unit 79 can then be adapted to analyze a color balance matrix corresponding to the inspection area 56. Advantageously, the detection (process step of measuring the colors 11, 12, 13) can be based on a synchronous demodulation with the UV excitation. This minimizes the impact of external (parasitic) light.
The sensors 71 to 75 are arranged on and supported by a printed circuit board (PCB) 81. The excitation device can be comprised by the PCB. The PCB also comprises a control unit 79 comprising additional electronic circuitry, which controls the excitation device 4 and the sensors 71 to 75, respectively. The control unit 79 can also comprise an energy supply such as a rechargeable battery.
The positioning means 61, 62 put the luminescent security feature 1 and the color sensors 71 to 75 into a defined spatial relation. In the present embodiment, the aperture 57 at the vertex of the parabolic mirror 51 is configured to be positioned on the luminescent security feature 1. A stiff connector 62 connects the PCB to the parabolic mirror 51. The reflector 51 and the lenses 591, 592 are configured to form a collimated spatial distribution of colors 11, 12, and 13. The UV cut off filter and/or color filter 53 is adapted to avoid parasitic light impairing the measurement.
The visual appearance of the spatial color distribution in the inspection area, as it is shown in FIG. 8b, is comparable to that of the first embodiment shown in FIG. 2b. A redundant recitation of features is therefore omitted. However, additionally shown are the color sensors 71 to 75 and the excitation device 4.
The eighth and ninth embodiments of FIGS. 9a and 9b are (in parts) comparable to the seventh embodiment. Therefore a detailed description of already described features has been omitted.
However, instead of a rotationally symmetric reflector 51, the eights and ninth embodiments comprise a mirror trough 51. Depicted in FIG. 9a is a mirror trough 51 comprising two inclined but flat mirrors (eighth embodiment). Alternatively, the mirror trough can have a concave or parabolic profile as can be seen in the cross sectional view of FIG. 8a (the ninth embodiment), as the cross sectional view from a rotationally symmetric parabolic mirror and a mirror-symmetric mirror trough comprising a parabolic profile are actually the same.
Also, the top view of FIG. 9b corresponds to both the eighth and the ninth embodiment. Accordingly, the two sides of the reflector 51 and the aperture 57 are of an elongated rectangular shape. Color sensors 77 are disposed in line following the mirror length. The arrangement of color sensors 77 can be considered to be a sensor matrix. UV emitters 4 are disposed in line in parallel to the color sensors. The lenses 591, 592 and filter means 58 are of rectangular shape (instead of rotationally symmetric). Authentication is based on the matrix of sensors 77 (or pixels if an image sensor 82 is used) and a corresponding color balance matrix. In the depicted embodiment, color sensors 72 and 75 will detect the second color 12, color sensors 71, 73, 74 and 75 will detect the first color 11 (or more precisely, the mixed color 13 comprising the first color 11 and the second color 12)
Other embodiment comprising an inspection device 5 comprising only a single (flat, prismatic or concave) mirror or an extreme wide angle lens (fish eye lens) are also disclosed (but not shown).
FIG. 10a shows a tenth embodiment of the invention. The tenth embodiment comprises basically the same technical configuration as the ninth embodiment. However, the matrix of color sensors 77 can be exchanged with an image sensor. The image sensor 82 can be of comparable or smaller in size as the matrix of color sensors 77. The image sensor can be accompanied by a respective lens optic / camera lens 593.
The control unit 79 comprises executable code to execute image recognition and/or image analysis methods (using the data output of the image sensor 82 as input) to recognize and analyze the spatial distribution of color information, i.e. parameters for shape, position and color.
The control unit 79, the PCB and the image sensor 82 can be parts of a handheld device such as a mobile phone or a personal digital assistant. The inspection device 5 can be configured to (mechanically) connect to the handheld device. Alternatively, the inspection device 5 can be separate and independent from the handheld device.
