Optical inspection of pulp bottles

Abstract

The invention relates to a device for optical inspection of pulp bottles. The device comprises a lighting device with an opening, wherein the mouth of the pulp bottle can be arranged below the opening of the lighting device; and a camera designed to record an image of at least a portion of the inner wall of the pulp bottle through or from the opening of the lighting device, where the pulp bottle can be arranged such that the portion of the inner wall of the pulp bottle is illuminated by the lighting device. The invention further relates to a method for the optical inspection of a pulp container, in particular of a pulp bottle.

Description

[0001] The present invention relates to a device and a method for optically inspecting a pulp bottle, in particular the inner wall thereof.

[0002] Before containers such as beverage bottles and beverage cans are used for filling, it must be ensured that they are free of damage, such as cracks or holes, and free of contamination. For this purpose, transparent containers can be checked from the outside for contamination, foreign matter or damage using transmitted light. For opaque cans, there is an inner wall inspection, where light is irradiated from a large area light source through a large can mouth that is approximately the same diameter as the can. Devices for optical inspection of containers, in particular beverage cans, are known, for example from DE 199 40 363 A1 and DE 102017 123684 A1.

[0003] Pulp containers are made of opaque, diffuse materials that make it difficult to inspect the interior from the outside. In addition, pulp bottles have narrow necks compared to cans, making it difficult for light to penetrate the bottle and illuminate the interior.

[0004] Therefore, the object of the present invention is to provide a device and a method which can reliably check whether a pulp bottle is contaminated or damaged. To this end, the present invention provides a device according to claim 1 and a method according to claim 11.

[0005] According to the present invention, a device for optically inspecting a pulp bottle comprises: an illumination device having an opening, wherein the mouth of the pulp bottle can be arranged below the opening of the illumination device; and a camera designed to record an image of at least a portion of the inner wall of the pulp bottle through or from the opening of the illumination device, wherein the pulp bottle can be arranged so that this portion of the inner wall of the pulp bottle is illuminated by the illumination device.

[0006] For example, unlike metal cans, pulp does not reflect incident light, but only diffuses it or absorbs part of it. In addition, the mouth of a can is usually larger in diameter than a bottle, so it is easier to introduce light into the interior of the can. In order to adequately illuminate the inner wall of a pulp bottle for optical inspection for damage and / or contamination, sufficient light must enter the interior of the pulp bottle through the mouth and the neck, and the light scattered from the interior must be recorded by a camera. This goal can be achieved by the configuration claimed in this case, which consists of a lighting device and a camera associated with a pulp bottle located below.

[0007] Pulp bottles can be made entirely or mainly of a mixture of water, fibers and possible other additives (called pulp). The fibers can be natural and biodegradable. The fibers can include lignin, banana leaves and / or quinine. The fibers can, for example, include cellulose fibers, conifer fibers, leafy shrub fibers and / or sycamore fibers and / or grass fibers, reed fibers and / or bamboo fibers or similar fibers. The fibers can include silk, spider silk, algae, natural fibers (such as Danube silk (DonauSilphie) fibers, corn, cotton), banana peels, orange peels, grass, straw, potato starch or treated cow dung. Cellulose fibers derived from artificial planting processes can also be provided. In the case of a shortage of wood, as the base material of the fluid substance with fibers, these alternative materials can replace the base material in whole or in part. Similarly, the fibers can include a fiber mixture made of non-wood materials, such as cotton and / or textile fibers.

[0008] The inner wall of the pulp bottle refers to the entire surface that limits the inner space or the interior of the pulp bottle. The surface includes, for example, the bottle bottom, the bottle sleeve and / or the bottle neck. The incident light is scattered by the surface and shines on the camera through the mouth of the pulp bottle.

[0009] The diameter of the opening of the pulp bottle is usually no more than 5 cm, especially no more than 3 cm. The pulp bottle is also characterized by: its diameter increases from the mouth to the bottom, and its height is greater than the diameter of the mouth and / or greater than the diameter of the bottom.

[0010] The lighting device may include one light source or multiple light sources. Furthermore, the lighting device may be annular or quasi-annular. Here, quasi-annular means that the portion of the lighting device surrounding the opening may be triangular, rectangular, other polygonal, or even irregular. Likewise, this portion of the lighting device may only surround a section of the opening. An annular lighting device particularly includes a circular or elliptical portion surrounding the opening. The center point of the lighting device is particularly arranged so that the mouth of the pulp bottle is located vertically below the center point. This configuration allows the inner wall of the pulp bottle to be particularly evenly illuminated, thereby achieving reliable inspection.

