Authentication device and authentication method for authentication of an elliptically polarizing security marking

Abstract

Authentication device for authentication of an elliptically polarizing security marking, comprising: a liquid crystal (LC) variable phase retarder for receiving elliptically polarized light reflected by the security marking and for converting the elliptically polarized light to linear polarized light at a selected wavelength by means of setting an appropriate retardance corresponding to a voltage applied to the LC variable phase retarder; a visible light (VIS) linear polarizer provided downstream of the LC variable phase retarder for transmitting a fraction of the linear polarized light, the fraction being in accordance with an angular difference between an orientation angle of the linear polarized light and an axis of the VIS linear polarizer; and an analysis unit for extracting a characteristic of the fraction of the linear polarized light as transmitted by the VIS linear polarizer, and for determining whether the security marking is authentic or not depending on the extracted characteristic.

Description

[0001] AUTHENTICATION DEVICE AND AUTHENTICATION METHOD FOR AUTHENTICATION OF AN ELLIPTICALLY POLARIZING SECURITY MARKING

[0002] TECHNICAL FIELD

[0003] The present invention relates to the technical field of authenticating an e I liptically polarizing security marking. In detail, the present invention relates to an authentication device for the authentication of an elliptically polarizing security marking, as well as to a corresponding authentication method.

[0004] BACKGROUND ART

[0005] In order to protect products (including documents) against counterfeiting, falsifying or illegal reproduction, it has been the conventional practice to incorporate security elements in these products. Typical examples of security elements include security threads, windows, fibers, planchettes, foils, decals, holograms, watermarks, security inks comprising optically variable pigments, magnetic or magnetizable thin-film interference pigments, interference-coated particles, thermochromic pigments, photochromic pigments, luminescent, infraredabsorbing, ultraviolet-absorbing or magnetic compounds. The present application focusses on the use of circular polarizing security markings (such as markings including a cholesteric liquid crystal polymer, CLCP), which selectively reflect one or both of left-handed and right-handed circular polarized light components. In other words, within a determined wavelength range, light having a determined circular polarization state (left- or right- handed) is predominantly reflected. Such circular polarizing security markings can for example be placed on vehicle license plates to ensure that the vehicle license plates are genuine.

[0006] To determine whether a product such as the vehicle license plate is genuine, distance authentication of the product can be performed by determining whether the security elements have the expected properties. For products including or carrying a circular polarizing security marking as described above, the authentication of the inks forming the circular polarizing security marking can include determining whether the light reflected by the circular polarizing security marking has the expected properties and polarization.

[0007] Human eyes cannot differentiate polarized light, and therefore, appropriate optical components are required to sense the difference between random polarization (natural light, sun for example) and circular polarization of light.

[0008] The document DE 102 11 310 A1 describes an example for such optical components. Namely, it discloses a device for verifying a security marking containing liquid-crystalline material with a cholesteric reflection band. The device illuminates the security marking by a lighting unit with one or more spectrally restricted light sources and checks the light reflected or transmitted by the security marking for circular polarization in a detection unit. The device does not have any moving parts for detecting circular polarization and changing the color of the lighting unit. To achieve this purpose, DE 102 11 310 A1 provides an analyzer including a switchable liquid crystal (LC) cell with a quarter wavelength plate, allowing switching between the two polarization states by switching between applying and not applying an electrical voltage to the LC cell.

[0009] Depending on an ink used in the circular polarizing security marking, the wavelength range of the predominantly reflected light can vary. It is therefore desirable to provide an authentication device that is easily adaptable to different wavelength detections. The purpose of the present invention is the facilitated and reliable authentication of different elliptically polarizing security markings, in particular circular polarizing security markings.

[0010] SUMMARY OF THE INVENTION

[0011] According to a first aspect, an authentication device for authentication of an elliptically, in particular circular, polarizing security marking is provided, the authentication device comprising: a liquid crystal (LC) variable phase retarder for receiving elliptically polarized light reflected by the security marking and for converting the elliptically polarized light to linear polarized light at a selected wavelength by means of setting an appropriate retardance corresponding to a voltage applied to the LC variable phase retarder; a visible light (VIS) linear polarizer provided downstream of the LC variable phase retarder for transmitting a fraction of the linear polarized light, the fraction being in accordance with an angular difference between an orientation angle of the linear polarized light and an axis of the VIS linear polarizer; and an analysis unit for extracting a characteristic of the fraction of the linear polarized light as transmitted by the VIS linear polarizer, and for determining whether the security marking is authentic or not depending on the extracted characteristic.

[0012] The retardance applied to the detected light can be varied quickly in the visible light range, for example with a frequency of variation of less than three seconds, in particular less than one second. This for example allows to bring out a specific wavelength of the ink and / or to scan over a retardance range of interest and bring out a behavior of the ink / marker. This behavior is specific to the elliptically (in particular circular) polarized light reflected by the circular polarizing security marking and hence allows determining whether the elliptically polarizing security marking is authentic or not from a distance.

[0013] The elliptically polarizing security marking can selectively reflect one or both (left- and right-handed) elliptically polarized light components. In other words, within a determined wavelength range, light having a determined elliptically polarization state (left- or right-handed) is predominantly reflected. The circular polarizing security marking can include a cholesteric liquid crystal polymer (CLCP). Cholesteric liquid crystal materials are characterized by an internal helical supramolecular texture, which causes a periodic modulation of the refractive index, wherein the periodicity is comparable with the wavelength of visible light. As a consequence, such materials act as optical diffraction gratings and reflect light of particular wavelengths, appearing colored to the eye. The helical sense of the texture (right or left) causes light of one elliptically polarization state to be predominantly reflected. Examples of CLCPs are for example described in the patent EP 3 057 801 B1 .

[0014] The term “authentication” here designates the verification as to whether or not the elliptically polarizing security marking is genuine, i.e. valid. This is only the case if the light reflected by the elliptically polarizing security marking has the expected properties, in particular if within a determined wavelength range, light having a determined circular polarization state (left- or right-handed) is predominantly reflected. Wherever used herein, the term “elliptically polarized” includes circular polarized. Preferably, the term “elliptically polarized” designates “circular polarized”.

[0015] In the light path of the light reflected by the circular polarizing security marking, the LC variable phase retarder and the VIS linear polarizer are provided in this order. The VIS linear polarizer may be provided directly after the LC variable phase retarder along the light path, without additional optical components therebetween. Alternatively, as will be described in later embodiments, it is also possible to provide further optical components between the LC variable phase retarder and the VIS linear polarizer, which may modify the light transmitted therebetween. The light reflected by the circular polarizing security marking is elliptically (in particular circular) polarized light. Depending on the polymer used in the circular polarizing security marking, the reflected polarized light is left-handed, right-handed or both left- and right-handed polarized light.

[0016] The LC variable phase retarder is a transmissive element with an electrically tunable optical phase retardance. It can be compared to a variable waveplate which, when addressed with the right voltage, can provide any phase shift from near zero to several times the light wavelength. A power supply unit (which is part of the authentication device or not) can be used to apply the voltage to the LC variable phase retarder. The LC variable phase retarder transforms the circular polarized light reflected by the circular polarizing security marking to linear polarized light at the selected retardance. The selected retardance, which is chosen according to the need of the application, can be selected in accordance with a voltage applied to the LC variable phase retarder (typically a rectangular wave at approximately 1 kHz, the amplitude being variable between -10V and 10V or between - 2.5V and +2.5V, for example). This voltage and hence retardance change can take place at high speed. This is particularly advantageous in applications in which the product to be authenticated moves at high speed, which is for example the case of a vehicle license plate on a moving vehicle. At the same time, the voltage-induced retardance modulation requires no mechanically moving component, which would not be suitable for image acquisition at a high frame rate. In particular, the retardance is the phase shift between light polarizations as introduced by the LC variable phase retarder. The retardance may be linked through a known calibration curve to the voltage applied to the LC phase retarder. Each retardance corresponds to a specific wavelength of the linear polarized light.

[0017] The VIS linear polarizer transmits light uniformly vibrating in a single plane while absorbing the orthogonal plane. The VIS linear polarizer is in particular only effective for light in the visible wavelength spectrum, i.e. light outside the visible range is not affected by the VIS linear polarizer. According to the angular difference between the linear polarized light and the axis of the VIS linear polarizer, i.e. a difference between the axes of the linear polarized light and the VIS linear polarizer, the quantity (fraction) of light passing the VIS linear polarizer changes. When the axes are perfectly parallel, 100% of the light passes through. When the axes are orthogonal, 0% of the light passes through. For an intermediate angular difference, the fraction (percentage) of light passing through varies between 0% and 100%, depending on the angular difference. When switching between different retardances by varying the voltage supplied to the LC variable phase retarder, the VIS linear polarizer switches between blocking either right-handed elliptically polarized light or left-handed elliptically polarized light, allowing for a robust authentication.

[0018] This solution is advantageous in that it allows to change the retardance quickly in the visible light range. This for example allows to perform a scan over a retardance range of interest and bring out a retardance at which the intensity captured by the analysis unit is maximum or minimum (example for characteristics). From these minimum and maximum intensity values and from the retardances at which they were obtained, the appropriate polarization handedness (left or right) and a corresponding main intensity wavelength (or other colorimetric characteristic) of the ink / marker can be derived. The intensity at a specific retardance (as well as the main wavelength, an observed hue or other characteristics) is specific to the polarized light reflected by the security marking and hence allows determining whether the security marking is authentic or not. In particular, it is possible to differentiate right-handed elliptical polarization from left-handed elliptical polarization depending on the (changeable) retardance.

[0019] Optionally, the authentication device includes a light sensorwhich receives the light transmitted by the VIS linear polarizer and captures an intensity thereof. Alternatively, the light sensor may be provided as a separate entity from the authentication device, in which case the light sensor can be part of a readily available camera and / or of a smartphone camera. This is advantageous in that the authentication device can be provided as an add-on for an existing camera. The light sensor may capture red-green-blue (RGB) intensities of the light that reaches it. The authentication can also be based on RGB values of images inks or also on conversions to HSV (hue, saturation, value), Lab or other colorimetric spaces.

