Apparatus and corresponding method for inspecting a surface of a transparent object

Abstract

The invention relates to a device for inspecting a foil-like transparent object (10) to determine the distribution and size of particles on the top side or the bottom side of the object, respectively, the foil-like transparent object having a first surface (11) on the top side of the transparent object and a second surface (12) on the bottom side of the transparent object. To this end, the device comprises a camera (40) and at least one light source (20), wherein the light source (20) is designed such that electromagnetic radiation emitted by the light source illuminates a linear region (15) of the first surface (11) of the object from above or a linear region (15) of the second surface (12) of the object from below, wherein the illumination takes place at a predetermined angle (a) to the respective illuminated surface (11), wherein the camera (40) is designed to detect the intensity of the electromagnetic radiation reflected back in at least one portion of the linear region (15). The predetermined angle (a) is less than or equal to 15°, the electromagnetic radiation emitted by the light source (20) is predominantly linearly polarized and s-polarized. The invention also relates to an inspection method.

Description

Technical Field

[0001] This invention relates to surface inspection of foil-type transparent objects (e.g., thin flat glass or transparent film) and corresponding methods thereof. Background Technology

[0002] Thin flat glass or transparent films, ranging from 0.01 mm to a few millimeters thick, are primarily used in the optics, electronics, and display industries, such as in the manufacture of liquid crystal displays (LCDs) or LCD screens. Thin flat glass is also known as thin glass, display glass, or microsheets. Thin flat glass or films have a surface on a top side and a surface on a bottom side opposite the top side, both of which have a relatively large extension relative to the thickness of the object. The purity of these surfaces is an important criterion in the production of such objects. Therefore, during the production process of flat glass, for example, it is necessary to monitor, for example, the presence of particles on the glass surface and how these particles are distributed on the surface. These particles can be very small; that is, their diameter can be only a few micrometers.

[0003] Recently, optical inspection of glass and thin films has become known. Typically, camera-based methods with strong dark-field illumination are used to detect contamination. This method simultaneously detects particles on both the top and bottom of thin, flat glass. This method is called AB-Side. However, it cannot distinguish between particles on the top and bottom sides.

[0004] However, in order to further process transparent objects, such as to perform targeted cleaning of flat glass, it is desirable to be able to determine whether the contaminant is on one of the two sides (i.e., the top side or the bottom side). Summary of the Invention

[0005] Therefore, the object of the present invention is to provide an apparatus by which the size and distribution of particles on only one side (i.e., the top side or the bottom side) of a transparent article can be determined separately. Similarly, the object is to provide a corresponding method.

[0006] The above tasks are accomplished by means of apparatus and methods having the following characteristics.

[0007] The apparatus for inspecting foil-type transparent objects according to the invention specifically includes a camera and at least one light source. The foil-type transparent object has a first surface on its top side and a second surface on its bottom side. The foil-type transparent object can have a thickness (dimension) of, for example, from 0.1 mm to several millimeters. Here, the top and bottom sides of the transparent object are two opposing sides with the greatest extension. Therefore, "above" refers to the space above the top side, and "below" refers to the space below the bottom side. The thickness is the material dimension between the top and bottom of the object.

[0008] The term "transparency" is used to describe the optical properties of an article's material, specifically its transmittance to electromagnetic radiation. In particular, the present invention applies to objects having the following characteristics: the object has at least 50% transparency to electromagnetic radiation in a sub-region of the wavelength range between 300 nm and 3 µm, preferably between 380 nm and 780 nm (visible light). A light source is arranged such that electromagnetic radiation emitted by the light source illuminates a linear region of a first surface of the object from above or a linear region of a second surface of the object from below. The wavelength of the electromagnetic radiation used by the light source is, for example, in the wavelength range between 300 nm and 3 µm, preferably between 380 nm and 780 nm (visible light). Thus, the light source illuminates the light-facing surface of the transparent object in the linear region, which may, for example, have a length ranging from 1 cm to 10 m along the line and a width ranging from a few µm to a few mm in the transverse direction of the line. The linear region illuminated by the light source is slightly larger than or has the same size as the detection area of ​​the camera, which is designed, for example, as a line scan camera and detects electromagnetic radiation reflected from the linear region. In this case, the camera is positioned such that it detects the intensity of the reflected electromagnetic radiation (i.e., the intensity of electromagnetic radiation reflected in the direction of the light source in at least a portion of the linear region) at a predetermined angle to the corresponding irradiated surface. Therefore, the camera is positioned on the same side of the object, i.e., the side facing the light source. The predetermined angle is further set to be less than or equal to 15°, and the electromagnetic radiation is primarily linearly polarized and s-polarized (i.e., transversely electrically polarized). Preferably, the predetermined angle is in the range of 3° to 12°, more preferably in the range of 5° to 10°. Here, s-polarization refers to the polarization direction of the electric field vector, whose oscillation direction is constant (linear polarization). Here, s-polarization means that the electric field of the electromagnetic radiation is perpendicular to the plane of incidence. The expression "primarily s-polarized" means that the degree of polarization (the proportion of s-polarized radiation) is at least 75%, preferably at least 90%, and particularly preferably at least 95%. Since electromagnetic radiation reflected from an uncontaminated surface is reflected away from the light source, the camera only detects the intensity of electromagnetic radiation reflected back from particles on the surface toward the camera or the light source. Specifically, the camera detects the intensity of electromagnetic radiation reflected back from a linear region of the surface illuminated in the direction of at least one light source. If the camera detects a high intensity of reflected radiation in a sub-region of the linear region, it is assumed that particles contaminating the surface are present on the surface.

