Lighting for optical surveillance devices

Abstract

An optical monitoring apparatus (10) and method are disclosed. The apparatus comprises an imaging device (11) for monitoring a material (4) moving continuously through a defined imaging area (12). The apparatus further comprises an illumination unit comprising a light source (15) and a concave freeform reflector (14) configured to reflect light from the light source in three dimensions to provide uniform illumination across the defined imaging area.

Description

[Technical field]

[0001] The present disclosure relates to an apparatus and method for providing illumination, and more particularly, for providing illumination for imaging devices used in optical monitoring of materials in industrial processes.

[0002] Industrial processes may involve the processing of large amounts of material flowing through the process. Various stages of the process may require constant monitoring of the material. Monitoring may be performed for various reasons and may be applied to various materials. Non-limiting examples of materials include food substances, such as grains, legumes, nuts, fruits, vegetables, etc., polymeric materials, metallic materials, and wood materials, mixed materials, waste materials, and any other physical material flowing through the process. Monitoring may be performed for the purpose of sorting the material and / or analyzing the material. As used herein, the term sorting encompasses a wide variety of material processing tasks that may be performed based on monitoring. For example, sorting may encompass any type of inspection, grading or classification, quality control, detection of defects, various analyses of materials, measurements such as volume, quantity, moisture measurements, and separation between organic and inorganic materials, for example when processing waste. Other property-related parameters may also be monitored, such as color, polymer type, damaged goods, etc. Sorting has many applications and the above examples are not presented as an exhaustive list of sorting applications. Material analysis applications provide information, such as quality analysis and reporting, of the monitored materials.

[0003] Material monitoring may be performed by specific equipment configured for the task of bulk material sorting and analysis. Optical equipment may be used for monitoring. Monitoring may be based on signals from an imaging device, such as an imaging device such as a digital camera or scanner, optical reader, etc. The signals from the imaging device may be input to a data processing device for analysis. Appropriate process control measures may be taken based on the analysis.

[0004] Optical instruments may require the use of high power broadband visible and infrared illumination. Currently, the main types of light sources include incandescent halogen bulbs and LEDs (light emitting diodes). Like most light sources, incandescent halogen bulbs have isotropic emission. Optical sorters use, for example, line scan cameras with a field of view or imaging field / area that is typically less than a few millimeters wide and may be hundreds of millimeters long. To provide a sufficiently high irradiance at the imaging area of ​​the line scan camera, cylindrical concentrating reflectors are used. An example of a cylindrical collinear reflector is disclosed in US Pat. No. 1,092,832,8 B2. The cylindrical reflector focuses the light in one direction along the line of the imaged strip-like area. Although concentrating cylindrical reflectors reduce the amount of energy required for illumination, a significant amount of electrical and optical power may still be required to provide sufficient illumination. Energy may also be wasted as heat. Heat may cause environmental and / or health and safety problems for the monitored material. Light bulbs have been used to increase the light level on the line, but as camera systems with higher resolution emerge, they require more light power.

[0005] The low light efficiency of current systems can be particularly problematic in applications where the cost of light power per inspected material volume is high. Applications that require very high light power due to narrow spectral bands of detection can also be problematic. Also, performance and signal-to-noise can be compromised when operating at lower light levels than they could be. Especially in the infrared range, cameras can only reach about 20% of their saturation level. The possible dynamic range of the detection system can be limited by poor uniformity of light with current systems.

[0006] Some of the wasted light may also affect the noise level of the surveillance system through stray light, reflections and scattering. Any additional optical power and spurious light may also be a potential health and safety hazard. A particular example of an application is a multi-module sorter where the foreground lighting may be modular. In such applications crosstalk may be an issue.

[0007] The aspects described herein aim to address at least some of these problems.

[0008] According to one aspect, an optical monitoring apparatus is provided comprising an imaging device for monitoring material moving continuously through a defined imaging area, and an illumination unit comprising at least one light source and a concave freeform reflector configured to reflect light from the at least one light source in three dimensions to provide uniform illumination across the defined imaging area.

[0009] According to another aspect, a method for optical monitoring of materials is provided, the method including imaging material flowing through a defined imaging area with a digital imaging device, emitting light from at least one light source, and reflecting the light in three dimensions with a concave freeform reflector to provide uniform illumination across the defined imaging area.

