Device and method for detecting reflected and / or emitted light from an object

Abstract

Device for detecting reflected and / or emitted light from an object (1), comprising the device: at least one lighting device (2) that illuminates the object (1) with pulsed light, at least one sensor (4, 6) that detects the light reflected and / or emitted by the object (1), a transport device that transports the object (1) relative to the lighting device (2) and past the sensor (4, 6) in the direction of transport, a power supply (16, 17, 18, 19, 20, 21, 22) of the lighting device (2), which supplies the lighting device (2) with a current that is a periodic function over time, wherein a period has at least two current pulses (23, 24) of different magnitudes, of which a first current pulse has a greater magnitude than a second current pulse; the device is further arranged such that • if saturation of at least one sensor (4, 6) is reached during the first current pulse, only the reflected and / or emitted light with respect to the second current pulse is evaluated from the at least two current pulses; and / or • if the detection limit of the at least one sensor (4, 6) is undercut during the second current pulse, only the reflected and / or emitted light with respect to the first current pulse is evaluated from the at least two current pulses.

Description

[0001] The invention relates to a device and a method for detecting reflected and / or emitted light from an object, in particular a flat object.

[0002] Such devices are used to inspect objects. This includes, for example, detecting, checking, verifying, and testing the authenticity of objects and identifying counterfeits. These objects include, in particular, valuables or documents such as banknotes, checks, shares, papers with security imprints, certificates, admission or travel tickets, vouchers, but also credit or debit cards, identification, or access cards. Devices for detecting reflected and / or emitted light from an object are often part of a multi-component system for processing flat objects. Devices for detecting reflected and / or emitted light serve to distinguish counterfeit from genuine objects.To distinguish counterfeit items from genuine ones, the items—especially banknotes, security documents, ID cards, or valuables—are printed with suitable security inks. These inks convey a specific color impression to the viewer in the visible spectral range. Furthermore, when exposed to light in the invisible spectral range, such as the UV or IR range, they exhibit characteristic reflection, fluorescence, or phosphorescence behavior. Since commercially available and conventional printing inks do not exhibit this behavior, counterfeits can be distinguished from genuine items by examining the reflection, fluorescence, and phosphorescence of light by objects.

[0003] When testing the reflection, fluorescence, and phosphorescence of an object, it is problematic that the intensity of the light reflected or emitted by the object is either so high that the detector used for detection saturates, or so weak that the detector cannot detect the effect. Since the objects to be tested exhibit differences in their optical properties, particularly with regard to their reflection, fluorescence, and phosphorescence behavior, the intensity of the reflected or emitted light cannot be limited to a predetermined value or a narrow range, and the detector or sensor cannot be adjusted to this range.

[0004] US 2007 / 0057198 A1 describes a method for measuring fluorescence lifetime, which requires repeatedly switching the intensity of the excitation light. A microscopic imaging system is provided, which has a scanning optics for scanning a stationary object to be irradiated.

[0005] EP 1 220 165 A2 discloses a UV / fluorescence detection system by means of which a paper sheet quality can be determined by projecting ultraviolet radiation onto a paper and detecting fluorescence or ultraviolet radiation reflected by the paper.

[0006] US 2007 / 0031043 A1 describes a microscope system with an infeed table and an outfeed table.

[0007] In contrast, the device with the features of claim 1 has the advantage that it is equipped with a power supply for a lighting device, which supplies the lighting device with a temporally periodic current, wherein one period of the temporal profile has at least two current pulses of different magnitudes. The different current pulses of the power supply lead to different intensities of the light pulses of the lighting device. Each area of ​​the object is thus irradiated with a strong and a weak light pulse. The frequency of the pulsed light and the temporal resolution of a sensor that detects the light reflected by the object and / or emitted by fluorescence and phosphorescence is so high compared to the speed of the transport device that the movement of the object between two light pulses is negligible.It can therefore be approximately assumed that the object is at rest between illumination with a strong and a weak light pulse.

