Method and device for the optical verification of a component of an identity, value or security document

Abstract

The invention relates to a device (100) for the optical verification of at least one component (102) of an identification, security, or other document (102), comprising a detection chamber (108) into which at least the component (102) of the identification, security, or other document can be placed, a set of light emitters (104) configured to emit light in the direction of the component (102) of the identification, security, or other document, wherein at least one first light emitter (104) is configured to emit light from a first wavelength range, and wherein at least one second light emitter (104) is configured to emit light from a second wavelength range that differs from the first wavelength range, and a photodetector (106) directed towards the detection chamber (108), which is configured to detect light transmitted through the component (102) of the identification, security, or other document.to detect light reflected from or deflected by it. The invention also relates to a method for optically verifying the authenticity of at least one component (102) of an identification, value, or security document.

Description

[0001] The invention relates to a device and a method for the optical verification of at least one component of an identification, value or security document, in particular for the optical testing of the quality of a hologram film exposed with a hologram.

[0002] Identification, valuation, or security documents serve to verify the identity of a person or object, or a claim, such as for payment of a sum of money, delivery of a product, or provision of a service. To this end, it must be ensured that the document cannot be imitated, forged, or falsified, or only with considerable effort. The document therefore often contains security features that are extremely difficult or even practically impossible to replicate. For example, like banknotes, the document may be made of a material that is not readily available. Additionally or alternatively, security features can be created using special inks, such as luminescent or optically variable inks; optical elements, such as ambiguous images, kinegrams, lens or prism arrays; as well as guilloches, melier fibers, security threads, and other methods.Furthermore, it is often necessary that the identification, value or security documents are easy to produce and that they can be read by visually impaired persons.

[0003] Although many security features exist, many forgeries of identity documents, valuables, or security documents are still successful.

[0004] One way to forge an identification, security, or other document is to alter person- and / or document-specific areas by adding or removing layers. To prevent subsequent manipulation, a final transparent top layer is applied, which can be further secured by incorporating holograms (volume reflection holograms). The efficiency of light diffraction at the holographic film is a measure of its visual perceptibility. This efficiency is spectrally unevenly distributed and has a maximum near the exposure wavelength or master matrix. It is determined by the spatial configuration of the exposed, embossed, or inscribed interference patterns in the substrate.

[0005] US patent 2011 / 0052082A1 describes a system and method for detecting adhesive tape on a security document in the form of a banknote. This system uses three different light sources to illuminate the banknote: two white light sources and one UV light source. A line scan camera acts as a photodetector, performing a line scan as the banknote is sequentially illuminated by the light sources.

[0006] EP 1 867 977 A1 describes a device for testing holograms on identification, security, or other documents. This device incorporates three different light sources, which are focused into a single beam via deflecting mirrors. This beam is then directed through a lens onto the document to be tested, using a deflecting mirror. The reconstructed light from the security document is then detected by three photodetectors positioned at different angles.

[0007] GB 2 355 522 A and US 2010 / 0128964 A1 describe an arrangement for testing banknotes, each consisting of multiple light emitters used to irradiate the banknote with different wavelengths. The transmitted and reflected components of the light are optically detected, and the banknote is then tested for counterfeiting.

[0008] It is therefore an object of the present invention to provide a device and a method that allow for a rapid and reliable estimation of the spectral position of the peak wavelength(s) and a rapid and reliable characterization of the diffraction efficiency for such hologram films. The diffraction efficiency is defined as the ratio of the intensity diffracted by a diffractive optical element (DOE) into a specific solid angle to the intensity incident on the element.

[0009] This problem is solved by a device having the features of claim 1 and by a method having the features of claim 9. Advantageous embodiments with expedient further developments of the invention are specified in the dependent claims.

[0010] The device according to the invention for the optical verification of at least one component of an identification, security, or other document, in particular for testing a hologram film exposed with a hologram, has a detection chamber into which at least the component, in particular the hologram film, can be inserted. The device is provided with a set of light emitters configured to emit light in the direction of the component of the identification, security, or other document, of which at least one first light emitter is configured to emit light from a first wavelength range, and of which at least one second light emitter is configured to emit light from a second wavelength range that differs from the first. A photodetector is directed towards the detection chamber and is configured to detect light transmitted through the component of the identification, security, or other document.