[0010] According to a more particular aspect, the concave reflector is shaped to reflect illumination onto the imaging area such that the imaging device provides a uniform response with optimized efficiency.

[0011] The width of the illuminated area may be 50mm or less for an illumination distance in the range of 150mm to 1200mm. The length of the illuminated area may be in the range of 150mm to 600mm. The width to length ratio of the illuminated area may be in the range of about 1:10 to 1:60.

[0012] The concave reflector may be shaped to provide a sharp cutoff in illumination at each end of the imaging area.

[0013] The apparatus may comprise at least two illumination units and associated imaging regions, wherein a concave reflector of at least one of the illumination units may be shaped to control illumination crosstalk with adjacent imaging regions.

[0014] The apparatus may also include at least two lighting units per imaging area, the concave reflectors of the at least two lighting units may be shaped such that a sum of illumination from the reflectors of the at least two lighting units results in a uniform irradiance or a uniform response of the imaging device over the imaging area.

[0015] The device may be configured to monitor bulk material moving through a defined imaging region.

[0016] The irradiation may be directed at the material being guided by a chute or a conveyor. The material may be a bulk material.

[0017] The device may comprise a sorting and / or analysis device.

[0018] Computer software products may be used to design and manufacture the reflector components described herein. [Brief description of the drawings]

[0019] Certain aspects will now be described in further detail, by way of example only, with reference to the following examples and the accompanying drawings, in which: [Figure 1] 1 shows a side view and a top view of a schematic embodiment of a system including an optical monitoring device; [Diagram 2] 1 shows a side view and a top view of a schematic embodiment of a system including an optical monitoring device; [Figure 3A] 1 is a schematic example of a reflection of light rays by a three-dimensional reflector formed in accordance with the principles disclosed herein; [Figure 3B] 1 is a schematic example of a reflection of light rays by a three-dimensional reflector formed in accordance with the principles disclosed herein; [Figure 3C] 1 is a schematic example of a reflection of light rays by a three-dimensional reflector formed in accordance with the principles disclosed herein; [Figure 3D] 1 is a schematic example of a reflection of light rays by a three-dimensional reflector formed in accordance with the principles disclosed herein; [Figure 4A] FIG. 2 shows a perspective view of a reflector according to an embodiment. [Figure 4B] FIG. 2 shows a perspective view of a reflector according to an embodiment. [Diagram 5] 1 shows a flowchart of an operation according to one embodiment. [Figure 6] 1 shows an embodiment of two coordinated lighting units. [Figure 7A] The results of the comparison test and the simulation are shown. [Figure 7B] The results of the comparison test and the simulation are shown. [Figure 7C] The results of the comparison test and the simulation are shown.

[0020] The following description provides an exemplary illustration of some possibilities for implementing the present invention. Although the specification may refer to "an", "one" or "some" examples or embodiments in several places in the text, this does not necessarily mean that each reference is to the same example of an embodiment or that a particular feature applies only to a single example or embodiment. Single features of different examples and embodiments may be combined to provide other embodiments.

[0021] Figure 1 shows a side view of a schematic embodiment of a processing system 1 including a monitoring device 10. Figure 2 shows a top view of the processing system. Processing system 1 includes a chute 2 that guides material 4 flowing through the system as indicated by arrow 3. The chute 2 guides downwardly moving material 4 towards a material processing device 8. Instead of or in addition to the chute, the processing system may include conveyors or other devices that effect material flow within the system.

[0022] The material processing equipment is controlled by a controller system comprising data processing equipment 6. The material processing equipment may process materials in various manners. For example, the materials may be subjected to quality analysis and / or control actions that may accept / reject the materials by appropriate sorting equipment based on detection of impurities, irregular objects, variations, etc. Other non-limiting examples include grading or sorting of materials, where different grades of materials or various materials are separated based on control instructions from a control system. For example, food materials or waste materials may be split into different material streams by material processing equipment 8 configured to sort the materials.

[0023] The monitoring device 10 comprises an imaging device, e.g. a digital camera 11, configured to image an imaging area 12. In this example, the imaging area extends across the width of the material flow between the end of the chute 2 and the subsequent processing device 8. The material is allowed to fall freely between the end of the chute and the subsequent processing device, such as a sorter. The term imaging area is used in this specification to refer to the field of view of the camera on the imaging plane. In line-scanning applications, the imaging area is provided as a substantially narrow strip. This allows imaging for line-scanning applications and obtaining a direct 2D image of the imaging area extending across the width of the material falling from the chute.