[0008] The sensor detects the light reflected and / or emitted by the object in relation to both the strong and the weak light pulse. If the sensor reaches saturation with the strong light pulse, only the reflected and / or emitted light with respect to the weak light pulse is evaluated. However, if the reflected and / or emitted light is too weak to be detected by the sensor due to the weak light pulse, only the reflected and / or emitted light with respect to the strong light pulse is evaluated. This expands the dynamic range of the optical measurement system. This enables quantified detection without prior knowledge of the strength of the optical properties to be detected.

[0009] If the resolution is too low due to two current pulses of different strengths, the number of different current pulses per period of the temporally repetitive current can be increased. The current strengths of the current pulses and the duration of the current pulses relative to the duration of the zero current, known as the duty cycle, can be specified depending on the objects being examined. This also applies to the period duration or frequency of the temporally repetitive current.

[0010] In this case, temporally periodic current means that the current is a periodic function over time and thus has a periodicity over time.

[0011] The method according to the invention with the features of claim 10 is characterized in that the illumination device irradiates the object with pulsed light, wherein at least two light pulses of different intensities are generated within one period of the pulsed light. This occurs by supplying the illumination device with a pulsed current via a power supply, wherein each period comprises at least two current pulses of different current strengths. The method can be carried out using the device according to claim 1.

[0012] According to an advantageous embodiment of the invention, the illumination device comprises at least one light-emitting diode (LED). Although excitation lamps, such as fluorescent lamps and gas discharge lamps, can also be used instead, LED light-emitting diodes are characterized by a compact design, lower manufacturing costs, faster response times and thus a higher frequency of the light pulses, as well as a lower susceptibility to failure and repair. Illumination with monochromatic light or at least light within a narrow spectral range is advantageous in any case. This makes it easier to distinguish between the fluorescence and phosphorescence of genuine objects and counterfeit objects.

[0013] According to a further advantageous embodiment of the invention, the light-emitting diode (LED) is a UV light-emitting diode (UV-LED). UV light has the advantage that fluorescence and phosphorescence occur in the visible spectral range or close to the visible spectral range and can therefore be easily detected with optical sensors.

[0014] According to a further advantageous embodiment of the invention, the device is equipped with at least one first sensor for detecting the light reflected by the object and with at least one second sensor for detecting the light emitted by the object through fluorescence and / or phosphorescence. The first and second sensors are located at different positions. Preferably, the illumination device, in particular the light-emitting diode (LED), is arranged with its optical axis at an angle different from 0° to the transport direction of the transport device.The first sensor for detecting the light reflected from the object is arranged with its optical axis at the same angle to the object's surface as the illumination device, but symmetrically to a plane that runs perpendicular to the object's surface and through the intersection point between the optical axis of the illumination device and the object's surface. This takes advantage of the fact that the angle of incidence and angle of reflection of the light are identical during reflection. The second sensor can be located at any position, for example, vertically above the object's surface. This means that its optical axis is aligned perpendicular to the object's surface. Since the wavelength of the reflected light is different from that of the emitted light, different sensors are used.The wavelength of the reflected light matches the wavelength of the light from the illuminating device. The wavelength of the emitted light is shorter than that of the light from the illuminating device.

[0015] According to a further advantageous embodiment of the invention, the second sensor is an RGB sensor. RGB stands for red, green, and blue. This sensor is based on the trichromatic theory, in which the entire color space is constructed from the superposition of the colors red, green, and blue. A separate sensor element is used for each of the three primary colors.

[0016] According to a further advantageous embodiment of the invention, an optical shield is arranged between the illumination device and the second sensor. This prevents the light from the illumination device from impairing the second sensor. Additionally, a filter can be arranged on the illumination device to filter out the typical wavelengths of fluorescence and phosphorescence from the light from the illumination device.

[0017] According to a further advantageous embodiment of the invention, the power supply of the lighting device is equipped with at least two input resistors connected in parallel and a differential amplifier. Furthermore, the power supply has a voltage source that supplies at least two pulsed input voltages. The number of pulsed input voltages corresponds to the number of current pulses per power supply period. With two input voltages, the frequency of one input voltage is twice the frequency of the other input voltage. With a number n of input voltages, the highest frequency is n times the lowest frequency. The maximum of the input voltages can be the same or different. The phase shift between the input voltages is 0.This particularly simple wiring with cost-effective components reliably generates a periodic current with at least two different current pulses per period.