[0009] The device according to the invention has the advantage that, due to the use of a very small illumination angle (a predetermined angle less than or equal to 15°), only a relatively small amount of light penetrates the transparent object. The proportion of reflected light depends on the illumination angle. At a small illumination angle, most of the incident light is reflected in a direction away from the light source (and thus away from the camera), and a very small proportion penetrates the transparent object. The same effect occurs on the opposite side of the transparent object from the illumination surface. Only a very, very small portion of the incident light reaches contaminant particles located on the opposite surface of the illumination surface.

[0010] Furthermore, it is known that electromagnetic radiation is partially reflected and partially refracted into the second medium (here, the transparent material) at the interface between two media (i.e., at the irradiated surface of the transparent object). Here, the proportions of reflected and refracted radiation of s-polarized light differ from those of p-polarized light. This law is used in a creative way, such that primarily s-polarized electromagnetic radiation is used for illumination. As is currently the case, at small illumination angles, a large proportion of s-polarized electromagnetic radiation is reflected, and the proportion transmitted is relatively small. This effect also occurs for p-polarized light, but it only occurs at extremely small angles, which, due to spatial constraints, requires considerable effort to achieve in practice. At smaller angles, the reflected portion of p-polarized light decreases very rapidly. If only s-polarized light is irradiated, the p-polarized portion of the incident electromagnetic radiation is very small, and the total irradiated portion refracted into the transparent body is very small. In contrast, most of the incident s-polarized radiation is reflected, and only a small portion is refracted into the transparent body. Therefore, only a very small portion of the light reaches the surface of the transparent object opposite the irradiated surface.

[0011] As can be seen from the two effects described above, the proportion of electromagnetic radiation passing through a transparent object is very small. Therefore, very little light reaches the contaminants on the opposite side of the irradiated surface. Consequently, in the corresponding camera image, individual particles on the surface facing the camera are detected with a significantly greater intensity than particles on the side facing away from the camera, thus distinguishing contaminant particles on the camera-facing surface from those on the opposite side of the object. The camera detects the light reflected back in the direction of at least one light source, and this is used as the basis for determining the location and size of the surface contaminant particles.

[0012] In one embodiment, the optical path of the camera at the object-side end adjacent to the illuminated linear region of the corresponding surface forms an angle of less than 20°, preferably less than 10°, and particularly preferably less than 5° with the optical path of the electromagnetic radiation emitted by the light source. By arranging the light source and camera at the same angle or such a very small angle, the negative impact of changes in the position of the object surface at the camera focus and the intensity of the illumination on the inspection is largely prevented. Variations in height within the material do not cause the illuminated linear region of the illuminated surface to shift out of the camera's detection range. In this embodiment, the optical path of the camera has an angle of less than 5° with the optical path of the electromagnetic radiation emitted by the light source (i.e., radiation with the incident light). Preferably, this angle is less than 3°, more preferably less than 1°. In one embodiment, the optical path of the camera can be arranged such that it propagates in a plane with the optical path of the light source at the end adjacent to the illuminated linear region of the corresponding surface.

[0013] The apparatus and method according to the invention are particularly suitable for objects made of glass (e.g., silicate glass) or plastic. The object may have a strip-like structure, i.e., pass through the apparatus according to the invention at a predetermined speed, or be inspected as a single piece.

[0014] In one embodiment, two or more light sources are provided. In another embodiment, one, two, or more light sources are in the form of lasers with linear optics, where the linear optics extend the beam exiting the laser to form a line. The resulting linear laser beam illuminates a linear region of a first surface from above or a linear region of a second surface from below with high intensity (e.g., optical power in the range of 1 mW to 10 mW per line millimeter). The electromagnetic radiation emitted by the light source is extended in the direction of the object due to the linear optics, resulting in minimal space requirements for the light source and the emitted radiation on the side facing the light source. For example, lasers with frequencies in the visible light wavelength range (e.g., diode lasers) can be used as lasers, which also have the advantage of emitting only polarized light. They then only need to be arranged in a manner where the light is s-polarized relative to the incident direction on the surface.