A separate excitation 4 device can be used, which can be separate from the inspection device 5 or alternatively can be integral to the inspection device 5. However, alternatively the built in camera flash (e.g. flash tube or LED) can be used to excite the luminescent security feature 1.
If the inspection device 5 is separate from the handheld device, the executable code can control a signaling device (such as a loudspeaker or a display) to signal the correct alignment of inspection device 5, security feature 1 and handheld device.
An advantageous aspect of this embodiment is the automated recognition of hidden anamorphous images.
FIGS. 11a to 11e depict 5 sequential steps in a simplified side view of an eleventh embodiment.
In a first step, a luminescent security feature 1 is fed along a direction of travel into an apparatus 3 for authenticating the luminescent security feature 1. The apparatus comprises a positioning means 6 (not shown) for transporting the luminescent security feature 1 along the direction of travel. For example, the luminescent security feature 1 can be disposed on a banknote and the banknote can be transported on a conveyor belt or by air stream. The apparatus further comprises an inspection device 5 and an excitation device 4. The inspection device comprises a first sensor 71 positioned in an inspection area 56. Adjacent the first sensor is a first excitation means 41 (such as an UV- or IR emitter) arranged on an upstream side of the first sensor 71. A second excitation device 42 is arranged on a downstream side of the first sensor 71. Spatial excitation radiation distributions 91, 93 are sketched for the first and second excitation device 41, 42, respectively. A color sensor spatial sensitivity distribution 92 is also depicted in a simplified manner. The respective features are depicted but additional reference signs are omitted for clarity, at least for non-moving features of the embodiment. The relative position of the components, the spatial light sensitivity and the trajectory of the luminescent security feature 1 determine a sequence of detected colors. The sequence of detected colors is further processed by the control unit to authenticate the security feature 1.
In a second step, as shown in FIG. 11b, the luminescent security feature 1 enters the spatial excitation radiation distribution 91 of the first excitation device 41. The luminescent security feature 1 is thereby excited to emit light of a first color in first emission angles in a substantially radial (annular) ring shape 94, 96. The luminescent security feature 1 is - at the same time - excited to emit light of a second color in a range of second emission angles in a substantially cone shape 95. The light 96 of the annular shape (having the first color) is detected by the first color sensor 71.
In a third step, as shown in FIG. 11c, the luminescent security feature 1 is moved directly under the first color sensor 71. The first and also the second excitation device 41, 42 excite the security feature 1. The first sensor 71 is illuminated by light rays of the second color. The second color (the color of the cone shape) is detected by the first color sensor 71.
In a fourth step, as shown in FIG. 11d, the luminescent security feature 1 is further moved along the direction of travel. The luminescent security feature is now excited by the excitation radiation 93 of the second excitation device 42. The first sensor 71 detects, again, the first color emitted at the first range of emission angles (the color of the annulus shape).
In a fifth step, as shown in FIG. 11e, the luminescent security feature 1 has left the spatial distribution of the excitation device 42. Therefore, the luminescent security feature is not excited. Accordingly, the luminescent security feature 1 does not emit light of the first and/or second color. The first sensor color neither detects light of the first nor of the second color.
Accordingly, the luminescent security feature can be authenticated by comparing the sensor output of the first color sensor (in timing as well as color value) to the following sequence: TABLE 1