[0024] The line of sight from the camera to the imaging area 12 is indicated by dashed line 13. The imaging or observation area 12 in an embodiment is a narrow strip extending across the width of the material emerging from the chute 2, and the camera 11 is configured to image the entire length of the imaging area such that all material 4 passing by the monitoring device 10 is imaged. Images captured by the camera optics are converted to digital signals for input to the data processing device 6.

[0025] According to an embodiment in which a single light source is provided to deliver optical power to a segment of light, the aspect ratio of the length to width of the illumination area covering the imaging area 12 may be in the range of about 10:1 to 60:1. For example, the strip-shaped illumination area may be approximately 600 mm by 10 mm in size or a multiple thereof. The imaging area may be narrower than the illumination area.

[0026] The camera 11 is connected to the data processing device 6 via a data connection. The connection can be any suitable wired or at least partially wireless data connection and can communicate data from the camera based on any suitable data communication protocol. The data processing device 6 can comprise any suitable computing device, e.g. a server, configured to perform the monitoring, analysis and control functions required by the process control. The data processing device 6 can be connected to / part of a wider data communication system, such as an intranet or the Internet. The data processing device can be configured for monitoring and control of large scale production installations, such as factories or food processing plants. For example, the data processing device can be configured for food factories, different types of food processing applications, recycling plants, different raw material processing installations, etc. The functionality of the data processor device can be at least partly provided in a cloud or virtualized environment. Suitable gateways can be provided between the different environments.

[0027] The data processing device may comprise any suitable computing arrangement comprising at least one processor, memory, software and internal circuits and components necessary to perform tasks configured to implement at least some of the features described herein. Figure 1 shows an example of the internal components of a data processing device arranged on a board comprising a processor and one or more memories, as well as the connections between the elements of the device and an interface for connecting the data processing device to a camera. The at least one processor may be configured to execute suitable software code to implement one or more of the following aspects. The software code may be stored in the at least one memory.

[0028] The monitoring device further comprises a collinear illumination unit comprising a light source 15 and a reflector 14. The light source 15 may comprise, for example, a single incandescent light bulb. In other embodiments, multiple light bulbs can be provided. The light source may also comprise at least one LED. Each detection module may also have multiple illumination units, each comprising a reflector and at least one light bulb. Such a configuration may be desirable, for example, for high power applications. The light of the multiple illumination units may overlap or be stacked to form a uniform illumination over the imaging area.

[0029] The reflector is configured to have a concave shape to reflect the light 16 emitted from the light source 15 in three dimensions to provide uniform illumination across the imaging area 12. The design of the reflector can be based on freeform optics, the basic principles of which are described later in this specification.

[0030] Lines 17 and 18 show the extremes of the useful light rays that emerge from the reflector 14 and are reflected in a predetermined manner by the shape of the reflector to the imaging area. The plan view of FIG. 2 shows diagrammatically how the light rays 20 are reflected at the end 22 of the reflector 14 to the imaging area 12. As shown, the light energy is also utilized at the end by reflecting the light towards the imaging area and preventing the light rays from escaping at the sides of the conveyor 2 and being so wasted. To ensure uniformity over the length of the imaging area 12, the reflector is shaped and the light distribution is optimized so that the light energy is uniformly distributed on the pixels across the line of sight of the camera. The provided illumination area can be configured to substantially coincide with the imaging area 12.

[0031] 3A-3D are schematic side views showing non-limiting examples of different reflection scenarios. As shown, the geometry of the reflector can be shaped to provide different distributions of light rays. Regardless of the design selected, the objective is to provide a uniform mapping of light energy in the light rays to the plane of the imaging area, i.e., the reflector in each case is designed so that pixels on the imaging area receive a uniform amount of light. In FIG. 3A cross-convergence is performed, while FIG. 3B shows cross-divergence. FIG. 3C shows non-cross-convergence, and FIG. 3D shows non-cross-divergence. Any of these can be used to implement the reflector designs described herein.

[0032] By careful mapping, a reflector design can be made where there is substantially no variation in the amount of light received by each pixel across the imaging area, thereby achieving good dynamic range for the optical system.