[0015] In one embodiment, two light sources may be provided, each illuminating a portion of a linear region of the surface. In this embodiment, a single camera is provided to observe both portions of the linear region. More than one camera may also be provided.

[0016] In one embodiment, the object-side end of the camera's optical path is defined at one end by a deflector and at the opposite end by the object's illumination surface. In other words, the object-side end of the camera's optical path is formed between the object's camera-facing surface and the deflector, wherein the camera's optical path is angled at the deflector. For example, the deflector deflects the camera's viewing angle by 90°, and other angles within the range of 20° to 170° are also conceivable. This allows for space-saving camera placement and also allows for the placement of additional deflectors within the camera's optical path.

[0017] The extremely simple structure of this device can be achieved by placing at least one light source next to the aforementioned deflector, which is arranged in the optical path of the camera. Here, "adjacent" means that the at least one light source is placed at a small distance in a direction transverse to the optical path of the camera, within the linear area of ​​the irradiated surface and the plane spanned by the camera's optical path. This allows light from at least one light source to pass next to the deflector, and from the deflector, the optical paths of the camera and the light source propagate in a common plane or at the aforementioned small angle (less than 5°). Therefore, the irradiation direction of the light source and the viewing angle direction of the camera are the same. In another embodiment, one light source is arranged on each side next to the deflector.

[0018] Alternatively or additionally, the deflector can be designed to be partially transparent, and at least one light source can be positioned behind the deflector when viewed from a transparent object. The electromagnetic radiation emitted by the light source then passes through the deflector and subsequently lies in the same plane as the camera's optical path; for example, the optical paths of the camera and the light source are identical between the deflector and the surface of the transparent object. Therefore, in this case, the direction of illumination and the camera's viewing angle are also the same.

[0019] In one embodiment, the camera is designed as a line scan camera, which detects the intensity of reflected electromagnetic radiation pixel-by-pixel and along the illuminated linear region. The line scan camera may have a CCD, NMOS, InGaAs, and / or CMOS sensor. Line scan cameras are advantageous for the device according to the invention compared to two-dimensional sensors with multiple lines. Although line sensors have approximately the same pixel size as area sensors, the line length can be much larger. A line scan camera can have 17,000 pixels or more, instead of the maximum 1,000 to 4,000 pixel width of an area sensor. Therefore, line scan cameras provide better spatial resolution for a given object field. Furthermore, the readout speed of lines is much faster than that of areas. In particular, line scan cameras are advantageous if they provide higher light source radiation intensity to achieve a generally significantly higher clock rate compared to area scan cameras. Another major advantage is the use of the exact same illumination geometry in the composite image of each line in the longitudinal direction (i.e., the line / width transverse to the object).

[0020] The method for inspecting foil-type transparent objects according to the invention is implemented, in particular, using a device comprising a camera and at least one light source. The foil-type transparent object has a first surface on the top side and a second surface on the bottom side. Electromagnetic radiation emitted by at least one light source is used to irradiate a linear region of the first surface of the object from above or a linear region of the second surface of the object from below, at a predetermined angle to the respective irradiated surface. Furthermore, the camera detects the intensity of electromagnetic radiation reflected back toward at least one light source in at least a portion of the linear region, wherein the predetermined angle is less than or equal to 15°, and the electromagnetic radiation emitted by the light source is primarily linearly polarized and s-polarized. The degree of contamination on the irradiated surface is determined based on the intensity of the reflected electromagnetic radiation detected by the camera (i.e., according to the detected intensity data). This process is based on the inventors' aforementioned discovery regarding the advantageous reflection behavior of electromagnetic radiation at small incident angles, whereby the electromagnetic radiation is primarily linearly polarized and s-polarized.

[0021] In embodiments with the aforementioned advantages, the camera is oriented such that the optical path of the camera at the object-side end adjacent to the linear region of illumination on the corresponding surface forms an angle of less than 5° with the optical path of the electromagnetic radiation emitted by the light source. The embodiments of the arrangement of the light source and camera optical paths given above are applied in the same manner to the method according to the invention.

[0022] In one embodiment, for example, the intensity of electromagnetic radiation reflected from a linear region of an illuminated surface is detected pixel by pixel using a line scan camera.