[0020] The analysis unit can be a specifically programmed module or chip for performing analysis of the light intensity transmitted by the VIS linear polarizer, in particular of the data captured by the light sensor. The analysis unit is capable of extracting at least one characteristic (such as an intensity at the selected retardance, a maximum and / or minimum intensity, wavelength corresponding to specific intensities, hue values, or the like). The extracted characteristic can then be used to determine whether or not the security marking is authentic. In particular, the analysis unit compares the extracted characteristic(s) with one another and / or with reference characteristic(s) and deduces, based on the comparison results, whether the security marking is authentic or not.

[0021] The analysis unit can be connected to an output unit which outputs the intensity transmitted by the VIS linear polarizer, the reference intensity, the comparison result therebetween and / or the authentication result. The output unit may be a display.

[0022] The described authentication device in particular allows for an automatized authentication of security markings from a distance, for example from a distance of at least one meter, preferably from a distance of at least five meters, more preferably for distances of up to fifty meters from the authentication device. This is particularly useful to check the authenticity of a security marking on a vehicle license plate, since the authentication can be performed while the vehicle is moving on a road, for example a highway, without having to slow down or stopping. This renders the authentication of vehicle license plates more convenient and safer for all involved parties.

[0023] According to an embodiment, the authentication device further comprises: a power supply unit for applying the voltage to the LC variable phase retarder, the power supply unit being configured to switch between different voltages applied to the LC variable phase retarder and / or to swipe over a range of voltages applied to the LC variable phase retarder, so that the LC variable phase retarder converts the elliptically polarized light to linear polarized light at selected retardances corresponding to the voltages applied to the LC variable phase retarder; wherein the analysis unit is configured to extract the characteristics of the fraction of the linear polarized light as transmitted by the VIS linear polarizer for each selected retardance, and to determine whether the elliptically polarizing security marking is authentic or not depending on the extracted characteristics. The power supply unit may be used to control a voltage supplied to the LC variable phase retarder, and accordingly to control the selected retardance. By varying the voltage using the power supply unit, the LC variable phase retarder can provide different selected retardances in accordance with the respective voltages. The voltages and hence selected retardances can be varied continuously (in a swipe, also referred to as scan) or discontinuously (discrete values). The power supply unit can be programmed such as to change the provided voltage in an autonomous manner, without requiring any manual switching by a user. Accordingly, the voltage switching / swiping can be performed very fast, at frequencies that are a lot smaller than one second.

[0024] For each selected retardance, an optional light sensor of the authentication device can capture the resulting intensity or another characteristic of the light exiting the VIS linear polarizer, along the lines described above. Each of these characteristics can be compared to a reference characteristic for the selected retardance corresponding to said measurement. The determination as to whether the security marking is authentic or not can be based on the result of all these individual comparisons. For example, the authenticity of the security marking is only confirmed if all measured intensities are equal to the corresponding reference intensity (within a predefined range).

[0025] When enough intensity values are captured or if a scan is performed, a spectrum over the scanned voltages and / or retardances can be obtained. This measured spectrum can be compared to a reference spectrum (which is made of the reference intensities) to determine the authenticity.

[0026] According to a further embodiment, the authentication device further comprises: a visible and infrared (VIS-IR) linear polarizer provided between the LC variable phase retarder and the VIS linear polarizer, the VIS-IR linear polarizer receiving the linear polarized light from the LC variable phase retarder and transmitting a fraction of the linear polarized light, the fraction being in accordance with an angular difference between the orientation angle of the linear polarized light received from the LC variable phase retarder and an axis of the VIS-IR linear polarizer; and a liquid crystal display (LCD) provided between the VIS-IR linear polarizer and the VIS linear polarizer, the LCD having two states selected in accordance of whether or not a voltage is applied to the LCD, wherein in a first state, in which a voltage is applied to the LCD, the LCD is configured to act neutrally and let the linear polarized light from the VIS-IR linear polarizer through without changes, and wherein in a second state, in which no voltage is applied to the LCD, the LCD is configured to act as a half wave retarder to shift by half a wave the linear polarized light from the VIS-IR linear polarizer, thereby providing a half wave shifted linear polarized light; wherein the linear polarized light leaving (exiting) the LCD in its unchanged state or in its half wave shifted state forms the linear polarized light entering the VIS linear polarizer.

[0027] In particular, when the LCD is in the first state, the VIS linear polarizer transmits visible light and infrared light, and when the LCD is in the second state, the VIS linear polarizer transmits only infrared light.

[0028] This embodiment allows to differentiate right-handed elliptical polarization from left-handed elliptical polarization depending on the (changeable) retardance and specific active infrared (IR) filter. For this, the VIS-IR linear polarizer is provided between the LC variable phase retarder and the VIS linear polarizer which were previously described. The linear polarized light exiting the LC variable phase retarder passes through the VIS-IR linear polarizer and subsequently through the LCD, before passing through the VIS linear polarizer. The VIS-IR linear polarizer functions in a similar way to the VIS linear polarizer, except that it also filters IR light in accordance with its orientation. Accordingly, the VIS-IR linear polarizer transmits a fraction of the linear polarized light received from the LC variable phase retarder in accordance with a difference between the angle of an axis of the visible and of the IR light in the linear polarized light received from the LC variable phase retarder and the main axis of the VIS-IR linear polarizer. The term “IR” as used herein includes near-infrared (NIR). In particular, anywhere where “IR” is used in the present description, the term “NIR” could be used instead.

[0029] Depending on whether a voltage (typically between 3 and 3.5V) is applied to the LCD or not, it acts as a halfwave retarder or as a neutral optical element which does not impact the light passing through it. When the LCD is in its first state, in which it acts neutrally, the light exiting the LCD is in an unchanged state. When the LCD is in its second state, in which it acts as a half-wave retarder, the light exiting the LCD is shifted by half a wave (A / 2) and designated as being in the “half wave shifted state”. The LCD may be powered by its own power supply unit, which may be preprogrammed and / or synchronized with the power supply unit of the LC variable phase retarder.

[0030] When the LCD is not powered (second state) and acts as a half wave retarder, the polarization state of the linear polarized light changes by passing through the LCD, it shifts by 90°. This brings the polarized light exiting the LCD in the same orientation as the axis of the VIS linear polarizer. In this case, 100% of the polarized light leaving the LCD is transmitted by the VIS linear polarizer and reaches the optional light sensor. The light sensor used in this embodiment is preferably sensitive to IR light.

[0031] When the LCD is powered (first state), it acts neutrally. The linear polarized light in the half wave shifted state is orthogonal to the axis of the VIS linear polarizer. All visible light (VIS) is stopped but not the IR light because the VIS linear polarizer is transparent in IR light. The combination of optical elements in this embodiment thus allows creating an active VIS filter.

[0032] With this implementation, it is possible to operate in the visible range (with the LCD unpowered and proper settings in the LC variable phase retarder to produce required VIS retardations, in particular calibration settings yielding a maximum intensity at a retardance of (3A / 4)+n*A for right-handed polarization, and a maximum intensity at a retardance of (A / 4)+n*A for left-handed polarization, A being the wavelength in the visible range of 400-700nm and n being any integer) and obtain two images of the product with the security marking: one at lefthanded elliptical polarization and another at right-handed elliptical polarization (one of them forming the reference intensity) that can be compared by the analysis unit to authenticate the product. With the same configuration, it is also possible to operate in the IR or more specifically NIR range (with the LCD powered and appropriate settings in the LC variable phase retarder to produce required IR retardations in particular calibration settings yielding a maximum intensity at a retardance of (3A / 4)+n*A for right-handed polarization, and a maximum intensity at a retardance of (A / 4)+n*A for left-handed polarization, A being the wavelength in the IR range of 700-1 OOOnm and n being any integer) and obtain two images of the product with the security marking in the IR: one at left-handed elliptical polarization and another at right-handed elliptical polarization (one of them forming the reference intensity) that can be compared by the analysis unit to authenticate the product. With those four images, the analysis unit and / or a user of the authentication device can identify the presence of the genuine inks on the product in the expected locations. According to a further embodiment, the analysis unit is configured to:

[0033] (i) extract, as the characteristic, an intensity of the fraction of the linear polarized light as transmitted by the VIS linear polarizer for the selected retardance and compare it with a reference intensity for the selected retardance, the analysis unit being configured to determine whether the security marking is authentic or not depending on the result of the comparison between the intensity of the fraction of the linear polarized light for the selected retardance with the reference intensity for the selected retardance;

[0034] (ii) extract, as the characteristic, a first intensity and a second intensity of the fraction of the linear polarized light as transmitted by the VIS linear polarizer for respectively a first retardance and a second retardance as the selected retardances, the analysis unit being configured to determine whether the security marking is authentic or not depending on the result of the comparison between the first intensity and the second intensity;

[0035] (iii) extract, as the characteristic, a width of an intensity curve around the maximum and / or minimum intensity, the analysis unit being configured to determine whether the security marking is authentic or not by comparing the width of the intensity curve around the maximum and / or minimum intensity with a reference width; and / or

[0036] (iv) extract, as the characteristic, a parameter of any colorimetric space, such as the hue, saturation, or value, of the fraction of the linear polarized light as transmitted by the VIS linear polarizer and to compare the extracted parameter of the colorimetric space with a reference parameter of any colorimetric space, the analysis unit being configured to determine whether the security marking is authentic or not depending on the result of the comparison of the extracted parameter of any colorimetric space with the reference parameter of any colorimetric space.