[0011] In this way, it is possible to transmit different reconstruction wavelengths of a hologram exposed onto a carrier film towards the identification, security, or other document and to check whether the hologram is reconstructed at the desired peak wavelength and intensity. Instead of a white-light spectrometer, the present invention uses a number of light emitters with different emission wavelengths sequentially to illuminate a film sample or a component of an identification, security, or other document. This allows for an evaluation of the quality of the exposed film, enabling real-time statements about the quality of the film material and the exposure process.Since the hologram film is usually supplied on a film roll and passes through the exposure chamber in a continuous path, a rapid inspection of the exposed hologram film's quality is necessary to avoid unnecessary waste in subsequent processing steps. The device can thus be used in conjunction with an exposure unit during production. During verification, the transmission is measured; therefore, the device functions as a type of transmission photometer. A fast and reliable—albeit rough—determination of the peak wavelength within the selected spectral range and the diffraction efficiency can be performed without expensive specialized equipment such as spectrometers. The underlying material sample can therefore be irradiated with different wavelengths quasi-simultaneously, and its corresponding response recorded, for example, using a rapid sequence of single flashes.

[0012] To examine the exact same position of the sample, the invention provides that each of the light emitters comprises an axis of symmetry of its light emission, which intersects with the other axes of symmetry of the other light emission points at a common intersection point. This common intersection point is located upstream of the detection area. Such crossed or converging beam paths of the emitters allow the amplitude (intensity) and the spectral position to be determined photometrically. This makes it possible to infer the diffraction efficiency of the hologram at the position of the component or film section of the identification, security, or other document under investigation. The ratio of a first intensity at an unexposed area of ​​the component or hologram film to a second intensity at an exposed area, which is therefore being tested, can be calculated, resulting in an efficiently evaluable relative measurement.

[0013] Preferably, the intersection point lies on the surface or within the volume of an optical diffuser positioned upstream of the detection area. This diffuser element ensures that the component of the identification, security, or other document being inspected is diffusely illuminated by the different wavelengths of the light emitters. The diffuser element thus serves to homogenize the initially differing directions of incidence and intensity distributions of the light emitters.

[0014] Furthermore, to limit the exit angle from a chamber or diffuser, it is advantageous to have an optical tube positioned upstream of the detection chamber, particularly in the light emitter's beam path. This tube can be located between the diffuser and the detection chamber. The diffuser element, in conjunction with the subsequent tube, homogenizes the light emitted by the light emitters at the exit side. This reduces the angle of incidence observed by the photodetector. Typically, the LEDs used have an inhomogeneous light field with respect to direction and intensity distribution. Therefore, it is also beneficial to have an additional aperture downstream of the tube.

[0015] It has proven advantageous to associate a photoamplifier with the photodetector. The photoamplifier sequentially determines the respective intensity of the transmitted light from each of the light emitters, which can then be temporarily stored and compared with reference values, particularly predefined or previously calibrated ones. Specifically, this process converts the photocurrent generated by the photodetector, which is proportional to the illumination intensity, into a proportional voltage that can be easily digitized.

[0016] The light emitters are, for example, particularly narrowband light-emitting diodes (LEDs), for which the spectral characteristics, specifically peak wavelength and full width at half maximum (FWHM), are preferably known. Alternatively, laser light sources or broadband light sources with suitable filters can also be used as light emitters.

[0017] A photodiode is preferably used as the photodetector. Since the supplied voltage is not proportional to the generated photocurrent, an I / V converter is preferably employed. The photocurrent is proportional to the intensity. In this context, the photoamplifier is also available as a photodiode amplifier, which demonstrates its advantages in its efficient interaction with the photodiode. The photodiode amplifier is further characterized by its speed, linear response, and adaptability to the input voltage range of an analog-to-digital converter (A / D converter).

[0018] To minimize the influence of ambient light, it is advantageous to mount the set of light emitters in a chamber wall in such a way that the light is emitted into the interior of the chamber. From this chamber, the light is then directed exclusively towards the detection area. Furthermore, it is possible to determine the dark current when the emitters are switched off and subtract it from the measurement signal during flash operation, ensuring that a certain amount of residual light, which remains constant over time, does not interfere with the measurement or verification of the component.

[0019] It is advantageous if the device also includes its own control system that is set up: to determine the optical transmission caused by a component of the identification, value or security document, in particular by an exposed hologram film, based on the light detected by the photodetector, to compare the determined optical transmission with a predetermined reference value for transmission, and to detect manipulation or at least a defect in the component of the identification, value or security document if the determined optical transmission and the predetermined reference value deviate from each other by a minimum value.