[0011] The spectrum of the lighting device may be in the visible range of the electromagnetic spectrum (400nm to 800nm). However, it is advantageous to use ultraviolet light, for example between 200nm and 400nm, to induce fluorescence in the pulp so that the pulp bottle itself glows. In the case of contamination, this may increase the contrast between the pulp and the contaminants, making it easier to identify the contaminated pulp bottle. As an alternative, infrared light may also be used, for example between 800nm ​​and 1500nm. This helps to identify holes or cracks on the inner wall of the pulp bottle, as the contrast between the pulp and the metal mold surrounding the pulp bottle will be higher.

[0012] The camera may include an objective lens and an imaging chip, wherein the objective lens gathers scattered light passing through the mouth of the pulp bottle and images the light onto the imaging chip.

[0013] The device may also include another objective lens arranged between the pulp bottle mouth and the camera. The other objective lens may be, for example, a so-called inner wall objective lens. In addition, the objective lens may have a large depth of field so that a large area between the bottleneck and the bottom of the bottle can be clearly imaged.

[0014] Optionally, the objective has a double cross beam path, wherein the strong cone of light emitted from the objective or its focus is located in the area of ​​the mouth rubber ring. This ensures that the light path remains clear during transport, i.e. is not blocked by the container mouth, despite possible displacement of the pulp bottle.

[0015] The camera may be arranged in an opening of the lighting device.

[0016] First, the camera is arranged in the opening of the lighting device, which can embody a compact design, so that the camera and the lighting device can also be arranged closer to the mouth of the pulp bottle. In particular, if the lighting device is annular or quasi-annular, light can be symmetrically projected into the pulp bottle, so that the inner wall of the pulp bottle is evenly illuminated.

[0017] The lighting device may be arranged such that light from the lighting device is incident at an angle between 60° and 90°, in particular between 70° and 90°, relative to the mouth plane of the pulp bottle.

[0018] The angle setting is based on the assumption of ray optics. The closer the angle is to 90°, the better the light will penetrate through the neck of the pulp bottle into the interior. Therefore, the largest possible angle (less than or equal to 90°) ensures good illumination and helps to reliably identify damage and / or contamination optically. If the angle is too small, the light will not penetrate deep enough into the pulp bottle and parts of the inner wall (especially the bottom) will not be adequately illuminated. This reduces the reliability of the inspection, because damage and / or contamination cannot be identified due to poor illumination and low contrast.

[0019] The device may further include an anti-reflection protection window, wherein the lighting device and the camera are arranged on a side of the protection window away from the pulp bottle, and the camera does not directly contact the protection window.

[0020] The protective window protects the lighting device and the camera from any contamination, thus ensuring that the optical inspection can be carried out reliably. Avoiding direct contact between the camera and the protective window also ensures that the camera is not damaged by impact. The anti-reflective protective window has, for example, an anti-reflective coating, which has a low reflectivity in the wavelength range of the light used, for example in the range of 1% or less. This reduces artifacts caused by reflections, which could lead to an erroneous recognition of dirt or cracks in the pulp bottle.

[0021] A protective element made of an elastic light-proof material (eg, sponge rubber) may be arranged between the camera and the protective window.

[0022] The element has two functions here. Firstly, due to its elasticity, it absorbs the impact force and thus protects the camera arranged behind the protective window from damage by the impact force. Secondly, the light-proof element prevents the light from the lighting device from directly entering the camera, which would cause the recorded image to be distorted, the contrast to be reduced, and thus impair the reliability of the inspection.

[0023] The lighting device may comprise a plurality of light sources, such as chip-on-board LEDs (COB-LEDs). In the case where the lighting device is annular or quasi-annular and the camera is arranged in an opening of the lighting device, the LEDs may in particular be arranged around the camera (objective).

[0024] COB-LEDs facilitate the compact arrangement of a large number of individual LED chips on a small surface. This allows a higher power density to be achieved and a higher light power to be supplied to the interior of the pulp bottle. In the case of the lighting device being annular or quasi-annular as described above, a higher light power can be supplied to the pulp bottle from all sides, in order to achieve a uniform illumination of the inner wall. The corresponding recorded images thus have a higher contrast and the detection of damage and / or contamination is particularly reliable.