[0037] According to (i), the analysis unit can compare the intensity of the light leaving the VIS linear polarizer (extracted characteristic) with a reference intensity and determine whether the circular polarizing security marking is authentic or not based on this comparison. The reference intensity can be a previously sensed intensity and / or a prestored intensity (which can be seen as a threshold) which is uniformly used to determine the authenticity of a type of security marking. For example, it is determined that the elliptically polarizing security marking (also referred to as “security marking” herein) is authentic if the captured intensity is within a certain range from the reference intensity. The reference intensity is in particular an intensity that is specific to the selected retardance. In this case, the reference intensity is an intensity that is expected for the specific selected retardance.

[0038] Option (i) is particularly relevant when the authentication device of the first aspect is used in combination with another camera, for instance a standard security camera. In such case, the authentication device of the first aspect could be operated only at one specific retardance yielding a specific polarization at that wavelength. Such an image could be compared with an image from a standard camera to authenticate the security marking.

[0039] According to (ii), by selecting two specific retardances (first and second retardances) for which significantly different intensities are expected and applying voltages corresponding to the first and second retardances to the LC variable phase retarder, first and second intensity values are obtained. The result of the comparison between these first and second intensity values is an indicator for the authenticity of the security marking.

[0040] According to (iii), the analysis unit analyses and compares the width of an intensity curve (which represents the intensity as a function of the retardance) with the reference retardance. The width of the intensity curve can be defined as the full width at half maximum, for example. The width of the intensity curve determined by the analysis unit may be compared with a reference width. The reference width was for example determined shortly before for the other of the left- or right-handed elliptical polarization or prestored.

[0041] According to (iv), a parameter (such as the hue, saturation, value, Lab intensity values or parameters of any other colorimetric space) of the light transmitted by the VIS linear polarizer can be extracted and compared with a corresponding reference value to determine the authenticity of the security marking.

[0042] The functionalities (i) to (iv) described above allow improving the accuracy with which the authentication device determines the authenticity of the security marking.

[0043] According to a further embodiment, the analysis unit is configured to: extract, as a characteristic, a maximum intensity retardance which is the selected retardance for which a maximum intensity of the fraction of the linear polarized light is observed, and / or a minimum intensity retardance which is the selected retardance for which a minimum intensity of the fraction of the linear polarized light is observed.

[0044] Namely, the analysis unit may determine the peak intensity, i.e. a maximum intensity, and a corresponding retardance (maximum intensity retardance) based on the result of the scan over the retardance range. The analysis unit may further determine the minimum intensity, and a corresponding retardance (minimum intensity retardance) based on the result of the scan over the retardance range. This maximum intensity retardance and / or maximum intensity retardance may be set as the selected retardance and compared with a reference intensity (as described above) to determine the authenticity of the security marking. For example, if the minimum and / or maximum intensity are observed at the expected or similar retardances, the authenticity of the security marking is confirmed.

[0045] Considering the maximum intensity retardance, the minimum intensity retardance and / or the width of the intensity curve allows determining the authenticity of the security marking more accurately.

[0046] According to a further embodiment, the analysis unit is configured to: fora security marking which is expected to reflect right-handed elliptically polarized (RHCP) light, calculating a right-handed maximum intensity wavelength being a wavelength corresponding to the determined maximum intensity retardance using the formula 3A / 4, A being the wavelength, and / or calculating a right-handed minimum intensity wavelength being a wavelength corresponding to the determined minimum intensity retardance using the formula A / 4; and / or for a security marking which is expected to reflect left-handed elliptically polarized (LHCP) light, calculating a left-handed maximum intensity wavelength being a wavelength corresponding to the determined maximum intensity retardance using the formula A / 4, and / or calculating a right-handed minimum intensity wavelength being a wavelength corresponding to the determined minimum intensity retardance using the formula 3A / 4; wherein the analysis unit is further configured to determine whether the security marking is authentic or not based on a result of a comparison of the right-handed maximum intensity wavelength with a corresponding reference right-handed maximum intensity wavelength, the right-handed minimum intensity wavelength with a corresponding reference right-handed minimum intensity wavelength, the left-handed maximum intensity wavelength with a corresponding reference left-handed maximum intensity wavelength, and / or the left-handed minimum intensity wavelength with a corresponding reference left-handed minimum intensity wavelength.

[0047] For RHCP light, the maximum intensity is expectedly obtained at a maximum intensity retardance of 3A / 4. Knowing the maximum intensity retardance, this equation can be rearranged to calculate the wavelength A, which forms the right-handed maximum intensity wavelength. Similarly, for RHCP light, the minimum intensity is obtained at a minimum intensity retardance of A / 4. Knowing the minimum intensity retardance, this equation can be rearranged to calculate the wavelength A, which forms the right-handed minimum intensity wavelength.

[0048] For LHCP light, the maximum intensity is expectedly obtained at a maximum intensity retardance of A / 4. Knowing the maximum intensity retardance, this equation can be rearranged to calculate the wavelength A, which forms the left-handed maximum intensity wavelength. Similarly, for LHCP light, the minimum intensity is obtained at a minimum intensity retardance of A / 4. Knowing the minimum intensity retardance, this equation can be rearranged to calculate the wavelength A, which forms the left-handed minimum intensity wavelength.

[0049] The different intensity wavelength values can be compared with respective selected wavelengths to determine whether the authenticity of the security marking.

[0050] According to a further embodiment, the authentication device further comprises: an observation angle determination unit for determining an angle of observation of the elliptically polarized light reflected by the security marking on the LC variable phase retarder; wherein the analysis unit is configured to determine whether the security marking is authentic or not additionally under consideration of the angle of observation determined by the observation angle determination unit by either

[0051] (i) comparing the extracted characteristic with a reference characteristic which is specific to the selected retardance and to the angle of observation determined by the observation angle determination unit, and / or by

[0052] (ii) using Bragg-Snell's law to deduce, from a maximum intensity retardance for which a maximum intensity of the fraction of the linear polarized light is sensed and from the angle of observation determined by the observation angle determination unit, a maximum intensity retardance at a predetermined observation angle, and by comparing the deduced maximum intensity retardance at the predetermined observation angle with a reference maximum intensity retardance at the predetermined observation angle.

[0053] Since the light reflected by the security marking can undergo Bragg-Snell diffraction, a maximum intensity of the light reflected from the security marking is obtained at different maximum retardances depending on an angle of the light striking the security marking and / or an angle between the security marking and the authentication device.

[0054] To account for Bragg-Snell’s diffraction when determining the authenticity of the security marking and thereby increase the reliability of the authenticity determination, the angle of observation is determined and taken into account. The angle of observation is an angle between a normal to the security marking plane and the optical axis of the camera setup. In other words, it is the angle at which the security marking is observed from the camera. At an angle of observation of zero, Bragg-Snell’s diffraction is smallest (inexistant) while it is maximum for an angle of observation of 90°. In order to account for Bragg-Snell’s diffraction, there are two options. Either the analysis unit compares the extracted characteristic with a reference characteristic which is not only specific to the selected retardance but is further specific to the determined angle of observation and accordingly determines the authenticity. Alternatively, the analysis unit deduces a maximum intensity for a predetermined observation angle (for example at 0°) based on the measured angle of observation and compares this deduced maximum intensity retardance with a reference maximum intensity retardance associated with the predetermined observation angle, in particular by comparing their respective retardances.

[0055] Considering Bragg-Snell’s diffraction along the lines above allows to increase the reliability of the authentication determination, even in situations in which the authentication detection is not performed with the ideal angle of observation (0°). This is for example advantageous to check a security marking on a vehicle license plate on vehicles moving past a control station, in which the security marking is usually at an oblique observation angle.

[0056] According to a further embodiment, the observation angle determination unit is configured to collect positions of reference points located on the e 11 iptica I ly polarizing security marking and to determine the angle of observation from the positions of the reference points.

[0057] On a genuine product, the reference points (which can be black dots, for example) may be located in predefined positions on or around the security marking. The reference points form an expected pattern, such as a square. When the product is view from an angle, the pattern is distorted and the observation angle determination unit can for example deduce the angle of observation from the observed distortion of the pattern formed by the reference points.

[0058] According to a further embodiment, an angle between a main axis of the LC variable phase retarder and a main axis of the VIS linear polarizer is 45°, an angle between the main axis of the LC variable phase retarder and a main axis of the VIS-IR linear polarizer is 45°, and / or an angle between the main axis of the VIS-IR linear polarizer and the main axis of the VIS linear polarizer is 90°.

[0059] In the previously described embodiments, an order of the optical elements (LC variable phase retarder, VIS linear polarizer, VIS-IR linear polarizer, LCD) along the light path is important and should not be changed as the authentication could otherwise become impossible. In addition, certain of these components are preferably orientated at a certain angle with respect to one another to ensure a reliable authentication.

[0060] The main axis of the VIS linear polarizer and of the VIS-IR linear polarizer is preferably an axis along which light aligned therewith is transmitted at 100%. The main axis of the LC variable phase retarder is preferably an axis along which the linear polarized light leaves the LC variable phase retarder. The LCD may be oriented in any direction without influencing the authentication device.

[0061] Placing the main axis of the LC variable phase retarder at 45° of the main axis of the VIS linear polarizer allows to perfectly convert RHCP light to linear polarization at 45 degrees (aligned with the VIS linear polarizer) if a retardation of (3A / 4) + n*lambda is set. This alignment also allows to perfectly convert LHCP to linear polarization at -45 degrees (at 90° with respect linear polarization, so the light does not pass through if a retardation of (A / 4) + n*A is set). This is true for wavelengths A in the visible range, as the VIS linear polarizer does not act as a polarizer beyond 700 nm.

[0062] Placing the main axis of the LC variable phase retarder at 45° of the main axis of the VIS-IS linear polarizer allows to do as above, but for wavelengths in visible and also beyond visible range, namely in the NIR.

[0063] Placing the main axis of the VIS polarizer at 90° of the main axis of the VIS-IR linear polarizer allows to stop light that both VIS and VIS-IR linear polarizer can polarize, which is light in VIS range, but not the NIR light, which will pass through.