[0020] In this way, it is possible to transmit different reconstruction wavelengths of a hologram exposed onto a carrier film towards the film of the identification, security, or other document, and to check whether the hologram reconstructs at the desired peak wavelength and intensity. Furthermore, by using different wavelengths, it is possible to very quickly determine at which wavelength and amplitude the diffraction maximum of a reconstructing hologram lies.

[0021] In this context, it is advantageous if the control system is configured to cause the light emitters to emit light sequentially towards the detection area. This sequence of on / off commands (signals) for the light emitters can be initiated very quickly by the control system, which, in turn, is typically a CPU or microcontroller and can very quickly process the response signal detected by the photodetector or photoamplifier. A single-chip solution can be used for this purpose; that is, a so-called SoC ("System on a Chip"), which includes integrated analog-to-digital converters and standard controller peripherals.

[0022] The advantages and beneficial effects described in connection with the device according to the invention apply equally to the method according to the invention, in particular when it is carried out with the device according to the invention.

[0023] The inventive method for optically verifying the authenticity of at least one component of an identification, value or security document comprises in particular the following steps: Positioning the component of the identification, valuable, or security document, in particular the exposed hologram film, relative to a set of light emitters and a photodetector in a detection space such that light emitted by the light emitters strikes the component of the identification, valuable, or security document and the photodetector detects light emanating from the component of the identification, valuable, or security document; emitting light from a first wavelength range by means of a first light emitter and receiving the light emanating from the component by means of the photodetector; emitting light from a second wavelength range, different from the first wavelength range, by means of a second light emitter and receiving the light emanating from the component by means of the photodetector; determining a wavelength range emanating from the component of the identification, valuable, or security document.Optical transmission of valuables or security documents is achieved by measuring the light detected by the photodetector, comparing the measured optical transmission with a predetermined reference value, and detecting manipulation or at least a defect in the document component if the measured optical transmission deviates from the predetermined reference value by a minimum value in at least one of the two emitted wavelength ranges.

[0024] In this way it is possible to send spectrally different beams of light onto a carrier film with an exposed hologram and to check whether the hologram is reconstructed at the desired peak wavelength and intensity.

[0025] Preferably, each light emitter has a symmetry axis of its light emission that intersects the other symmetry axes of the other light emission points at a common intersection, with the light rays meeting at this common intersection point even before they enter the detection area. In this way, predefined conditions for authentication are met. The photodetector can thus very efficiently detect each individual light source.

[0026] To achieve more uniform illumination / transmission of the identification, value or security document, the intersection point is located particularly on the surface or in the volume of an optical diffuser positioned in front of the detection area.

[0027] It is advantageous if the set of light emitters is embedded in a wall of a chamber, and if the light is emitted into the interior of the chamber. In this way, the wall can be used to block out ambient light.

[0028] Preferably, the light emitters emit light sequentially towards the detection area. Therefore, to perform a simple – and thus very efficient – ​​relative measurement, the respective intensity of the transmitted light from each of the light emitters can be sequentially determined and temporarily stored using a photointensifier. This is first done on an unexposed area of ​​the sample and then on an exposed area, resulting in the relative measurement.

[0029] Examples of acceptable identification, security, or other documents include a passport, identity card, driver's license or other ID card or access control card, vehicle registration certificate, vehicle title, visa, check, other means of payment, in particular a banknote, check, bank, credit or cash payment card, customer card, health card, chip card, company ID, proof of authorization, membership card or other ID document.

[0030] When an identification, security, or other document is tested using transmission measurement, the individual component can be examined in the document's unassembled form, for example, as an exposed and fixed film within a photopolymer sheet. However, the component or hologram can also be subjected to transmission measurement even if the identification, security, or other document is already a laminated unit with a window through which the transmission measurement can be performed.

[0031] Preferably, the identification, valuables, or security document is in ID 1, ID 2, ID 3, or any other format, for example, in booklet form, similar to a passport-like item. The identification, valuables, or security product is generally a laminate of several document layers that are precisely bonded together under heat and increased pressure. These products should meet standardized requirements, such as ISO 10373, ISO / IEC 7810, and ISO 14443.