[0025] An example is a ring-shaped lighting device with a diameter of 14 mm, on which 130 LED chips operating at a pulse power of 130 watts can be arranged.

[0026] The LEDs can be arranged directly on the protective window. This allows for a very compact design, since no separate component is required to mount the LEDs.

[0027] The device may further comprise a beam splitter and a light source. The beam splitter may be arranged between the mouth of the pulp bottle and the camera. The light source may be designed for uniform surface illumination and may at least partially enter the interior of the pulp bottle through the mouth by means of the beam splitter. An optical element may be arranged between the beam splitter and the light source. The optical element may comprise a lens element and / or a polarization filter.

[0028] As an alternative to the above configuration, the device may further include a beam splitter and a pattern projector. The beam splitter may be arranged between the mouth of the pulp bottle and the camera, and the pattern projector may be designed to generate an optical pattern and may be arranged so that the pattern generated by the pattern projector is partially reflected by the beam splitter and enters the interior of the pulp bottle through the mouth.

[0029] The aforementioned illumination of the inner wall of the pulp bottle which is as uniform as possible is intended to detect damage and / or contamination, while the optical pattern can be used to detect whether there are material distribution errors. Inhomogeneities in the wall thickness of the pulp bottle lead to distortions in the optical pattern (compared to a flat surface), which can be detected in the corresponding images recorded by the camera.

[0030] The beam splitter may be a 50:50 beam splitter, reflecting approximately 50% of the incident radiation / power and allowing the remainder to pass through. However, other beam splitters with different reflection ratios (25% or 40%) are also contemplated.

[0031] If the device comprises a further objective lens, the latter is in particular arranged between the beam splitter and the mouth of the pulp bottle. In this way, the further objective lens can be used both to couple the optical pattern into the pulp bottle and to collect light scattered from the pulp bottle.

[0032] The pattern generated by the pattern projector may be in the form of a grid and / or may include concentric circles.

[0033] The two types of patterns mentioned above help to identify material distribution errors as clearly as possible. If the wall thickness of a pulp bottle is uneven due to material distribution errors, the optical pattern will bend at the wall thickness deviations. This bending is a deviation from the regular pattern of a grid or concentric circles and can be evaluated in the image recorded by the camera. Deviations from straight lines or circles (in the grid) can be detected particularly reliably.

[0034] The present invention also provides a system, which includes a device for manufacturing pulp bottles and the above-mentioned device for optically inspecting pulp bottles. The system adopts a design in which the pulp bottles are transported to the device for optical inspection after they are manufactured.

[0035] The method for optically inspecting a pulp container, in particular a pulp bottle, according to the present invention comprises: illuminating a part of the inner wall of the pulp container through the mouth of the pulp container with a lighting device, and recording light scattered from the inside of the pulp container with a camera, thereby recording an image through the mouth of the pulp container with the help of the camera.

[0036] The method solves the technical problem mentioned at the beginning of the present document, namely to reliably check whether pulp containers, such as pulp bottles, are damaged and / or contaminated.

[0037] The light from the lighting device may be incident at an angle between 60° and 90°, in particular between 70° and 90°, relative to the mouth plane of the pulp bottle. As described above in conjunction with the device, the given angle range helps to achieve reliable inspection.

[0038] The method may further comprise illuminating a portion of the inner wall of the pulp container with an optical pattern, for example generated by a pattern projector, and recording a further image by recording light scattered from the interior of the pulp container with a camera.

[0039] This evaluation of two images allows the pulp container to be checked both for damage and / or contamination and for uneven material distribution. This allows for a more complete characterization of the pulp container and a more reliable inspection.

[0040] The order of the two images can be arbitrary. It is also possible to first record the image with the optical pattern and then the image with the most uniform illumination possible (ie without pattern), or vice versa.

[0041] The pattern used may be in the form of a grid and / or include concentric circles. Please refer to the advantages of these particular patterns mentioned above.

[0042] The time interval between recording a (first) image and recording a further image may be shorter than 100 μs, in particular shorter than 50 μs.