[0064] Normally Quarter Wave Plates (QWP) and thus LC variable phase retarders have a fast and a slow optical axis (in terms of propagation of light). If one sends unpolarized light through a linear polarizer at 45 degrees and then through a quarter wave plate with horizontal slow axis and vertical fast axis, then light will get retarded (quarter wave) in the horizontal slow axis and will therefore be converted to circular polarized light. It means that the fast and slow axis of retarders are advantageously at + / - 45 degrees with respect to linear polarizers placed right before or after.

[0065] According to a further embodiment, the authentication device further comprises a light sensor provided downstream of the VIS linear polarizer for sensing the intensity of the fraction of light transmitted by the VIS linear polarizer. The light sensor can be part of a camera and / or of a smartphone camera.

[0066] According to a further embodiment, the authentication device further includes an illumination device for illuminating the elliptically polarizing security marking, the illumination device comprising: a light source for emitting light towards the elliptically polarizing security marking; an illumination linear polarizer which is a visible (VIS) linear polarizer or a visible and infrared (VIS-IR) linear polarizer and is provided downstream of the light source for receiving the light emitted by the light source and for transmitting a fraction of the received light as linear polarized light; and an illumination liquid crystal (LC) variable phase retarder provided downstream of the illumination linear polarizer, the illumination LC variable phase retarder being configured to receive the linear polarized light from the illumination linear polarizer and to convert it into specific elliptically polarized light at a selected wavelength corresponding to a voltage applied to the illumination LC variable phase retarder; wherein the elliptically polarized light reflected by the security marking and reaching the LC variable phase retarder includes light emitted by the illumination device onto the security marking.

[0067] An illumination device is added to the authentication device, said illumination device being not only for illuminating the security marking (which is useful at nighttime, for example), but more specifically to illuminate the security marking with a light with a specific polarization and retardance as selected through the illumination device. The optics included in the illumination device are similar to those of the detector device (including the VIS linear polarizer and the LC variable phase retarder) of the authentication device described above. Namely, as described above, the combination of an LC variable phase retarder and a VIS light polarizer as part of a detector allows to select a specific retardance of reflected light and analyze said light to determine whether the security marking is authentic or not. Similarly, in the illumination device, the combination of an illumination linear polarizer (VIS or VIS-IR) and of an illumination LC variable phase retarder in front of the light source allows emiting light at a specific retardance towards the security marking, allowing to verify the authenticity thereof. Namely, when the security marking reflects only the light at a specific and predetermined retardance (corresponding to a specific wavelength) and polarization, then it is considered to be genuine.

[0068] All features and properties described in view of the VIS linear polarizer and the LC variable phase retarder above equally hold for the illumination linear polarizer and for the illumination LC variable phase retarder, respectively.

[0069] The illumination device and more specifically, a power supply thereof providing the illumination LC variable phase retarder with voltages in a predefined range (for example between 0 and 10V), can be configured to vary between different voltages, leading to different retardances. For example, the illumination device may alternate between a retardance corresponding to a wavelength that gets reflected by the security marking and one that does not get reflected by the security marking. The alternation between these two states leads to an alternation between a reflection of the light by the security marking and no reflection (given that the security marking is genuine). Observing this alternation using the detector device allows determining the authenticity of the security marking. A similar alternation process can be used with even more than two states, for example if the security marking includes different regions which reflect different wavelengths and / or polarizations, wherein each of these regions is illuminated with a corresponding wavelength and polarization in alternation to detect whether the security marking is genuine or not.

[0070] The light exiting the illumination device can illuminate the security marking and depending on the nature of the security marking, it may get reflected. The analysis unit may be configured to compare the extracted characteristic with a reference characteristic, and for determining whether the security marking is authentic or not depending on the result of this comparison. The analysis unit can be synchronized with the illumination device, in particular with the voltage switches of the voltage to the LC variable phase retarder, such as to optimize the detection of the authenticity of the security marking.

[0071] According to a second aspect, an authentication device for authentication of an elliptically polarizing security marking is provided, the authentication device of the second aspect comprising: an illumination device for illuminating an elliptically polarizing security marking, the illumination device comprising: a light source for emiting light towards the elliptically polarizing security marking; an illumination linear polarizer which is a visible (VIS) linear polarizer or a visible and infrared (VIS-IR) linear polarizer and is provided downstream of the light source for receiving the light emited by the light source and for transmiting a fraction of the received light as linear polarized light; and a liquid crystal (LC) variable phase retarder provided downstream of the VIS linear polarizer or of the VIS-IR linear polarizer, the LC variable phase retarder being configured to receive the linear polarized light from the VIS linear polarizer or from the VIS-IR linear polarizer and to convert it into specific elliptically polarized light at a selected wavelength by means of seting an appropriate retardance corresponding to a voltage applied to the LC variable phase retarder, the specific elliptically polarized light illuminating the elliptically polarizing security marking; and an analysis unit for extracting a characteristic of the light reflected by the elliptically polarizing security marking, and for determining whether the security marking is authentic or not depending on the extracted characteristic. The authentication device of the second aspect relies on the same optics and principles as the authentication device of the first aspect. Namely, in the authentication device of the first aspect, the combination of an LC variable phase retarder and a VIS light polarizer in front of a detector allows to select a specific retardance of reflected light and analyze said light to determine whether the security marking is authentic or not. Similarly, in the authentication device of the second aspect, the combination of a linear polarizer and of a LC variable phase retarder in front of the light source allows emitting light at a specific retardance towards the security marking, allowing to verify the authenticity thereof. Namely, when the security marking reflects only the light at a specific and predetermined wavelength (which translates into a corresponding retardance) and polarization, then it is considered to be genuine. For the detection of the light reflected by the security marking, a normal, readily available camera can be used, which can be connected to the analysis unit for analysis of the detected intensities.

[0072] All features and definitions described in view of the authentication device of the first aspect or any embodiment thereof also hold for the authentication device of the second aspect. For example, an angle between the main axis of the linear polarizer and the LC variable phase retarder can be 45°. The LC variable phase retarder can be powered using a power supply specific to the illumination device, which for example provides voltages between 0 and 10V.

[0073] The illumination device and more specifically, the power supply thereof, can be configured to vary between different voltages, leading to different illumination retardances. For example, the illumination device may alternate between a retardance corresponding to a wavelength that is reflected by the security marking and one that does not get reflected by the security marking. The alternation between these two states lead to an alternation between a reflection of the light by the security marking and no reflection (given that the security marking is genuine). Observing this alternation using a camera allows determining the authenticity of the security marking. A similar alternation process can be used with even more than two states, for example if the security marking includes different regions which reflect different wavelengths and / or polarizations, wherein each of these regions is illuminated with a corresponding retardance corresponding to a wavelength and polarization in alternation to detect whether the security marking is genuine or not.

[0074] The illumination device may include or be used together with a light sensor and / or an analysis unit. The light sensor is for capturing an intensity of light emitted by the illumination device and reflected by the security marking. The light sensor can be a standard camera. The analysis unit may be configured to extract a characteristic as defined above and to perform adequate comparisons with reference wavelengths to determine whether the security marking is authentic or not. The light sensor and analysis unit can be synchronized with the illumination device, in particular with the voltage switches of the voltage to the LC variable phase retarder. In some embodiments, an element converting the elliptically polarized light into linear polarized light is arranged in front of the analysis unit, in order to distinguish between RHCP and LHCP.

[0075] According to a third aspect, an authentication method for authentication of an elliptically polarizing security marking, in particular using the authentication device according to any one of claims 1 to 9, is provided. The authentication method of the third aspect comprising: providing a liquid crystal (LC) variable phase retarder for receiving elliptically polarized light reflected by the security marking and for converting the elliptically polarized light to linear polarized light at a selected wavelength by means of setting an appropriate retardance corresponding to a voltage applied to the LC variable phase retarder; providing a visible light (VIS) linear polarizer downstream of the LC variable phase retarder, the VIS linear polarizer being for transmitting a fraction of the linear polarized light, the fraction being in accordance with an angular difference between an orientation angle of the linear polarized light and an axis of the VIS linear polarizer; extracting a characteristic of the fraction of the linear polarized light as transmitted by the VIS linear polarizer for the selected retardance; and determining whether the security marking is authentic or not depending on the extracted characteristic.

[0076] The embodiments and features described with respect to the authentication device of the first aspect also hold for the authentication method of the third aspect and vice versa.

[0077] According to an embodiment, the authentication method further comprises: varying the voltage applied to the LC variable phase retarder so as to obtain intensity values for both right-handed elliptically (in particular circular) polarized (RHCP) light reflected by the security marking and lefthanded elliptically (in particular circular) polarized (LHCP) light reflected by the security marking.

[0078] This voltage variation is in particular performed using the power supply unit connected to the LC variable phase retarder. For example, the power supply unit repetitively changes the applied voltage to repetitively switch between the RHCP and the LHCP light, in particular at a frequency of less than one second or of less than 3 seconds. Switching between RHCP and LHCP allows comparing the intensities captured for RHCP and LHCP (in this case, the reference intensity may be the intensity captured at the other one of RHCP and LHCP). This is particularly relevant in cases where the ink used in the security marking exhibits different properties for RHCP and LHCP. For example, the used ink can be reflective for RHCP and non-reflective for LHCP (or the opposite).

[0079] According to a further embodiment, the authentication method of the third aspect uses the authentication device including a VIS-IR linear polarizer and an LCD to: switch the voltage applied to the LCD between ON and OFF such as to switch the LCD between its first and second state, wherein when the LCD is in the first state, the VIS linear polarizer transmits visible and infrared light and when the LCD is in the second state, the VIS linear polarizer transmits only infrared light.