[0033] The concave reflector shape prevents light rays 20 from escaping at the reflector end 22 and being directed in the desired manner towards the imaging area 12. Note that the presentation is very schematic and only a few light rays 20 are shown reflected at one end of the reflector to explain the principle. In a practical implementation, the light source can be positioned differently, for example outside the reflector dome, and the light emitted from the light source 15 is distributed throughout the reflector volume and is reflected uniformly in three dimensions across the imaging area 12.

[0034] 4A and 4B are schematic top and bottom perspective views of an embodiment of a reflector having a three-dimensional (3D) reflecting interior volume. FIG. 4A shows the outer shape of the reflector, and FIG. 4B shows the inner shape of the reflector. The shape of the reflector can be adjusted using appropriate optimization tools and techniques to more precisely focus the light uniformly across the imaging area while preventing light leakage at the edges. The illumination on the imaging area is adjusted to allow the imaging device to give a uniform response with maximum efficiency.

[0035] 5 shows an example for using a three-dimensional (3D) reflector in a method for optical monitoring of materials. In this method, light is emitted from a light source at 100, which is then reflected in three dimensions at 102 by a collinear concave reflector for uniform illumination across a defined imaging area. The reflector is designed to provide substantially the same amount of light to pixels across the imaging area. At 104, a bulk material flows continuously through the defined imaging area. While the light is being reflected in three dimensions by the collinear concave reflector, at 106, an imaging device is used to image the material within the defined imaging area.

[0036] In the following, the specific features and achievable advantages are described in more detail in the context of examples in relation to providing foreground illumination for an optical sorter with a collinear illumination configuration. A three-dimensional (3D) "freeform" shaped reflector is provided so that the light is focused in a defined segment in the imaging area, rather than in a line. The use of a 3D reflector results in an increase in light efficiency and does not require the use of additional optical elements such as lenses. It has been found that in certain applications a three-fold increase is achievable over a cylindrical reflector. The 3D shape of the reflector can be optimized so that the irradiance is the same at any point along the length of the camera's viewing area. The illumination on the imaging area can be performed in smaller segments. Depending on the possibilities, multiple lighting units, each comprising at least one light source and a reflector, can be arranged to cover the length of the imaging area. By using several coordinated lighting units, the total power can be increased while maintaining a flat response over the entire area.

[0037] 6 shows an embodiment in which two coordinating lighting units 60, 61 are provided to illuminate the imaging area 12. The concave reflectors of the two lighting units are shaped such that the sum of the illumination from the reflectors of the two lighting units results in a uniform irradiance or uniform response of the camera. More than two coordinating lighting units can be provided.

[0038] A freeform or anamorphic reflector can be configured to provide tailored illumination to achieve better focus and control over the light. A single light source, such as a single halogen bulb, can be used to provide flattened uniformity over the viewing area. The reflector shapes the isotropic radiation of an extended-size light source, such as a halogen bulb, and focuses the light into a concentrated, uniform, segment-shaped illumination area. This increases the amount of light available at the camera's viewing area and reduces the overall energy used.

[0039] Concave reflectors can be used to manage heat generated by high power foreground lighting. Freeform reflectors can reduce the number of light sources for the same final image plane irradiance compared to cylindrical 2D reflectors. With reduced heat, failures caused by excessive heat can also be avoided. Reducing the amount of heat generated can be important, for example, for safety and product quality reasons. Reducing the heat generated within the lighting assembly and / or lighting unit housing is considered to be a significant advantage.

[0040] The freeform three-dimensional surface of the reflector to optimally bring light to the desired location can be designed using software-assisted optimization. A semi-ellipsoid with the appropriate length, width, and height can be used as a starting point for the iterative process. The imaging area, the location and position of the ellipsoid, and the information of the light source can be used as input. The software-assisted optimization algorithm can be run on a computer to iteratively shape the reflecting surface until a shape is found that achieves the desired uniform illumination over the imaging area. A CAD / CAM model can be created for the manufactured freeform reflector based on the designed shape and model. Freeform optics constitute a recent technology that drives significant changes in illumination design. Freeform surfaces can be understood as optical surfaces without linear or rotational symmetry. Freeformity offers high design freedom, which can be used to avoid the limitations of the reflector surface shape and create efficient designs. The design of the reflector can be freed from the constraints of traditional optical surface shapes. The design method of the freeform illumination optics includes a zero-etendue algorithm based on the assumption of an ideal light source and an extended light source design algorithm. Zero-etendue algorithms include ray mapping, Monge-Ampere equations, and support quadratic methods (SQM). Extended source algorithms include illumination optimization, feedback design, and simultaneous multiple surface methods. The use of freeform surfaces can lead to new designs that cannot be achieved by using spherical or aspheric surfaces. The design of freeform illumination optical systems can be formulated as follows: given the input from the source and the output from the reflector as a given illumination, one or more freeform surfaces should be designed to redirect the light rays emitted from the source to produce the given illumination on the target surface, which is the inverse problem.