[0043] The method is usually implemented in such a way that the lighting device and the camera are stationary and the pulp container passes by them in a single track on the transport device. The exposure (uniform illumination and optical pattern) of the inner wall of the pulp container should be approximately the same for both images. For this purpose, the relative position between the mouth of the pulp container and the lighting device and between the mouth of the pulp container and the camera should change as little as possible. At normal transport speeds of the pulp container, the distance moved by the pulp container in a given time is less than one millimeter and therefore significantly less than the mouth of the pulp container. As a result, the exposure of the two images is sufficiently consistent and the conditions for recording the images are also sufficiently consistent.

[0044] It will be appreciated that the method may be implemented with the aid of the apparatus for optical inspection of pulp bottles as described herein.

[0045] Further features and advantages are explained below by means of the exemplary drawings, in which:

[0046] Figure 1 is a schematic diagram of an apparatus for optically inspecting a pulp bottle according to a first embodiment;

[0047] Figure 2is a schematic diagram of an apparatus for optically inspecting a pulp bottle according to a second embodiment;

[0048] Figure 3A is a schematic diagram of an apparatus for optically inspecting a pulp bottle according to a third embodiment;

[0049] Figure 3B Schematic diagram of a device for optically inspecting a pulp bottle according to a fourth embodiment Figure 4 is a simplified schematic top view of a device according to a third embodiment.

[0050] In the following text and in the drawings, unless otherwise specified, the same reference numerals are used for the same or corresponding elements in different embodiments.

[0051] Figure 1 A schematic side view of a device 1 for optically inspecting a pulp bottle 20 according to a first embodiment of the present invention is shown. The device 1 comprises an illumination device 10 designed to emit light L. When a pulp bottle 20 having a bottleneck 21 and a mouth 22 is placed under the device 1, the light L emitted by the illumination device 10 is emitted into the interior of the pulp bottle through the mouth 22. In this way, at least a portion of the inner wall of the pulp bottle 20 is illuminated. The light L is emitted into the mouth at an angle α to the plane of the mouth. For example, the angle α may be between 60° and 90°, in particular between 70° and 90°. It should be noted that, unlike, for example, a beverage can, the narrow bottleneck 21 of the bottle may increase the difficulty of light L entering the interior of the pulp bottle 20. The device 1 solves this problem.

[0052] The device 1 further comprises a camera 11. Light is scattered at the illuminated portion of the inner wall of the pulp bottle 20 and is at least partially reflected back through the mouth 22. This portion of the reflected light is captured by the camera 11, which then records / generates an image of the illuminated portion of the inner wall of the pulp bottle 20.

[0053] In order to image the scattered light onto the camera 11, the device 1 may have a further objective lens 12, which is arranged in the optical path between the bottle mouth 22 and the camera 11. For this case, the optical path is schematically shown in the figure with a dotted line. For example, the further objective lens 12 can be an inner wall objective lens with a large depth of field. This has the advantage that a large area of ​​the inner wall of the pulp bottle can be clearly imaged, thereby achieving efficient optical inspection. As an alternative, the camera 11 itself can have an objective lens, so that the further objective lens 12 is not needed, or the camera 11 and the further objective lens 12 can be regarded as a unit.

[0054] The lighting device 11 has an opening (such as Figure 3B). The camera 11 is arranged in this opening. If the device 1 includes another objective lens 12, the other objective lens 12 is arranged in this opening. If the camera 11 and the other objective lens 12 are regarded as a unit, the unit is arranged in the opening of the lighting device 10. The lighting device 10 can be, for example, in the form of a ring or a ring-like shape, so that the camera 11 and / or the other objective lens 12 are partially surrounded. This arrangement above the pulp bottle 20 enables as much light L as possible to enter the mouth 22 of the pulp bottle 20, and the inner wall can be evenly illuminated, so that a well-exposed image can be recorded for optical inspection.

[0055] Figure 2 An inventive device 1 is shown according to a second embodiment for optically inspecting a pulp bottle 20. Since the second embodiment is substantially identical to the first embodiment, only the differences between the two embodiments are described here.

[0056] Firstly, the device comprises a pattern projector 16 designed to generate and emit an optical pattern.

[0057] This optical pattern is, for example, a pattern with regular and / or periodic elements, such as a grid or concentric circles.