[0080] The VIS linear polarizer is active on the VIS light and it can let VIS light to pass through or not, but the NIR light passes through all the time. This VIS linear polarizer seeks to suppress all the VIS light reaching the camera that can be disturbing when focusing on IR or NIR light. This is due to the fact that for typical sensors, the sensitivity is low in the IR and thus sensors may be saturated by existing VIS light. If one wants to image details in IR with lots of ambient VIS light, it is more difficult, and thus, it is preferrable to suppress VIS contribution.

[0081] This voltage variation is in particular performed using the power supply unit connected to the LCD. For example, the power supply unit repetitively changes the applied voltage to repetitively switch between the visible and the infrared light, in particular at a frequency of less than one second or of less than 3 seconds. Switching between visible and infrared light on one hand and only infrared light on the other hand allows comparing the intensities captured for visible and infrared (in this case, the reference intensity may be the intensity captured at the other one of visible and infrared). This is particularly relevant in cases where the ink used in the security marking exhibits different properties for visible and infrared light. For example, the used ink can be reflective for visible and non-reflective for infrared light (or the opposite, or visible and infrared light may reflect different peak intensities or the like).

[0082] According to a further embodiment, the authentication method of the third aspect, further comprises: extracting a characteristic of the fraction of the linear polarized light as transmitted by the VIS linear polarizer with the LCD being in the first state; extracting a characteristic of the fraction of the linear polarized light as transmitted by the VIS linear polarizer with the LCD being in the second state; and determining whether the security marking is authentic or not depending on the characteristics extracted with the LCD being in the first state and with the LCD being in the second state.

[0083] A visible ink reflecting RHCP (or LHCP) light at a specific wavelength (for example green light) can be authenticated by operating the LCD in its first state (VIS an IR light are transmitted). By sweeping voltages (or using at least two voltage values), it is possible to determine the retardances causing a maximum or a minimum in RGB (or HSV or Lab) and thus deduce a main reflected wavelength (or another equivalent parameter in HSV or Lab space). An angle of observation may induce a shift into this wavelength due to Bragg-Snell’s law.

[0084] Now, if the ink (or eventually another ink on the same security marking) reflects RHCP (or LHCP) light in an IR range, then the LCD can be operated in its second state, so that all VIS reflected light are suppressed and so that it is focused on IR light. Once the LCD is in the second state, we can repeat the authentication technique: sweep voltages, determine retardances causing a maximum and a minimum in RGB values and compare the obtained main wavelength with an expected one.

[0085] According to a fourth aspect, an authentication method for authentication of an elliptically circular polarizing security marking, in particular using the authentication device according to the second aspect, is provided. The authentication method of the fourth aspect comprises: illuminating an elliptically polarizing security marking, the step of illuminating comprising: emitting light towards the elliptically polarizing security marking using a light source; providing an illumination linear polarizer with is a visible (VIS) linear polarizer or a visible and infrared (VIS-IR) linear polarizer and is provided downstream of the light source for receiving the light emitted by the light source and for transmitting a fraction of the received light as linear polarized light; and providing a liquid crystal (LC) variable phase retarder provided downstream of the VIS linear polarizer or VIS-IR linear polarizer, the LC variable phase retarder being configured to receive the linear polarized light from the VIS linear polarizer or from the VIS-IR linear polarizer and to convert it into specific elliptically polarized light at a selected wavelength by means of setting an appropriate retardance corresponding to a voltage applied to the LC variable phase retarder; extracting a characteristic of the light reflected by the elliptically polarizing security marking for the selected retardance; and determining whether the security marking is authentic or not depending on the extracted characteristic. The present invention will be described more fully hereinafter with reference to the accompanying figures in which like numerals represent like element throughout the different figures, and in which prominent aspects and features of the invention are illustrated.

[0086] BRIEF DESCRIPTION OF THE FIGURES

[0087] Fig. 1 shows an authentication device according to a first embodiment;

[0088] Fig. 2 shows a captured intensity as a function of retardance at an observation angle of 90°;

[0089] Fig. 3 shows an authentication device according to a second embodiment;

[0090] Fig. 4 shows an authentication device according to a third embodiment;

[0091] Fig. 5 shows the concept of Bragg-Snell diffraction;

[0092] Fig. 6 shows a captured intensity as a function of retardance at an observation angle of 45°;

[0093] Fig. 7 shows an example of an illumination device;

[0094] Fig. 8 shows an authentication device according to a fourth embodiment;

[0095] Fig. 9 shows an authentication device according to a fifth embodiment;

[0096] Fig. 10 shows an example of a system including two authentication devices; and

[0097] Fig. 11 shows an example of a system including one authentication device and a mirror.

[0098] DETAILED DESCRIPTION

[0099] The detailed description provided in the following focusses on the authentication of vehicle license plates with a circular polarizing security marking. However, the same devices and methods as the ones described below in view of the license plates can also be used to authenticate any type of product, especially from a distance (of at least several meters). Such products for example include casino tokens, tax stamps, jewelry, merchandise, injection molding products or the like. Further, the examples described below can be generalized to any elliptically polarizing security marking.

[0100] The approach described in the following uses a camera-based system that includes active and passive optical components to exploit the specific polarization properties of cholesteric liquid crystal polymer (CLCP) inks used in security markings. The inks can use some markers which can reflect left or right-handed circular polarization in different wavelengths. Human eyes cannot differentiate polarized light, and therefore, appropriate optical components are required to sense the difference between random polarization (natural light, sun for example) and circular polarization of light. The optical components of the setups described in the following allow to select different polarization states of the light reflected off the license plate and by comparing the reflected intensities sensed by the camera at those different polarization states, the inks present on the license plate can be authenticated, thereby allowing for an authentication of the license plate.

[0101] To authenticate a license plate, an authentication device 100 can be used. A first embodiment of such an authentication device 100 is shown in Fig. 1. The authentication device 100 shows an alignment of optical components forming the authentication device 100. Namely, the authentication device 100 aligns, in this order along a light path 107 along which light reflected by the security marking of the license plate travels towards a light sensor 105, an LC variable phase retarder 101 and a VIS linear polarizer 102. Not part of the authentication device 100 but provided after the VIS linear polarizer along the light path 107, a light sensor 105 is provided. Although not shown in Fig. 1 , the optical elements 101 , 102 of the authentication device 100 are held in place by appropriate holders, for example by a frame stacking the different components one behind another.

[0102] In the example of Fig. 1 , the light reflected by the security marking is sunlight or light from a standard lamp such as a streetlamp. The reflected light is circularly polarized in accordance with the properties of the inks used in the security marking. This reflected circular polarized light first passes through the LC variable phase retarder

[0103] 101 . The LC variable phase retarder 101 comprises two electrical terminals 108 to which a power supply unit 109 is connected. The power supply unit 109 can be part of the authentication device 100 or of an external entity. The power supply unit 109 variably provides a voltage between 0 and 10V. A pattern according to which the power supply unit 109 switches voltages is preprogrammed in the power supply unit 109. The power supply unit 109 is here continuously and repeatedly swiping from 0V to 10V within a few seconds. This high-speed swipe is appropriate for vision system, for example.

[0104] In accordance with the applied voltage, the LC variable phase retarder 101 outputs a linear polarized light at a specific retardance. The voltage swipes allow to scan over a predefined retardance range, i.e. to output, by the LC variable phase retarder 101 , all retardances of the range in a same scan.

[0105] After exiting the LC variable phase retarder 101 , the linear polarized light passes through the VIS linear polarizer

[0106] 102. According to the angle difference between the linear polarized light and a main axis of the VIS linear polarizer 102 (which is aligned with the linear polarizer axis 115 provided on the VIS linear polarizer 102 in Fig. 1 ), the quantity of light passing through the VIS linear polarizer 102 changes. When the axes are perfectly parallel, 100% of light passes through the VIS linear polarizer 102. When the axes are orthogonal, 0% of light passes through the VIS linear polarizer 102. At intermediate angular differences, a corresponding intermediate percentage (fraction) passes through the VIS linear polarizer 102. Finally, the fraction of the linear polarized light exiting the VIS linear polarizer 102 comes through an objective 103 and onto a camera sensor 104, which jointly form a light sensor 105. Here, the light sensor 105 is not part of the authentication device 100 and forms a standard RGB camera. However, in alternative embodiments, the light sensor 105 may be part of the authentication device 100.

[0107] The light sensor 105 captures an intensity of the light reaching it. The light sensor 105 transmits the captured intensity value to an analysis unit 106 of the authentication device 100 which is connected to the light sensor 105. Since the LC variable phase retarder 101 scans over all retardances, the intensity sensed by the light sensor 105 keeps evolving and accordingly, the analysis unit 106 receives multiple subsequent intensity values, which form characteristics extracted by the analysis unit 106. These intensity values are used to plot a curve showing an evolution of the intensity of the sensed light as a function of the retardance selected by the LC variable phase retarder 101.

[0108] An example of such a curve is shown in Fig. 2. As shown therein, the light intensities I are plotted as a function of the linear retardance LR for each of the red R, green G and blue B components as sensed by the RGB- sensitive camera 104 of the light sensor 105. Fig. 2 shows a curve labelled “A” corresponding to an average intensity for each retardance LR. A peak average intensity of curve “A” is labelled as Imax and is obtained for a maximum intensity retardance LRmax. For a non-authentic security marking reflecting unpolarized light, the intensity would remain constant even upon changing the voltage (and hence the retardance and wavelength). In order to determine whether the security marking is authentic, the analysis unit 106 analyses the obtained curve. Namely, the analysis unit 106 compares the measured curve as shown in Fig. 2 with a reference intensity curve (which shows reference intensities as a function of the retardance) which is prestored and characteristic of a genuine security marking. In detail, the analysis unit 106 compares the sensed and the reference intensities for each color component (red R, green G and blue B, or other colorimetric conversions such as HSV or Lab) as well as for the average A and determines if they are all within a predefined range (for example 5%) from one another. If this is the case, the authenticity of the security marking is confirmed. The analysis unit 106 optionally also considers the peak (maximum) intensity values of the curves R, G, B and A (or other colorimetric conversions such as HSV or Lab) and / or a width of the peak intensity and compares it with reference values in order to determine the authenticity of the security marking with more accuracy.