[0018] The sensors convert the light reflected or emitted by the object into an electrical signal proportional to the light intensity. These can be photodiodes or CCDs, for example. Several such components can be arranged in a row or in an array. The sensor is also equipped with an optical system, in particular a lens system. In addition, the sensor can have a filter to block out those wavelengths of light that are to be detected by the other sensor. For example, the second sensor for detecting light based on fluorescence and phosphorescence is equipped with a filter that absorbs light in the wavelength range of the illumination device.

[0019] Further advantages and advantageous embodiments of the invention can be found in the following description, the drawings and the claims. drawing

[0020] The drawing shows an embodiment of the device according to the invention. It shows: Fig. 1 basic structure of the device, Fig. 2 Device according to Fig. 1 with additional optical shielding and a filter, Fig. 3 Longitudinal section through a device with the basic structure according to Fig. 1, Fig. 4 Detail from Fig. 3, Fig. 5 Circuit diagram for the device according to Fig. 1 to 4, Fig. 6 Time course of the input voltages to the circuit diagram according to Fig. 5, Fig. 7 Time course of the two input voltages according to Fig. 6 resulting current at the UV LED. Description of the embodiment

[0021] Fig. 1 and Fig. 2 show the basic structure of a device for detecting reflected and emitted light from an object 1. The object is a banknote. The object 1 is irradiated with light generated by an illumination device 2. The illumination device 2 is a UV LED. The optical axis of the illumination device 2 is Fig. 1 by an arrow 3. The light reflected from the surface of the object 1 is detected by a first sensor 4. The optical axis of the first sensor 4 is marked by the arrow 5. Furthermore, the object 1 irradiated with the light from the illumination device 2 emits light due to fluorescence and phosphorescence, the wavelength of which differs from the incident light of the illumination device. To detect this emitted light, a second sensor 6 is arranged above the object 1. The optical axis 7 of this second sensor 6 runs perpendicular to the surface of the object 1. The reflected light detected by the first sensor 4 is in Fig. 1 symbolized by an arrow 8. The light emitted by fluorescence or phosphorescence is in Fig. 1 symbolized by arrow 9.

[0022] Fig. 2 shows the same schematic structure as Fig. 1. In addition, Fig. 2 shows an optical shield 10 between the illumination device 2 and the second sensor 6, as well as a filter 11 in front of the illumination device 2. The filter is a UV transmission filter that filters out the visible components of the light from the illumination device, particularly the blue components. The second sensor 6 is an RGB sensor. The optical shield 10, in the form of a partition, ensures that the UV radiation directly reflected by the object 1 does not reach the second sensor.

[0023] Fig. 3 shows a complete device which is constructed according to the principle of Fig. 1 and Fig. 2. The device consists of two illumination devices 2, two first sensors (not visible in the drawing), and two second sensors 6. Both illumination devices 2 are equipped with UV LEDs and a filter 11. They are each located in a housing 12, which also serves as an optical shield against the two second sensors 6. The illumination devices 2 and the second sensors 6 are arranged on a circuit board 13, which is equipped with further electrical components. The circuit board and the components arranged thereon are surrounded by a housing 14. In order to allow the light from the illumination devices 2 and the light emitted by an object to pass through to the second sensors 6, the housing 14 is equipped with a protective glass 15 that is permeable to this light. Fig. 4 is the detail of the device according to Fig. 4 with the two lighting devices 2 and the second sensors 6 shown enlarged.

[0024] Fig. Figure 5 shows a circuit diagram of the power supply of the lighting device to the device according to Fig. 1 to 4. Input resistors 18 and 19 of a differential amplifier 20 are provided at inputs 16 and 17 of the circuit. The two input resistors are connected in parallel. The input voltages U1 and U2 are generated by a digital component (not shown), for example, a microcontroller, an FPGA, or a CPLD. The differential amplifier determines the base current of a transistor 21, which is connected to the UV LED of the illumination device 2. The current through the diode is limited by a resistor 22.