[0032] Preferably, the product layers consist of a carrier material suitable for lamination. The identification, value, or security document may, however, preferably be formed from a polymer selected from the group comprising polycarbonate (PC), in particular bisphenol A polycarbonate or a polycarbonate formed with a geminal disubstituted bis(hydroxyphenyl) cycloalkane, polyethylene terephthalate (PET), its derivatives such as glycol-modified PET (PETG), polyethylene naphthalate (PEN), polyvinyl chloride (PVC), polyvinyl butyral (PVB), polymethyl methacrylate (PMMA), polyimide (PI), polyvinyl alcohol (PVA), polystyrene (PS), polyvinylphenol (PVP), polypropylene (PP), polyethylene (PE), thermoplastic elastomers (TPE), in particular thermoplastic polyurethane (TPU), acrylonitrile butadiene styrene copolymer (ABS), and its derivatives, and / or paper and / or cardboard and / or glass and / or metal and / or ceramics.Furthermore, the product can also be made from several of these materials. Preferably, it consists of PC or PC / TPU / PC. The polymers can be either filled or unfilled. In the latter case, they are preferably transparent or translucent. If the polymers are filled, they are opaque. The foregoing refers both to films to be bonded together and to liquid formulations that are applied to a precursor, such as a protective or topcoat. Preferably, the product is made from three to twelve, more preferably four to ten films. The films can also bear printed layers. A laminate formed in this way can finally be coated on one or both sides with the protective or topcoat or with a film. The film can, in particular, be a scratch-resistant film. Overlay layers formed in this way protect an underlying security feature and / or give the document the required abrasion resistance.

[0033] Preferably, the identification, valuables, or security document has a PE substrate onto which the photopolymer film, exposed with the hologram, is applied. The photopolymer can, for example, be formed from a one-component monomer selected from benzoyl ether, acetophenone, benzoyl oxime, and acylphosphine. Alternatively, the photopolymer can be based on a two-component system whose components are selected from benzophenone, xanthone, and quinone.

[0034] Further features, properties, and advantages of the present invention are described in more detail below with reference to exemplary embodiments and the accompanying figures. All features described so far and below are advantageous both individually and in any combination. The exemplary embodiments described below are merely examples and do not limit the scope of the invention. The figures show: Fig. 1 a sectional view through a device for optical authentication of an identity, value or security document, Fig. 2 the wavelengths / wavelength ranges emitted by the different light emitters of the device, and Fig. 3 a device according to Figure 1 , supplemented by an aperture.

[0035] In the Figure 1 and 3A device 100 for the optical verification of at least one component 102 of an identification, security, or other document, in particular for the optical quality control of an exposed hologram film, is shown in a sectional view. The device 100 comprises a detection chamber 108 into which at least one component 102 of the identification, security, or other document can be inserted. By way of example, the device 100 is formed with a housing that includes a feed slot for feeding the component 102 of the identification, security, or other document into the detection chamber 108. The device 100 also comprises a set of light emitters 104, which in this case are narrowband light-emitting diodes, in particular of known characteristics. The set of light emitters 104 is configured to emit light in the direction of the detection chamber 108 and thus in the direction of the component 102 of the identification, security, or other document.The figure shows three light emitters 104 as purely exemplary examples, although a different number of light emitters 104 is also possible. At least one of the light emitters 104 is configured to emit light from a first wavelength range (e.g., with peak wavelength λ1). At least one of the light emitters 104 is configured to emit light from a second wavelength range that differs from the first (e.g., with peak wavelength λ2). In the present case, all light emitters 104 present in the device 100 emit light from different wavelength ranges, i.e., light with different peak wavelengths. The device 100 also includes a photodetector 106 directed towards the detection space 108.The photodetector 106 shown in the figure is designed to detect the light transmitted through the component 102 of the identification, value or security document, which is why the device 100 forms a type of transmission photometer, because the photodetector 106 is arranged opposite the set of light emitters 104 with respect to the detection space 108.

[0036] An alternative configuration is possible – but not covered by the invention – in which the photodetector 106 is arranged on the same side as the set of light emitters 104 with respect to the detection area 108, so that the photodetector 106 is configured to detect the light reflected from the component of the identification, security, or other document 100. In this case, the device 100 functions as a type of reflection photometer. It is advantageous if the photodetector 106 is aligned in a direction corresponding to the reconstruction angle of the hologram to be tested.