[0109] In addition or alternatively, the analysis unit 106 may extract characteristic wavelengths (corresponding to the characteristics) from the intensity versus retardance curve shown in Fig. 2, to therefrom deduce the authenticity of the security marking. Namely, for a genuine security marking, one expects to obtain maximum intensities Imax every (3A / 4)+n*A for RHCP, wherein A is the maximum intensity wavelength and n is an integer. From this formula, the analysis unit 106 calculates the maximum intensity wavelength and compares it with a reference wavelength at which a maximum intensity is expected. Similarly, for a genuine security marking, one expects to obtain minimum intensities Imin every (A / 4)+n*A for RHCP. From this formula, the analysis unit 106 calculates the minimum intensity wavelength and compares it with a reference wavelength at which a minimum intensity is expected. Similar calculations can be performed for LHCP, for which the maximum intensity Imax is expected every (A / 4)+n*A and the minimum intensity Imin is expected every (3A / 4)+n*A, and said results for LHCP are also compared with adequate reference wavelengths. The analysis unit 106 determines the authenticity of the security marking based on all the results of the performed comparisons.

[0110] The analysis unit 106 may output the result of whether the security marking is authentic or not by visually indicating if yes or no, the security marking is authentic. The analysis unit 106 may specify a likelihood (percentage) of this result being correct. The output may include a visual or audio output of the result and / or may include storing the result in a database.

[0111] An advantage of the solution shown in Fig. 1 is to be able to change the retardance quickly in the VIS range. This allows to perform a scanning and bring out a spectrum of the ink / marker (Fig. 2). This spectrum is specific only for circular polarized light and allows determining whether the security marking and hence the license plate is authentic or not.

[0112] Fig. 3 shows an authentication device 110 according to a second embodiment. The authentication device 110 of Fig. 3 is mostly similar to the authentication device 100 of the first embodiment as shown in Fig. 1 , with the exception that it additionally includes a VIS-IR linear polarizer 111 and an LCD 112. The remaining components of the authentication device 110 are identical to those of the authentication device 100 and are hence not described again, as the above description still holds (in Fig. 3, the VIS linear polarizer 102 has a square shape while it has a circular shape in Fig. 1 , however this does not make a difference for the described invention) . The VIS-IR linear polarizer 111 and the LCD 112 are located along the light path 107 of the light reflected by the security marking, and are located in this order between the LC variable phase retarder 101 and the VIS linear polarizer 102. Light exiting the LC variable phase retarder 101 thus passes though the VIS-IR linear polarizer 111 and then through the LCD 112 before reaching the VIS linear polarizer 111.

[0113] An order of the optical components 101 , 102, 111 , 112, 105 is very important and is as shown in Fig. 3. The angular orientation between the LC variable phase retarder 101 and the VIS-IR linear polarizer 111 is at an angle of 45° between their respective main axes. An angular orientation between the VIS-IR linear polarizer 111 and the VIS linear polarizer 102 is of 90°, as indicated by a difference in angle between the linear polarizer axis 116, 115 (respectively of the VIS-IR linear polarizer 111 and the VIS linear polarizer 102 shown in Fig. 3). An orientation of the LCD 112 does not matter.

[0114] The light reflected from the license plate first comes through the LC variable phase retarder 101 which transforms the circular polarized light to linear polarized light at the specific retardance (as described above in view of Fig. 1 ). The voltage applied to the LC variable phase retarder 101 (0-10V) allows to choose the retardance according to the need of the application. This change can perform at high speed appropriate for vision system for example.

[0115] After the LC variable phase retarder 101 , the linear polarized light passes through the VIS-IR linear polarizer 111. According to the angle difference between the polarized lightand the main axis of the VIS-IR linear polarizer 111 , the quantity of light let through the VIS-IR linear polarizer 111 changes. When the axes are perfectly parallel, 100% of light pass through. When the axes are orthogonal, 0% of light pass through. At intermediate angular differences between the axes, a corresponding intermediate percentage (fraction) passes through the VIS-IR linear polarizer 111. The fraction of the linear polarized light exiting the VIS-IR linear polarizer 111 then passes through the LCD 112.

[0116] The LCD 112 is powered to be in one of two states using a power supply unit 114 connected to electrical terminals 113 of the LCD 112. The power supply unit 114 can either provide a voltage of 0V or a voltage of 3- 3.5V. When the LCD 112 is not powered (0V, the unpowered state being a second state), it behaves as halfwave (A / 2) retarder. The polarization state of the light passing the unpowered LCD 112 changes and shifts by 90°. This brings the polarized light in the same orientation as the VIS linear polarizer 102. In this case, 100% of polarized light reaches the light sensor 105. In the embodiment of Fig. 3, the camera 104 is sensitive to IR light.

[0117] When the LCD is powered (3.3-5 V, the powered state being a first state), it behaves neutrally. The result is that the linear polarized light is orthogonal to the VIS linear polarizer 102. In this case, all visible light is stopped but not IR light because the VIS linear polarizer 102 is transparent in IR light. This feature allows us to create an active IR filter.

[0118] With this implementation, one can operate in the visible range (LCD 112 unpowered and proper settings in LC variable phase retarder 101 to produce required visible retardations, namely calibration settings yielding a maximum intensity at a retardance of (3A / 4)+n*A for RHCP, and a maximum intensity at a retardance of (A / 4)+n*A for LHCP, A being the wavelength in the visible range of 400-700nm and n being any integer) and capture two images of the license plate. One of these images is at left-handed circular polarization LHCP and the other is at right-handed circular polarization RHCP. These images are compared by the analysis unit 106 to authenticate the license plate. The authentication device 110 further operates in the NIR (LCD 112 powered, with appropriate settings in the LC variable phase retarder 101 to produce required near IR retardations, namely calibration settings yielding a maximum intensity at a retardance of (3A / 4)+n*A for RHCP, and a maximum intensity at a retardance of (A / 4)+n*A for LHCP, A being the wavelength in the IR range of 700-1 OOOnm and n being any integer) to capture one image of the plate in the NIR corresponding to LCHP and another corresponding to RHCP. With those 4 images (VIS_LHCP, VIS_RHCP, NIR_LHCP and NIR_RHCP), the analysis unit can identify the presence of visible and near infrared circular polarizing security inks on the license plate and authenticate or even use their positions for a more robust authentication (if their positions were accurately fingerprinted during production).

[0119] In other words, the light sensor 105 here captures the intensities for LHCP and RHCP for both visible and NIR light. These intensities are plotted as curves, as shown in Fig. 2, to show the impact of retardance on the sensed intensity. The curves are transmitted to the analysis unit 106, which compares them with reference intensity curves, which are either prestored and captured intensity curves for genuine license plates, or which are the curves captured for the other states (i.e. VIS_RHCP, NIR_LHCP and NIR_RHCP). In other words, by switching between the states VIS_RHCP, NIR_LHCP and NIR_RHCP by applying appropriate voltages on the LC variable phase retarder 101 and on the LCD 112, the resulting intensity values or curves can be compared with one another (a previous measured intensity value or curve forming a reference intensity). As a result of this comparison, the analysis unit 106 indicates whether the license plate is authentic or not. In addition or alternatively, a comparison along the lines described in view of Fig. 1 is also possible to account for both visible and IR properties of the security marking.

[0120] Using the authentication device 110 of Fig. 3, it is possible to compare not only LHCP and RHCP light reflected off the license plate in the visible range but also in the near infrared range, therefore allowing for a more robust distant authentication. Further, specificities of the ink such as the presence of RHCP reflecting light and absence of LHCP reflecting light (or the opposite) can be detected.

[0121] Fig. 4 shows an authentication device 120 according to a third embodiment. The authentication device 120 of Fig. 4 is mostly identical to the authentication device 110 of the second embodiment as shown in Fig. 2, with the exception that it additionally includes an observation angle determination unit 121. The remaining components of the authentication device 120 are identical to those of the authentication device 110 and are hence not described again, as the above description still holds.

[0122] The authentication device 120 of the third embodiment allows to optimize the authentication of the license plate even in cases in which the license plate is viewed from an angle. This is illustrated in Fig. 4 by an experimental setup showing a license plate 200 on a rotating support 201 allowing to rotate the license plate 200 around a rotation axis 202. This simulates different angles of observation of a license plate 200 on a vehicle. The described authentication device 120 is particularly useful to reliably authenticate a license plate 200 observed obliquely, which can be the case when authenticating license plates 200 on moving cars. Obliquely here designates an observation direction (light path 107) that is not perpendicular to the surface of the license plate The authentication device 120 includes an observation angle determination unit 121 for determining the observation angle (i.e. the angle at which the plate is imaged) to optimize the authentication. The observation angle influences the intensity curve as shown in Fig. 2 because light reflected by the ink on the license plate 200 undergoes Bragg-Snell diffraction. Namely, the maximum (peak) intensity of the light reflected from the ink is obtained at a maximum intensity retardance which depends on an incidence angle of the light impinging onto the plate 200 as well as on the observation angle.

[0123] The Bragg-Snell diffraction phenomenon is illustrated in Fig. 5. Fig. 5 schematically depicts the reflection of a beam of light B incident at angle 6 on the surface of a license plate 200 . A first part of the beam B1 is reflected at a first surface 300 and interferes with a second part of the beam B2 which is reflected at a second surface / interface 301 , after passing through a dielectric layer 302 of physical thickness d and refraction index n2. The refraction index m of the outer medium is assumed to be 1 .0 (air).