[0025] In Fig. Figure 6 shows a schematic of the time course of the two input voltages U1 and U2. The frequency of the input voltage U1 is twice that of the input voltage U2. The phase shift is 0. Fig. 7 shows the circuit according to Fig. 5 temporal course of the current I resulting from these input voltages LED of the LED of the lighting device. Two current pulses 23 and 24 are generated within a period T. Current pulse 23 has a higher current intensity than current pulse 24. The duty cycle is 1 / 5. The intensity of the current pulses and the duty cycle depend on the input voltages U1 and U2 and the input resistances 18 and 19.

[0026] All features of the invention can be essential to the invention both individually and in any combination with one another. Reference numbers 1 item 2 Lighting device 3 optical axis of the lighting device 4 first sensor 5 optical axis of the first sensor 6 second sensor 7 optical axis of the second sensor 8 light reflected from the object 9 light emitted by the object by fluorescence or phosphorescence 10 optical shielding 11 filters 12 Housing of the lighting device 13 Circuit board 14 Housing of the device 15 protective glass 16 Power supply input 17 Power supply input 18 Input resistance 19 Input resistance 20 differential amplifiers 21 transistors 22 Resistance 23 current pulse 24 current pulses

Claims

[1] Device for detecting reflected and / or emitted light of an object (1), the device comprising: at least one lighting device (2) which illuminates the object (1) with pulsed light, at least one sensor (4, 6) which detects the light reflected and / or emitted by the object (1), a transport device which transports the object (1) relative to the lighting device (2) and past the sensor (4, 6) in the transport direction, a power supply (16, 17, 18, 19, 20, 21, 22) of the lighting device (2), which supplies the lighting device (2) with a current which is a periodic function over time, wherein a period has at least two current pulses (23, 24) of different magnitude, of which a first current pulse has a greater magnitude than a second current pulse; wherein the device is further configured such that • if saturation of the at least one sensor (4, 6) is reached during the first current pulse, only the reflected and / or emitted light from the at least two current pulses is evaluated with respect to the second current pulse; and / or • if a detection limit of the at least one sensor (4, 6) is undershot in the second current pulse, only the reflected and / or emitted light from the at least two current pulses is evaluated with respect to the first current pulse. [2] Device according to claim 1, characterized by that the lighting device (2) has at least one light-emitting diode. [3] Device according to claim 2, characterized by that the light emitting diode is a UV light emitting diode. [4] Device according to claim 1, 2 or 3, characterized bythat it is equipped with at least one first sensor (4) for detecting the light reflected by the object (1) and with at least one second sensor (6) for detecting the light emitted by the object (1) by fluorescence and / or phosphorescence. [5] Device according to claim 4, characterized by that the illumination device (2) is arranged with its optical axis at an angle different from 0° and 90° against the transport direction of the transport device, that the first sensor (4) for detecting the light reflected by the object (1) is arranged with its optical axis at the same angle to the surface of the object (1) as the illumination device (2), but symmetrical to a plane which runs perpendicular to the surface of the object (1) and through the intersection point between the optical axis of the illumination device (2) and the surface of the object (1). [6] Device according to claim 4 or 5, characterized by that the second sensor (6) is an RGB sensor. [7] Device according to claim 4, 5 or 6, characterized by that an optical shield (10) is arranged between the lighting device (2) and the second sensor (6). [8] Device according to one of the preceding claims, characterized by that the power supply for generating the temporally periodic current with at least two current pulses of different strengths per period has at least two parallel-connected input resistors (18, 19) and a differential amplifier (20). [9] Device according to claim 1, characterized by that the flat object is a security note. [10] Method for detecting reflected and / or emitted light of an object (1), the method comprising: Transporting the object (1) with a transport device past at least one lighting device (2) and at least one sensor (4, 6), Supplying the lighting device (2) with a current which is a periodic function over time, wherein a period has at least two current pulses (23, 24) with different magnitudes, of which a first current pulse has a greater magnitude than a second current pulse; Illuminating the object (1) with the pulsed light of the lighting device (2), Detecting the light reflected and / or emitted by the object (1) with the sensor (4, 6); where, • if saturation of the at least one sensor (4, 6) is reached during the first current pulse, only the reflected and / or emitted light from the at least two current pulses is evaluated with respect to the second current pulse; and / or • if a detection limit of the at least one sensor (4, 6) is undershot in the second current pulse, only the reflected and / or emitted light from the at least two current pulses is evaluated with respect to the first current pulse.

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