Detection Step Sensor Output of the First Sensor

1 Off

2 First Color (e.g. red)

3 Second Color (e.g. green)

4 First Color (e.g. red)

5 Off
FIGS. 12a to 12h show 8 sequential steps in a simplified side view of a twelfth embodiment.
In a first step, as shown in FIG. 12a a luminescent security feature 1 is fed along a direction of travel into an apparatus 3 for authenticating the luminescent security feature 1. The apparatus comprises a positioning means 6 (not shown) for transporting the luminescent security feature 1 along the direction of travel. For example, the luminescent security feature can be disposed on a banknote and the banknote can be transported on a conveyor belt or by air stream. The apparatus further comprises an inspection device and an excitation device. The inspection device comprises (in direction of travel) a first excitation means 41, a first color sensor 71, a second excitation means 42, a second color sensor 72, and a third excitation means 43 positioned in an inspection area 56.
Excitation radiation distributions 91, 93 and 98 are sketched for the first, second and third excitation means 41, 42 and 43, respectively. A first and second color sensor spatial sensitivity distribution 92 and 97 are also depicted in a simplified manner. The respective features are depicted but additional reference signs can be omitted for clarity, at least for non-moving features of the embodiment. The relative position of the components, the spatial light sensitivity and the trajectory of the luminescent security feature determine a sequence of detected colors. The sequence of detected colors is further processed by the control unit to authenticate the security feature 1. As the luminescent security feature is not excited yet, it does not emit light.
In a second step, as shown in FIG. 12b, the luminescent security feature 1 enters the spatial excitation radiation distribution 91 of the first excitation means 41. The luminescent security feature 1 is thereby excited to emit light of a first color in first emission angles in a substantially radial (annular) ring shape 94, 96. In the depicted position of the security feature 1, a right side 96 of the annular shape 94, 96 is excited and therefore emits light of a first color. The luminescent security feature 1 is - at the same time - excited to emit light of a second color in a range of second emission angles in a substantially cone shape 95. The light 96 of the annular shape (having the first color) is not yet detected by the first color sensor 71.
In a third step, as shown in FIG. 12c, the luminescent security feature 1 is fully excited by the first excitation means 41. The light 96 of the annular shape (having the first color) is detected by the first color sensor 71.
In a fourth step, as shown in FIG. 12d, the luminescent security feature 1 is moved directly under the first color sensor 71. The first and also the second excitation means 41, 42 excite the security feature 1. The first sensor 71 is illuminated by light rays of the second color. The second color (the color of the cone shape) is detected by the first color sensor 71.
In a fifth step, as shown in FIG. 12e, the luminescent security feature 1 is further moved along the direction of travel. The luminescent security feature 1 is now excited by the excitation radiation 93 of the second excitation means 42. The first color sensor 71 detects, again, the first color emitted at the first range of emission angles (the color of the annulus shape). Also, the second color sensor 72 detects the first color.
In a sixth step, as shown in FIG. 12f, the luminescent security feature 1 by the second and the third excitation means 42 and 43, respectively. Accordingly, the second color sensor 72 detects light of the second color.
In a seventh step, as shown in FIG. 12g, the luminescent security feature 1 is excited by the third excitation means 43. Therefore, the second color sensor 72 detects the first color.
In the eighth step, as show in FIG. 12h, the luminescent security feature 1 is still excited by the third excitation means 43. However, as the emitted light does not reach the second color sensor 72, no color is detected.

Accordingly, the luminescent security feature can be authenticated by comparing the sensor output of the first and second color sensors 71, 72 (in timing as well as color value) to the following sequence:

TABLE 2

Detection Step	Sensor Output of the First Sensor	Sensor Output of the Second Sensor
2	Off	Off
3	First Color (e.g. red)	Off
4	Second Color (e.g. green)	Off
5	First Color (e.g. red)	First Color (e.g. red)
6	Off	Second color (e.g. green)
7	Off	First Color (e.g. red)
8	Off	Off

All described embodiments can be utilized to perform the claimed method of authenticating a luminescent security feature.
While various embodiments have been described, the description is intended to be exemplary, rather than limiting and it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of the embodiments. Accordingly, the embodiments are not to be restricted except in light of the attached claims and their equivalents. Also, various modifications and changes may be made within the scope of the attached claims.

Claims

An apparatus for authenticating a luminescent security feature, wherein the luminescent security feature is configured to simultaneously emit light rays of different colors upon excitation, wherein the color of each of the emitted light rays depends on a respective emission angle, the apparatus comprising:
an excitation device configured to excite the luminescent security feature so that the luminescent security feature emits light rays;

an inspection device, wherein the inspection device is configured to inspect a first color and a second color of the emitted light rays, which are directed to an inspection area of the inspection device, wherein the first color light rays are emitted from the security feature within a first predefined range of emission angles and the second color light rays are emitted from the security feature within a second predefined range of emission angles; and