[0124] In the security marking of the license plate 200, several such layers of crystalline structures are stacked. There is a pitch or distance d between layers. According to the pitch d, and the observation angle of the light on the ink 6, the combination of Bragg’s law and Snell’s law of refraction tells us that constructive interference of waves producing the reflected light will follow the following rule: mA = 2d * sqrt((n_2)A2- cosA2 (6)), with m being the diffraction order (m=1 is the first order, m=2 is the second order and so on), and A is the wavelength. In the example of Fig. 5, a lower beam traverses an extra length of 2*n2*d*sin 6’ as compared to an upper beam. By using Snell’s law, one can easily substitute 6’ by 6 in such expression and obtain Bragg-Snell’s equation. When such path difference is an integer multiple of a certain wavelength, we obtain a constructive interference leading to a maximal reflection.

[0125] For non-normal incidences (6 < 90°), the right term of the equation above will reduce and thus the wavelength corresponding to the reflectivity maximum will also decrease (i.e.: will shift to Blue). As an example, for an ink with a pitch d = 187 nm and n_2 = 1.5 at normal incidence (6=90°), a green color (~560 nm, see Fig. 2) is reflected, but when the observation angle and incident angle are changed to 45°, the color shifts to a light blue (since A = 494 nm when using given values of d, n_2 and 6=45° in Bragg-Snell equation), which is what can be seen in Fig. 6. Namely, Fig. 6 shows a captured intensity as a function of retardance at an angle of incidence of 45° for the same license plate 200 as the curve shown in Fig. 2, which corresponds to an angle of observation of 90°.

[0126] Thus, the car plate 200 has a reflection maximum at a retardance (and hence at a corresponding wavelength) that changes according to observation angle and incidence angle 6. If the observation and incident angle are equal and known, the reflectivity maximum of the ink at normal observation angle can be inferred using Bragg- Snell’s law. If observation does not match incident angle, one can still deduce the reflection wavelength peak using Bragg-Snell taking 6 as the observation angle and allow for some margin to include peak deviations induced by incident angles different from 6.

[0127] In order to determine the angle of observation, the observation angle determination unit 121 extracts, from an image of the license plate 200 captured by the light sensor 105, positions of reference points located on the license plate 200. The reference points are positioned on the license plate 200 such as to form a known pattern and when the license plate 200 is viewed from an angle, the pattern is distorted. From this distortion, the observation angle determination unit 121 determines the angle of observation.

[0128] The determined angle of observation can be used in different ways to determine the authenticity of the license plate 200 more reliably. Firstly, for a given angle of observation, the captured intensity value can be compared with a reference intensity value which is specific to the same angle of observation, using the analysis unit 106. Based on this comparison, the authenticity of the license plate 200 is determined. Since the compared intensity values are specific to the same angle of observation, the authenticity determination is more reliable and not influenced by the angle of observation. Alternatively or in addition to the above authenticity determination, using Bragg-Snell’s law on the captured intensity value (or intensity vs. wavelength curve) and the angle of incidence, a wavelength for which a maximum intensity would be observed (maximum intensity wavelength) is calculated and used for comparison with a reference maximum intensity wavelength to determine the authenticity of the license plate 200 in a more reliable and observation angle independent manner.

[0129] The authentication device 120 allows to distinguish between a large variety of inks, in particular between inks with reflectivity maxima in different color ranges. In particular, if the intensity maximum at each angle corresponds to the expected intensity maximum for this ink and to Bragg-Snell’s law, then the ink on the license plate 200 can be authenticated as a genuine one.

[0130] In the embodiments described above, the security marking reflects ambient light, such as sun light, light from streetlamps or the like. The described authentication devices 100, 110, 120 focus on selecting different polarization states of the reflected lights. It is possible to instead or additionally select polarization states of the light used for illuminating the security marking. For this purpose, an illumination device relying on the same technical features as the authentication devices 100, 110, 120 described above can be used.

[0131] Fig. 7 discloses an illumination device 500 according to an example. The illumination device 500 can be used jointly with the authentication device 100, 110, 120 described above to form an authentication device 130 according to a fourth embodiment, as shown in Fig. 8. In this case, the security marking is illuminated by the light emitted by the illumination device 500 and the elements 101 - 106 form a detector device for detecting the intensity of the light reflected by the security marking at a specific retardance and polarization.

[0132] As an alternative, the illumination device 500 can be part of an authentication device 140 according to a fifth embodiment, which is shown in Fig. 9. This authentication device 140 includes the illumination device 500 as well as the analysis unit 106.

[0133] The illumination device 500 includes a light source 505 (for example an LED lamp) emitting light along a light path 507 incident onto the security marking. The light emitted by the light source 505 first passes through an illumination VIS or a VIS-IR linear polarizer 502 and subsequently through an illumination LC variable phase retarder 501 . The LC variable phase retarder 501 has a voltage applied thereto from a power supply unit 509 through electrical terminals 508 of the LC variable phase retarder 501 . The light exiting the LC variable phase retarder 501 is directed at the security marking of the license plate 200, which will reflect it or not, depending on the polarization state of the light. The light emited by the light source 505 is typically unpolarized. The linear polarizer 502 transmits a fraction of the light emited by the light source 505 towards the LC variable phase retarder 501 . By applying a voltage to the LC variable phase retarder, a circular polarized light at a specific selected retardance (in accordance with the applied voltage) is obtained and emited onto the license plate 200. Depending on the polarization state of the light, a different intensity of light will be reflected by the license plate 200. The intensity of this reflected light is analyzed by the analysis unit 106 and compared to a reference intensity allowing to determine the authenticity of the security marking.

[0134] As shown in Fig. 10, it is possible to arrange two or more authentication devices 100, 110, 120, 130 at different positions with respect to the security marking. One of these authentication devices 100, 110, 120, 130 is preferable placed at an observation angle of 90° and another at an observation angle of 30°. That way, it is possible to not only determined whether the light reflected by the security marking is RHCP or LHCP, but also that it presents a maximum (peak) at a certain wavelength when observed at near 90° and another when observed at nearly 30°, and that these values comply with Bragg-Snell’s law.

[0135] As an alternative to using two authentication devices 100, 110, 120, 130, which is shown in Fig.11 , it is also possible to use only one authentication device 100, 110, 120, 130 and to arrange a mirror 310 such as to detect both observation angles of 30° and of 90° with the same authentication device 100, 110, 120, 130.

[0136] The above disclosed subject-mater is to be considered illustrative, and not restrictive, and serves to provide a beter understanding of the invention defined by the independent claims.

[0137] REFERENCE NUMERALS

[0138] 100 authentication device

[0139] 101 LC variable phase retarder

[0140] 102 VIS linear polarizer

[0141] 103 objective

[0142] 104 camera

[0143] 105 light sensor

[0144] 106 analysis unit

[0145] 107 light path

[0146] 108 electrical terminal

[0147] 109 power supply unit

[0148] 110 authentication device

[0149] 111 VIS-IR linear polarizer

[0150] 112 LCD

[0151] 113 electrical terminal

[0152] 114 power supply unit

[0153] 115 linear polarizer axis

[0154] 116 linear polarizer axis

[0155] 120 authentication device

[0156] 121 observation angle determination unit 130 authentication device 140 authentication device

[0157] 200 license plate

[0158] 201 rotating support

[0159] 202 rotation axis 300 first surface

[0160] 301 second surface

[0161] 302 dielectric layer

[0162] 310 mirror

[0163] 500 illumination device 501 illumination LC variable phase retarder

[0164] 502 illumination linear polarizer

[0165] 505 light source

[0166] 507 light path

[0167] 508 electrical terminal 509 power supply unit

Claims

CLAIMS1 . An authentication device (100, 110, 120, 130) for authentication of an elliptically polarizing security marking, the authentication device (100, 110, 120, 130) comprising: a liquid crystal (LC) variable phase retarder (101 ) for receiving elliptically polarized light reflected by the security marking and for converting the elliptically polarized light to linear polarized light at a selected wavelength by means of setting an appropriate retardance corresponding to a voltage applied to the LC variable phase retarder (101 ); a visible light (VIS) linear polarizer (102) provided downstream of the LC variable phase retarder (101 ) for transmitting a fraction of the linear polarized light, the fraction being in accordance with an angular difference between an orientation angle of the linear polarized light and an axis of the VIS linear polarizer (102); and an analysis unit (106) for extracting a characteristic of the fraction of the linear polarized light as transmitted by the VIS linear polarizer (102) for the selected retardance, and for determining whether the security marking is authentic or not depending on the extracted characteristic.

2. The authentication device according to claim 1 , further comprising a power supply unit (109) for applying the voltage to the LC variable phase retarder (101 ), the power supply unit (109) being configured to switch between different voltages applied to the LC variable phase retarder (101 ) and / or to swipe over a range of voltages applied to the LC variable phase retarder (101 ), so that the LC variable phase retarder converts the elliptically polarized light to linear polarized light at selected retardances corresponding to the voltages applied to the LC variable phase retarder (101 ); wherein the analysis unit (106) is configured to extract the characteristics of the fraction of the linear polarized light as transmitted by the VIS linear for each of the selected retardances, and to determine whether the elliptically polarizing security marking is authentic or not depending on the extracted characteristics.

3. The authentication device according to claim 1 or 2, further comprising: a visible and infrared (VIS-IR) linear polarizer (111 ) provided between the LC variable phase retarder (101 ) and the VIS linear polarizer (102), the VIS-IR linear polarizer (111 ) receiving the linear polarized light from the LC variable phase retarder (101 ) and transmitting a fraction of the linear polarized light, the fraction being in accordance with an angular difference between the orientation angle of the linear polarized light received from the LC variable phase retarder (101 ) and an axis of the VIS-IR linear polarizer (111 ); and a liquid crystal display, LCD (112) provided between the VIS-IR linear polarizer (111 ) and the VIS linear polarizer (102), the LCD (112) having two states selected in accordance of whether or not a voltage is applied to the LCD (112), wherein in a first state, in which a voltage is applied to the LCD (112), the LCD (112) is configured to act neutrally and let the linear polarized light from the VIS-IR linear polarizer (111 ) through without changes, and wherein in a second state, in which no voltage is applied to the LCD (112), the LCD (112) is configured to act as a half wave retarder to shift by half a wave the linear polarized light from the VIS-IR linear polarizer (111 ), thereby providing a half wave shifted linear polarized light; wherein the linear polarized light leaving the LCD (112) in its unchanged state or in its half wave shifted state forms the linear polarized light entering the VIS linear polarizer (102).