[0041] To take a more detailed example, a freeform reflector can be designed using the SQM method in an intuitive way. A given irradiance distribution can be assumed to be a continuous function defined on a domain. The domain is pixelated and sample points are defined for each pixel. The amount of energy assigned to each sample point can be considered to be equal to the total energy contained in its corresponding pixel. The given illumination problem is transformed into a discrete illumination problem after pixelation of the domain. The discrete illumination problem can then be solved. The light emitted from a source S is expressed as i 番目 It is considered that the light is reflected by the surface patch of i and converges to a point Ti on the target surface in the imaging area. 番目 The surface patch of is an ellipsoidal mirror, and points S and Ti are the two foci of the ellipsoid. 番目 The total energy delivered by a surface patch of i is determined by the parameters of this ellipsoid. For a fixed focal length, i 番目 It is possible to iteratively adjust the parameters of the ellipsoid (e.g., the length of the major axis) such that the total energy delivered to a point Ti by a surface patch of is equal to a specified value. For discrete illumination problems, a set of ellipsoidal surface patches is calculated, the number of surface patches being equal to the number of sample points. The final freeform reflector is constructed from these surface patches. The SQM method can also be used to find the ray mapping for the ray mapping method.

[0042] The above are illustrative examples, and there are several possible design methods that can be used to achieve the desired freeform reflector design. A more detailed description of these can be found in the paper by Wu, Rengmao, Zexin Feng, Zhenrong Zheng, Rongguang Liang, Pablo Benitez, Juan C. Minano, and Fabian Duerr, "Design of Freeform Illumination Optics." Laser & Photonics Reviews 12, no. 7 (July 2018): 1700310.

[0043] The generation and optimization of the shape of a freeform reflector can be performed using software products specifically configured for this purpose. Commercially available ray tracing software can be used to evaluate the performance of the design between iterations. Examples of such are commercially available software products such as Speos, Lightools, Tracepro, and Zemax.

[0044] FIG. 1 shows a single monitoring device and a single camera. A monitoring system can comprise several monitoring devices, lighting units and cameras, and their respective imaging areas. All of these may be connected to a control system / data processing device 6. Multiple high power lighting units may need to be placed close to each other, or at least within the field of view, as they may cause interference. This can be addressed by shaping the reflectors of the lighting units to tailor the illumination to illuminate only the relevant imaging area in a limited spatial range. Reflected illumination can produce uniform illumination across the line of sight of the camera with sharp cutoffs at each end to avoid crosstalk with adjacent monitoring modules. Crosstalk can lead to limitations in the design and size of the equipment, for example due to the need for recessed illumination. Adjustments to the shape of the freeform reflector can be used to mitigate the effects of crosstalk and / or control it so that the distribution of illumination is uniform and appears flat by the imaging device. The use of interacting lighting units can also be used to reduce the overall size and complexity of the system architecture.

[0045] Comparative reflector performance tests and simulations were carried out between the freeform three-dimensional (3D) reflector design disclosed herein and an existing cylindrical two-dimensional (2D) design. The test results are shown in Figure 7A. The simulation results are shown in Figures 7B and 7C.

[0046] 7A shows the test results for an illumination length of 900 mm (x-axis) and the relative response on the y-axis on a scale of 0 to 1. The tests showed a relatively uniform response across the width of the illuminated area.

[0047] 7B and 7C, the horizontal axis represents the y-axis values ​​over the 600 mm length of the viewing area, and the vertical axis represents the determined incoherent irradiance.

[0048] FIG. 7B shows the simulation results of the 2D design with 2×300 bulbs and a baffle used. FIG. 7C shows the simulation results of the 3D design with a single 300W bulb and no baffle. The edge drop-off of the 3D design is clear, while the left edge of the 2D design is not as sharp even with the baffle. Testing also showed that the irradiance efficiency of the 3D design is higher, even though only a single bulb is used. Testing and simulation results are consistent in showing the improvement of the 2D reflector.