a positioning means configured to put the luminescent security feature into a defined spatial relation with the inspection device.
The apparatus according claim 1, wherein the light rays being emitted within the first predefined range of emission angles occupy a first portion of the inspection area, wherein the light rays being emitted within the second predefined range of emission angles occupy a second portion of the inspection area, and wherein the first portion of the inspection area is different from the second portion of the inspection area.
The apparatus according to claims 1 or 2, wherein the inspection device comprises a first refractive, diffractive or reflective optical element that is configured to deflect the light rays emitted within the first predefined range of emission angles towards the inspection area.
The apparatus according to claim 3, wherein the initial directions of propagation of the light rays emitted within the second predefined range of emission angles are directed towards the inspection area.
The apparatus according to claim 3, wherein the inspection device comprises a second refractive, diffractive or reflective optical element that is configured to deflect the light rays emitted within the second predefined range of emission angles towards the inspection area.
The apparatus according to anyone of claims 3 to 5, wherein the optical element(s) is/are either of:
a) a mirror, in particular a flat mirror or a prismatic mirror configured to reflect the emitted light rays of the first range and/or second range of emission angles towards the inspection area;

b) a concave or parabolic mirror, wherein the concave/parabolic mirror comprises an aperture at a vertex of the concave/parabolic mirror, such that the luminescent security feature may be positioned in or adjacent to the aperture;

c) a mirror trough comprising a concave or parabolic profile, wherein the mirror trough comprises an elongated aperture at a vertex of the mirror trough, such that the luminescent security feature may be positioned in or adjacent to the elongated aperture;

d) a convergent lens configured to deflect the emitted light rays of the first range of emission angles towards the inspection area; and/or

e) a convergent cylindrical lens configured to deflect the emitted light rays of the first range of emission angles towards the inspection area.
The apparatus according to claim 6, comprising an optical group, wherein alternatively
a) the optical group comprises a convergent lens and one out of a mirror and a concave/parabolic mirror; or

b) the optical group comprises a convergent cylindrical lens and a mirror trough.
The apparatus according to any one of claims 2 to 7, wherein the inspection means comprises a matrix of color sensors comprising at least a first and a second color sensor, wherein the first color sensor is arranged to detect the color of the light rays emitted within the first predefined range of emission angles and the second color sensor is arranged to detect the color of the light rays emitted within the second predefined range of emission angles.
The apparatus according to claim 8, wherein the matrix of color sensors is an image sensor.
The apparatus according to any one of claims 1 to 9, wherein the excitation device is suitable to excite the luminescent security feature by use of invisible light, in particular invisible light in the range of ultra violet wavelength and/or infra red wavelength, wherein the inspection device comprises a filter means, the filter means being arranged between the luminescent security feature and the inspection area, and the filter means being configured to filter the invisible light irradiating towards the inspection area.
The apparatus according to any one of claims 1 to 10, wherein the inspection device is configured to equalize a hidden anamorphous pattern in the luminescent security feature.
The apparatus according to claim 1, wherein the inspection device comprises at least one color sensor in the inspection area, wherein the positioning means is adapted to transport a banknote comprising the luminescent security feature along a direction traversing the inspection area at a predetermined speed, wherein a control unit is adapted to authenticate the banknote using a sequence of colors emitted by the luminescent security feature and detected by the color sensor and a recorded timing of the sequence to calculate respective emission angles associated with the emitted colors.
The apparatus according to any one of claim 1 to 12, wherein the inspection device comprises an anti scatter filter, and in particular a honey comb grid.
A method for authenticating a luminescent security feature, wherein the luminescent security feature is configured to simultaneously emit light rays of a first and a second color upon excitation, wherein the color of each of the emitted light rays depends on a respective emission angle, comprising the steps of:
a) positioning an inspection device in a predefined spatial relation with the luminescent security feature;

b) exciting the luminescent security feature such that the luminescent security feature emits light rays;

c) directing the emitted light rays towards an inspection area;

d) inspecting, in the inspection area, a first color of light rays emitted within a first predefined range of emission angles and a second color of light rays emitted within a second predefined range of emission angles;

e) comparing the first color of light rays emitted within the first predefined range of emission angles with a first predefined color;

f) comparing the second color of light rays emitted within the second predefined range of emission angles with a second predefined color; and

g) classifying the luminescent security feature as being authentic or counterfeit based on the result of comparing the first color of light rays emitted within the first predefined range of emission angles with the first predefined color and comparing the second color of light rays emitted within the second predefined range of emission angles with the second predefined color.
The method according to claim 14, additionally comprising the step of deflecting the light rays emitted within a second predefined range of emission angles towards the inspection area, such that the light rays emitted within the second predefined range of emission angles occupy a second portion of the inspection area that is different from a first portion of the inspection area.