4. The authentication device according to any one of claims 1 to 3, wherein the analysis unit (106) is configured to:(i) extract, as the characteristic, an intensity of the fraction of the linear polarized light as transmitted by the VIS linear polarizer (102) for the selected retardance and compare it with a reference intensity for the selected retardance, the analysis unit (106) being configured to determine whether the security marking is authentic or not depending on the result of the comparison between the intensity of the fraction of the linear polarized light for the selected retardance with the reference intensity for the selected retardance; and / or(ii) extract, as the characteristic, a first intensity and a second intensity of the fraction of the linear polarized light as transmitted by the VIS linear polarizer (102) for respectively a first retardance and a second retardance as the selected retardances, the analysis unit (106) being configured to determine whether the security marking is authentic or not depending on the result of the comparison between the first intensity and the second intensity;(iii) extract, as the characteristic, a width of an intensity curve around the maximum and / or minimum intensity, the analysis unit (106) being configured to determine whether the security marking is authentic or not by comparing the width of an intensity curve around the maximum and / or minimum intensity with a reference width; and / or(iv) extract, as the characteristic, a parameter of any colorimetric space of the fraction of the linear polarized light as transmitted by the VIS linear polarizer (102) and to compare the extracted parameter of the colorimetric space with a reference parameter of the colorimetric space, the analysis unit (106) being configured to determine whether the security marking is authentic or not depending on the result of the comparison of the parameter of the colorimetric space with the reference parameter of the colorimetric space.

5. The authentication device of any one of claims 1 to 4, wherein the analysis unit (106) is configured to: extract, as the characteristic, a maximum intensity retardance which is the selected retardance for which a maximum intensity of the fraction of the linear polarized light is observed, and / or a minimum intensity retardance which is the selected retardance for which a minimum intensity of the fraction of the linear polarized light is observed.

6. The authentication device of claim 5, wherein the analysis unit (106) is configured to: fora security marking which is expected to reflect right-handed elliptically polarized (RHCP) light, calculating a right-handed maximum intensity wavelength being a wavelength corresponding to the determined maximum intensity retardance using the formula 3A / 4, A being the wavelength, and / or calculating a right-handed minimum intensity wavelength being a wavelength corresponding to the determined minimum intensity retardance using the formula A / 4; and / or for a security marking which is expected to reflect left-handed elliptically polarized (LHCP) light, calculating a left-handed maximum intensity wavelength being a wavelength corresponding to the determined maximum intensity retardance using the formula A / 4, and / or calculating a right-handed minimum intensity wavelength being a wavelength corresponding to the determined minimum intensity retardance using the formula 3A / 4; wherein the analysis unit (106) is further configured to determine whether the security marking is authentic or not based on a result of a comparison of the right-handed maximum intensity wavelength with a corresponding reference right-handed maximum intensity wavelength, the right-handed minimum intensity wavelength with a corresponding reference right-handed minimum intensity wavelength, the left-handed maximum intensity wavelength with a corresponding reference left-handed maximum intensity wavelength, and / or the left-handed minimum intensity wavelength with a corresponding reference left-handed minimum intensity wavelength.

7. The authentication device of any one of claims 1 to 6, further comprising:an observation angle determination unit (121 ) for determining an angle of observation of the elliptically polarized light reflected by the security marking on the LC variable phase retarder (101 ); wherein the analysis unit (106) is configured to determine whether the security marking is authentic or not additionally under consideration of the angle of observation determined by the observation angle determination unit (121 ) by either (i) comparing the extracted characteristic with a corresponding reference characteristic which is specific to the selected retardance and to the angle of observation determined by the observation angle determination unit (121 ), and / or by (ii) using Bragg-Snell's law to deduce, from a maximum intensity retardance for which a maximum intensity of the fraction of the linear polarized light is sensed and from the angle of observation determined by the observation angle determination unit (121 ), a maximum intensity retardance at a predetermined observation angle, and by comparing the deduced maximum intensity retardance at the predetermined observation angle with a reference maximum intensity retardance at the predetermined observation angle.

8. The authentication device of any one of claims 1 to 7, wherein an angle between a main axis of the LC variable phase retarder (101 ) and a main axis of the VIS linear polarizer (102) is 45°, an angle between the main axis of the LC variable phase retarder (101 ) and a main axis of the VIS-IR linear polarizer (111 ) is 45°, and / or an angle between the main axis of the VIS-IR linear polarizer (111 ) and the main axis of the VIS linear polarizer (102) is 90°.

9. The authentication device of any one of claims 1 to 8, further comprising a light sensor (105) provided downstream of the VIS linear polarizer (102) for sensing the intensity of the fraction of light transmitted by the VIS linear polarizer (102).

10. The authentication device of any one of claims 1 to 9, further including an illumination device (500) for illuminating the elliptically polarizing security marking, the illumination device (500) comprising: a light source (505) for emitting light towards the elliptically polarizing security marking; an illumination linear polarizer (502) which is a visible (VIS) linear polarizer or a visible and infrared (VIS-IR) linear polarizer and is provided downstream of the light source (505) for receiving the light emitted by the light source (505) and for transmitting a fraction of the received light as linear polarized light; and an illumination liquid crystal (LC) variable phase retarder (501 ) provided downstream of the illumination linear polarizer (502), the illumination LC variable phase retarder (501 ) being configured to receive the linear polarized light from the illumination linear polarizer (502) and to convert it into specific elliptically polarized light at a selected wavelength by means of setting an appropriate retardance corresponding to a voltage applied to the illumination LC variable phase retarder (501 ); wherein the elliptically polarized light reflected by the security marking and reaching the LC variable phase retarder (101 ) includes light emitted by the illumination device (500) onto the security marking.

11. An authentication device (140) for authentication of an elliptically polarizing security marking, the authentication device (140) comprising: an illumination device (500) for illuminating an elliptically polarizing security marking, the illumination device (500) comprising: a light source (505) for emitting light towards the elliptically polarizing security marking;an illumination linear polarizer (502) which is a visible (VIS) linear polarizer or a visible and infrared (VIS-IR) linear polarizer and is provided downstream of the light source (505) for receiving the light emitted by the light source (505) and for transmitting a fraction of the received light as linear polarized light; and an illumination liquid crystal (LC) variable phase retarder (501 ) provided downstream of the illumination linear polarizer (502), the illumination LC variable phase retarder (501 ) being configured to receive the linear polarized light from the illumination linear polarizer (502) and to convert it into specific e 11 i ptica I ly polarized light at a selected wavelength by means of setting an appropriate retardance corresponding to a voltage applied to the illumination LC variable phase retarder (501 ), the specific elliptica lly polarized light illuminating the elliptically polarizing security marking; and an analysis unit (106) for extracting a characteristic of the light reflected by the elliptically polarizing security marking, and for determining whether the security marking is authentic or not depending on the extracted characteristic.

12. An authentication method for authentication of an elliptically polarizing security marking, in particular using the authentication device (100, 110, 120, 130) according to any one of claims 1 to 10, the authentication method comprising: providing a liquid crystal (LC) variable phase retarder (101 ) for receiving elliptically polarized light reflected by the security marking and for converting the elliptically polarized light to linear polarized light at a selected wavelength by means of setting an appropriate retardance corresponding to a voltage applied to the LC variable phase retarder (101 ); providing a visible light (VIS) linear polarizer (102) downstream of the LC variable phase retarder (101 ), the VIS linear polarizer (102) being for transmitting a fraction of the linear polarized light, the fraction being in accordance with an angular difference between an orientation angle of the linear polarized light and an axis of the VIS linear polarizer (102); extracting a characteristic of the fraction of the linear polarized light as transmitted by the VIS linear polarizer (102); and determining whether the security marking is authentic or not depending on the extracted characteristic.

13. The authentication method of any one of claims 11 or 12 using the authentication device (110, 120, 130) of claim 3, the method further comprising: switching the voltage applied to the LCD (112) between ON and OFF such as to switch the LCD (112) between its first and second state, wherein when the LCD (112) is in the first state, the VIS linear polarizer (102) transmits visible light and infrared light, and when the LCD (112) is in the second state, the VIS linear polarizer (102) transmits only infrared light.

14. The authentication method of claim 13, further comprising: extracting a characteristic of the fraction of the linear polarized light as transmitted by the VIS linear polarizer (102) with the LCD (112) being in the first state; extracting a characteristic of the fraction of the linear polarized light as transmitted by the VIS linear polarizer (102) with the LCD (112) being in the second state; and determining whether the security marking is authentic or not depending on the characteristics extracted with the LCD (112) being in the first state and with the LCD (112) being in the second state.

15. An authentication method for authentication of an elliptically polarizing security marking, the authentication method comprising: illuminating an elliptically polarizing security marking, the step of illuminating comprising: emitting light towards the elliptically polarizing security marking using a light source (505); providing an illumination linear polarizer (502) with is a visible (VIS) linear polarizer or a visible and infrared (VIS-IR) linear polarizer and is provided downstream of the light source (505) for receiving the light emitted by the light source (505) and for transmitting a fraction of the received light as linear polarized light; and providing an illumination liquid crystal (LC) variable phase retarder (501 ) provided downstream of the illumination linear polarizer (502), the illumination LC variable phase retarder (501 ) being configured to receive the linear polarized light from the illumination linear polarizer (502) and to convert it into specific elliptically polarized light at a selected wavelength by means of setting an appropriate retardance corresponding to a voltage applied to the illumination LC variable phase retarder (501 ); extracting a characteristic of the light reflected by the elliptically polarizing security; and determining whether the security marking is authentic or not depending on the extracted characteristic.

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