[0049] The reflector design provides increased light power in the relevant area. The need for baffles can be reduced or eliminated altogether. Controlled spatial brightness can be achieved. Heat can be reduced. The amount of stray light that can interfere with other vision systems can be reduced or eliminated. Modularity can be provided since the lighting unit can be adjusted to fit different sizes of machines and processes without the need for filler, thus reducing the machine footprint. Better control of light emission can be used to reduce optical safety issues. In general, improved uniformity of illumination over the observation area can be provided, thus improving the accuracy of imaging and the results of the analysis. An increase in the effective dynamic range of detection was observed. Illumination can be flattened, compensating for the loss of camera sensitivity at high angles. Cost, complexity, and energy usage can be reduced.

[0050] Although the above detailed examples have been described with reference to specific processes, applications, and machines, it should be noted that there are several variations and modifications that can be made to the disclosed solutions without departing from the scope of the present invention. In particular, different embodiments have been described by way of example. Different features from different embodiments can be combined.

[0051] The above description provides a complete and informative description of exemplary embodiments of the present invention, by way of illustrative and non-limiting examples. However, various modifications and adaptations may become apparent to those skilled in the art in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. All such and similar modifications of the teachings of this invention are within the scope of the present invention.

Claims

1. an imaging device (11) for monitoring a material (4) moving continuously through a defined imaging area (12); an illumination unit comprising at least one light source (15) and a concave freeform reflector (14) configured to reflect light from the at least one light source in three dimensions to provide uniform illumination over the defined imaging area; An optical monitoring device (10) comprising:

2. 2. The apparatus of claim 1, wherein the concave freeform reflector (14) is shaped to reflect the illumination onto the imaging area (12) such that the imaging device (11) provides a uniform response with optimized efficiency.

3. 3. The apparatus of claim 1 or 2, wherein the width of the illumination area is less than or equal to 50 mm for an illumination distance in the range of 150 mm to 1200 mm, and the length of the illumination area is in the range of 150 mm to 600 mm.

4. 4. The apparatus of claim 1, wherein the illuminated area has a width to length ratio of about 1:10 to 1:

60.

5. 5. Apparatus according to any one of claims 1 to 4, wherein the concave reflector (14) is shaped to provide a sharp cut-off of illumination at each end of the imaging region.

6. 6. An apparatus according to any one of claims 1 to 5, comprising at least two illumination units and associated imaging areas, at least one concave reflector (14) shaped to control illumination crosstalk with adjacent imaging areas.

7. 7. The apparatus of claim 1, further comprising at least two lighting units (60, 61) for each imaging area (12), the concave reflectors of the at least two lighting units being shaped such that the sum of the illumination from the reflectors of the at least two lighting units results in a uniform irradiance or a uniform response of the imaging device over the imaging area.

8. 8. An apparatus according to any one of claims 1 to 7, configured to monitor a bulk material (4) moving through the defined imaging area (12).

9. 9. Apparatus according to claim 8, wherein the illumination is directed towards the material (4) guided by the chute (2) or conveyor.

10. 10. Apparatus according to claim 8 or 9, wherein the material (4) comprises at least one of the following: food material, granular polymer material, wood material, bulk dry goods, waste and / or metal material.

11. 11. Apparatus according to any one of the preceding claims, comprising a sorting and / or analysis device (1).

12. 12. The apparatus of claim 11 configured to perform at least one of inspecting, grading, separating, rejecting and generating measurement and / or quality analysis of the material based on imaging.

13. 13. The device according to any one of the preceding claims, wherein the light source (15) comprises at least one halogen bulb and / or at least one LED.

14. A method for optical monitoring of a material (4), comprising: Imaging material flowing through a defined imaging area (12) by a digital imaging device (11); Emitting light (16) from at least one light source (15); reflecting said light in three dimensions by a concave freeform reflector (14) to provide uniform illumination across said defined imaging area; The method includes:

15. 15. The method of claim 14, comprising reflecting the light towards the defined imaging area (12) such that a sharp cut-off of illumination at each end of the imaging area is provided, and / or illumination crosstalk with adjacent imaging areas is controlled, and / or at least two lighting units provide a single uniformly illuminated area.

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