[0037] To exclude ambient light as far as possible, the light emitters 104 are embedded in a wall or lid of a housing or chamber 118 such that light emission occurs exclusively within the chamber. It can be seen that each of the light emitters 104 comprises an axis of symmetry 112 or a cone of light from its emission point, which intersects the other axes of symmetry 112 of the other emission points at a common intersection point 114. It should be noted that the optical axes 112 of the light emitters 104 can also be imprecise, so that their cones of light overlap in a plane to form a single illuminated surface.The intersection point 114, or the overlapping light surface, lies close to the surface or within the volume of an optical diffuser 116 positioned upstream of the detection chamber 108. This diffuser causes a diffuse distribution of the light and, together with the intersection point 114, ensures that each light emitter 104 illuminates precisely the same position on the sample. The diffuser 116 acts as a second light source whose light can be used for verification. It emits light towards the detection chamber 108 with a program-controlled wavelength, which is determined by the wavelength of the light emitter 104 illuminating the diffuser 116. An optical tube 120 is also located between the detection chamber 108 and the diffuser 116, which further limits the exit angles into the detection chamber 108.

[0038] The device 100 after Figure 3 differs from device 100 according to Figure 1This is achieved simply by inserting an aperture 121 between the tube 120 and the detection chamber 108. The optional aperture 121 allows the captured area of ​​the document and the angle of incidence of the light beam to be controlled. This design is particularly advantageous when checking holographic features, as it allows the direction of incidence of the light to be precisely controlled.

[0039] To increase the amount of light arriving at the detector and thus the signal-to-noise ratio of the measuring system, it is possible to insert one or more additional lenses into the tube to form a system that emits a beam of light with a limited angle into the detection area of ​​the detector.

[0040] All described devices 100 also include a controller 110, preferably a CPU or a microcontroller, which can perform evaluation tasks. The controller 110 is also configured to cause the light emitters 104 to emit light sequentially in the direction of the detection area 108. The angle of intersection of the light beam, approximately formed by beam shaping, with the film should be freely selectable; the drawing only suggests a perpendicular transit by way of example. In holography, a changed angle of incidence can improve the signal quality. For this purpose, the controller 110 is in a communication link (shown as dashed lines) with each individual light emitter 104. The controller is also in a communication link (shown as dashed lines) with the photodetector 106. Through the - if necessary -By means of a flash-like, sequential activation of the light emitters 104, different, discrete wavelength ranges can be coupled into the detection space 108 in a very short time, which are then detected by the photodetector 106. If a hologram film 102 of an identification, security, or other document is placed in the detection space 108, the peak wavelengths and intensities that lead to a reconstruction of an exposed hologram can be detected very easily; in the case of transmission, this is due to the lack of intensity on the exit side, whereby the reconstruction takes place in the direction of the incidence side and components of the light are therefore missing; in the case of reflection, the opposite is true: light of the respective wavelength is detected in the corresponding solid angle of the reconstruction.

[0041] A counterfeit or manufacturing defect exists, for example, if no peak wavelength is detected, or if the "wrong" peak wavelength is detected, or if there is merely an attenuation of the intensity at a "correct" wavelength. The measured value can "drift," usually by a few nanometers, so this drift or the associated color shift can also be taken into account during testing.

[0042] The control unit 110 determines the optical transmission caused by component 102 of the identification, security, or other document based on the light detected by the photodetector 106. The optical transmission thus determined is then compared with a predetermined reference value for transmission. Manipulation of component 102 of the identification, security, or other document is detected if the determined optical transmission and the predetermined reference value deviate from each other by a minimum value.

[0043] To improve the processing of the detected wavelength ranges and the amplification of the measured amplitudes, a photoamplifier is assigned to the photodetector 106. More efficient data processing can be achieved by designing the photodetector 106 as a photodiode and the photoamplifier as a photodiode amplifier.

[0044] The inventive method and the inventive device 100 have the advantage that, due to the rapid sequential switching between the light emitters 104, a multitude of different wavelengths are coupled into the detection space 108 simultaneously or "quasi"-simultaneously, whereby the transmission of these wavelengths can then be detected by the photodetector 106 without having to move the sample between the emitters or the emitters above the sample. The desired accuracy can be further increased by taking into account the influence of the spectral width of the emitters and the spectral sensitivity of the receiver.