[0058] This optical pattern is emitted into the interior of the pulp bottle 20 located below the device 1 and illuminates a portion of the inner wall. For this purpose, the device 1 further comprises a beam splitter 15, which is arranged in such a way that the light emitted by the pattern projector 16 is reflected by the beam splitter 15 and passes through the mouth 22 of the pulp bottle 20. For this purpose, the beam splitter 15 is arranged between the mouth 22 and the camera 11. The schematic optical path of the optical pattern is also indicated by a dotted line.

[0059] Like the light L emitted by the lighting device 10, the optical pattern is scattered on the inner wall of the pulp bottle 20 and recorded by the camera 11 to generate a corresponding inner wall image. The imaging process is similar to that of the first embodiment.

[0060] The beam splitter 15 is, for example, a 50:50 beam splitter, which can reflect 50% of the incident light intensity and allow 50% of the incident light intensity to pass through. In principle, other beam splitters (for example, 60:40 or 75:25) can also be used.

[0061] The second embodiment of the device 1 can record two images in sequence. When recording the first image, the inner wall of the pulp bottle 20 is illuminated as evenly as possible by the lighting device 10. This is used to identify severe shape defects, darkened stains and / or holes on the bottom or side walls of the pulp bottle 20. When recording the second image, the inner wall of the pulp bottle 20 is illuminated with an optical pattern. In this way, thickenings and / or depressions on the bottom or side walls can be found, because the optical pattern, which is preferably originally regular, is distorted at the thickenings and / or depressions. Thus, defects on the pulp bottle 20 can be reliably identified by means of the two images. It is understood that the two images can also be recorded in the opposite order.

[0062] The time interval between the two images should be as short as possible, for example shorter than 100 μs. The pulp bottle 20 is usually located on a transport device and is transported under the device 1. The transport is not stopped during the optical inspection. In other words, the pulp bottle 20 moves slightly between the two images. Only if the time interval is short enough and this spatial displacement is small enough, in the range of less than 1 mm, the illumination of the inner wall of the pulp bottle 20 and the viewing angle of the camera 11 through the mouth 22 will not change significantly. This means that the two images can be compared directly without any correction.

[0063] Figure 3A A third embodiment of the device 1 for optically inspecting a pulp bottle 20 is shown. In addition to the above-described components (illumination device 10, objective lens 12 and pulp bottle 20), this embodiment also comprises a protective window 13 and a protective element 14.

[0064] The protection window 13 is arranged between the objective lens 12 and the pulp bottle 20. Specifically, the protection window 13 is anti-reflective, that is, it has an anti-reflective coating on its surface to minimize the light component reflected at the protection window 13. In addition, the protection window 13 is also used to protect the objective lens from possible contamination or mechanical impact. In particular, the objective lens 12 does not directly contact the protection window 13.

[0065] The lighting device 10 has a plurality of light sources 10b arranged on a protective window 13. As shown in the figure, the lighting device 10 is annular or quasi-annular and surrounds an objective lens 12 arranged in an opening of the lighting device 10. A protective element 14 is also provided between the light source 10b and the objective lens 12. On the one hand, the protective element 13 can prevent light from directly entering the objective lens 12 from the light source 10b and then from being projected onto the camera. In addition, the protective element 14 is particularly made of an elastic material such as sponge rubber, thereby preventing the objective lens 12 from colliding with the protective window 13. It is understandable that the camera 11 can also be directly installed instead of the objective lens 12. The above description is also applicable to this case.

[0066] Figure 3B A fourth embodiment of the device 1 for optically inspecting a pulp bottle 20 is shown. Figure 3A The third embodiment shown differs in an alternative arrangement of the protective window 13. This protective window 13 can be inclined relative to the horizontal plane, for example between 5 and 20 degrees. In this way, the light from the lighting device 10 is not reflected in the direction of the camera, but mainly in the direction of the protective element 14 (not shown in this figure, but can be seen in the figure). Figure 3A and Figure 4 In particular, the protection element 14 may be black to suppress reflection of light at the protection element 14 .

[0067] The remaining components shown in this figure have been disclosed in the above description and will not be repeated here.

[0068] Figure 4 It is a top view of the device 1 of the third embodiment, that is, the device 1 is viewed from below through the protection window 13 .

[0069] The lighting device 10 includes a plurality of light sources 10b, such as LEDs arranged on the lighting device 10 in a ring shape. In this case, the number of light sources 10b shown and their arrangement should not be understood as limiting. For example, the light sources 10b can also be so-called chip-on-board LEDs, which are closely arranged to achieve high brightness.

[0070] The (ring-shaped) lighting device 10 has an opening 10a, which in the illustrated example is circular and concentric with the lighting device 10. Figure 3A As described above, the objective lens 12 is arranged in the opening. The protection element 14 shields the objective lens from direct light from the illumination device 10 .

[0071] It is understood that the embodiments described above can also be combined with each other in appropriate ways. For example, the pattern projector can be implemented together with the beam splitter and the protective window in the same embodiment. The same considerations also apply to the protective element, which can also exist independently of the protective window.

Claims

1. A device (1) for optically inspecting a pulp bottle (20), the device comprising: A lighting device (10) having an opening (10a), wherein the mouth of the pulp bottle (20) can be arranged below the opening of the lighting device (10); as well as A camera (11) is designed to record an image of at least a portion of the inner wall of the pulp bottle (20) through or from an opening of the lighting device, wherein the pulp bottle (20) can be arranged so that the portion of the inner wall of the pulp bottle (20) is illuminated by the lighting device (10).

2. The device (1) according to claim 1, wherein the camera (11) is arranged in an opening (10a) of the lighting device (10).

3. The device (1) according to claim 1 or 2, wherein the lighting device (10) is arranged so that: relative to the mouth plane of the pulp bottle (20), the light (L) from the lighting device (10) is incident at an angle (α) between 60° and 90°, in particular at an angle between 70° and 90°.

4. The device (1) according to any one of the preceding claims, further comprising an anti-reflection protection window (13), The lighting device (10) and the camera (11) are arranged on a side of the protection window away from the pulp bottle, and The camera (11) does not directly contact the protection window.

5. The device (1) according to any one of the preceding claims, wherein a protective element (14) made of an elastic light-proof material (such as sponge rubber) is arranged between the camera (11) and the protective window (13).

6. The device (1) according to any one of the preceding claims, wherein the lighting device (10) comprises a plurality of light sources (10b), such as chip-on-board LEDs.

7. The device (1) according to any one of the preceding claims, further comprising a beam splitter and a light source, wherein the beam splitter is arranged between the mouth of the pulp bottle (20) and the camera (11), The light source is designed to provide uniform surface illumination and enters the interior of the pulp bottle (20) at least partially through the mouth with the aid of the beam splitter, and In particular, an optical element is arranged between the beam splitter and the light source, which optical element comprises, for example, a lens element and / or a polarization filter.

8. The device (1) according to any one of claims 1 to 6, further comprising: A beam splitter (15) arranged between the mouth (22) of the pulp bottle (20) and the camera (11); as well as A pattern projector (16) is designed to generate an optical pattern, The pattern projector (16) is arranged so that the pattern generated by the pattern projector (16) is partially reflected by the beam splitter (15) and enters the interior of the pulp bottle (20) through the mouth (22).

9. Device (1) according to claim 8, wherein the pattern is grid-like and / or comprises concentric circles.

10. A system for manufacturing and inspecting a pulp bottle (20), the system comprising: An apparatus for manufacturing a pulp bottle (20), and Device (1) for optical inspection of pulp bottles (20) according to any one of the preceding claims, The system is designed so that after the pulp bottle (20) is manufactured, it can be transported to the device (1) for optical inspection.

11. A method for optically inspecting a pulp container, in particular a pulp bottle, the method comprising: Using a lighting device (10) to illuminate a portion of the inner wall of the pulp container through the mouth of the pulp container, and The light scattered from the interior of the pulp container is recorded by a camera (11), so that an image is recorded through the mouth of the pulp container by means of the camera (11).

12. The method according to claim 11, wherein the light (L) from the lighting device (10) is incident at an angle (θ) between 60° and 90°, in particular between 70° and 90°, relative to the mouth plane of the pulp bottle (20).

13. The method according to claim 11 or 12, further comprising: illuminating the portion of the inner wall of the pulp container with an optical pattern, for example generated by a pattern projector (16), and Another image is recorded by recording light scattered from the interior of the pulp container with the camera (11). The method of claim 13 , wherein the pattern is in the form of a grid and / or comprises concentric circles. 15 . The method according to claim 13 , wherein the time interval between recording the image and recording the further image is shorter than 100 μs, in particular shorter than 50 μs.

Images