[0045] In Figure 2Finally, and purely as an example for a set of five different light emitters 104, the recorded wavelength ranges with their corresponding peak wavelengths λ1 - λ5 are illustrated, from which the respective intensity (amplitude) can also be derived. The evaluation using the control unit 110 thus allows a photometric determination of the amplitude and the spectral position of the diffraction maximum at one and the same position of a film or material sample as a component 102 of the identification, value, or security document. REFERENCE MARK LIST

[0046] 100 Optical verification device 102 Component of an identification, security, or other document 104 Light emitter 106 Photodetector 108 Detection chamber 110 Control unit 112 Axis of symmetry 114 Intersection point 116 Diffuser 118 Chamber / Housing 120 Tube 121 Aperture

Claims

1. A device (100) of the transmission photometer type for the optical verification of at least one component (102) of an identity, value, or security document by means of a transmission measurement, with a detection space (108) into which at least the component (102) of the identification, value, or security document can be inserted, with a set of light emitters (104) that is designed to emit light in the direction of the component (102) of the identification, value, or security document, wherein at least a first light emitter (104) is designed to emit light from a first wavelength range, and wherein at least one second light emitter (104) is arranged to emit light from a second wavelength range that differs from the first wavelength range, and a photodetector (106) directed toward the detection space (108) and configured to detect light transmitted through the component (102) of the identification, value, or security document, wherein each of the light emitters (104) has an axis of symmetry (112) of its light exit, which intersects with the other axes of symmetry (112) of the other light exits at a common intersection point (114), characterized in that the common intersection point (114) is located upstream of the detection space (108) so that the light beams meet at the common intersection point before they enter the detection space.

2. The device (100) according to claim 1, characterized in that the intersection point (114) is located on the surface or within the volume of an optical diffuser (116) upstream of the detection space (108).

3. The device (100) according to claim 1 or 2, characterized in that an optical tube (120) upstream of the detection space (108) is present in the beam path of the light emitter (120).

4. The device (100) according to any one of claims 1 to 3, characterized in that a photoamplifier is associated with the photodetector (106).

5. The device (100) according to claim 4, characterized in that the photodetector (106) is in the form of a photodiode and the photoamplifier is in the form of a photodiode amplifier.

6. The device according to any one of claims 1 to 5, characterized in that the set of light emitters (104) is inserted into a wall of a chamber (118) in such a way that the light is emitted into the interior of the chamber (118).

7. The device (100) according to any one of claims 1 to 6, comprising a control unit (110) which is designed to: - determine an optical transmission caused by the component (102) of the identification, value, or security document based on the light detected by the photodetector (106), - compare the determined optical transmission with a predetermined reference value for transmission, and - detect a manipulation or at least an error of the component (102) of the identification, value, or security document if the determined optical transmission and the predetermined comparison value deviate from each other by a minimum value.

8. The device (100) according to claim 7, characterized in that the control unit (110) is configured to cause the light emitters (104) to emit light sequentially in the direction of the detection space (108).

9. A method for optically verifying at least one component (102) of an identification, value, or security document by means of a transmission measurement with a device according to any one of claims 1 to 8, comprising the steps of: - positioning the component (102) of the identification, value, or security document relative to a set of light emitters (104) and a photodetector (106) in a detection space (108) such that light emitted from the light emitters (104) strikes the component (102) of the identification, value, or security document and light emerging from the component (102) of the identification, value, or security document is detected by the photodetector (106), - emitting light from a first wavelength range by means of a first light emitter (104) and receiving the light emerging from the component (102) by means of the photodetector (106), - emitting light from a second wavelength range that differs from the first wavelength range by means of a second light emitter (104) and receiving the light emitted from the component (102) by means of the photodetector (106), - determining an optical transmission caused by the component (102) of the identity, value, or security document based on the light detected by the photodetector (106), - comparing the determined optical transmission with a predetermined reference value for the transmission, and - detecting a manipulation or at least an error of the component (102) of the identification, value, or security document if the determined optical transmission deviates from the predetermined comparison value by a minimum value in at least one of the two emitted wavelength ranges, wherein each of the light emitters (104) comprises an axis of symmetry (112) of its light exit that intersects with the other axes of symmetry (112) of the other light exits at a common intersection point (114), characterized in that the light beams meet at the common intersection point (114) before entering the detection space (108).

10. The method according to claim 9, characterized in that the intersection point (114) is located on the surface or in the volume of an optical diffuser (116) upstream of the detection space (108).

11. The method according to claim 9 or 10, characterized in that the set of light emitters (104) is inserted into a wall of a chamber (118) and that the light is emitted into the interior of the chamber (118).

12. The method according to any one of claims 9 to 11, characterized in that the light emitters (104) emit light sequentially in the direction of the detection space (108).

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