[0023] In another embodiment, a data processing device connected to the camera and receiving determined intensity data determines the position and / or size of contaminating particles on the irradiated surface of the transparent object based on the aforementioned intensity data (i.e., data on the intensity of reflected electromagnetic radiation determined by the camera from the linear region). For example, the camera determines a light intensity for each pixel between a value of 0 and a maximum intensity value (e.g., a value of 255). Furthermore, each pixel of the camera can be assigned a position on the irradiated side surface of the transparent object (i.e., a first surface or a second surface). This can be achieved, for example, by defining two-dimensional coordinates and assigning them to corresponding positions on the object surface. Through appropriate calibration, the data processing device knows which position with which coordinates is currently irradiated by the light source, and therefore accordingly knows from which position the intensity of reflected electromagnetic radiation currently detected by the camera originates. This is particularly due to the arrangement of the light source and / or camera, the (initial) position of the transparent object strip, and its feed speed (when inspecting the strip-shaped transparent object). Since the reflected electromagnetic radiation detected by the camera is located within the irradiated area, a position or coordinate on the object surface can be assigned to each pixel. If the detected intensity is higher than a predetermined first intensity threshold, it is concluded that particles on the contaminated surface exist at their corresponding assigned locations (represented by two-dimensional coordinates) on the surface associated with the light source and camera. To more precisely describe the size of the contaminant particles, the intensity of the reflected light can be evaluated. For example, intensity sub-ranges can be predefined, dividing the range of intensity between the first intensity threshold and the maximum intensity value into sub-ranges. For example, four intensity sub-ranges can be defined. By assigning the measured intensity to each pixel, the size of the corresponding particle can be determined, assuming that larger contaminant particles will produce higher measured intensities. Thus, a data processing device (e.g., a microprocessor) can determine the location and / or size of the contaminant particles on any given surface. In this respect, particles or aggregates of particles can extend across multiple pixels.

[0024] When inspecting the surface of a foil-type transparent object (e.g., a thin sheet of flat glass) along its entire longitudinal (i.e., transverse to the object's width) extension, the process involves, for example, moving the object relative to a device. In the direction transverse to the object's movement, an irradiated linear area is arranged to cover the entire width of the object, if possible. If desired, several devices according to the invention are arranged adjacent to each other to cover the entire width of the transparent object. By moving the object, the entire surface of one side of the object can be inspected. Alternatively, the device can be moved along the object for inspection.

[0025] This inspection allows for the separate inspection of contamination on the respective top or bottom sides of a planar object (i.e., an object with continuously formed surfaces). Attached Figure Description

[0026] In the following description, further advantages, features, and possible applications of the invention are described with reference to preferred embodiments and the accompanying drawings. All features described and / or illustrated herein constitute the subject matter of the invention, regardless of their summary in the claims and their references.

[0027] The figure shows:

[0028] Figure 1 This is a side perspective view of a portion of a first embodiment of the device according to the invention during the inspection of a foil-type transparent object;

[0029] Figure 2 It is based on Figure 1 A frontal perspective view of the example;

[0030] Figure 3 It is based on Figure 1 Another perspective view of the side of the embodiment; and

[0031] Figure 4 It is based on Figure 1 The embodiment shows the optical path of the camera and light source, as well as a side view of the transparent object. Detailed Implementation

[0032] Figures 1 to 4 A first embodiment of the apparatus for inspecting transparent objects according to the present invention is shown. For example, the object is a transparent flat glass 10, which passes through the apparatus at a constant speed in direction R in the form of a headband with a width B of, for example, 1 m. The flat glass 10 has a first surface 11 on the top side and a second surface 12 on the bottom side. The thickness (dimension) D of the flat glass 10 can be, for example, from 0.1 mm to several millimeters.

[0033] To inspect the first surface 11 of the flat glass for particulate contamination, the device includes two lasers 20 (e.g., diode lasers, with wavelengths in the visible light range, for example) arranged side-by-side. Each laser has linear optics and illuminates a linear region 15 with a length of 50 cm on the first surface 11. The width (lateral to length) of the linear region is within the aforementioned range. Both lasers emit linearly polarized light with s-polarization. The lasers 20 are mounted on a support plate 17, which is attached to a frame suspended on the flat glass strip. Due to the linear optics, the light exiting the lasers 20 is diffused into broad linear beams 22, such that when the beams strike the first surface 11 of the flat glass 10, only the linear region 15 is illuminated. In this configuration, the two lasers 20 are arranged adjacent to each other, such that each laser 20 illuminates a portion of half the length of the linear region 15.

[0034] Figure 4The diagram shows that the beam 22 of each light source 20 forms an angle α with the first surface 11 of the flat glass, such that the angle is, for example, less than 15°, preferably between 3° and 12°, and particularly preferably between 5° and 10°. Due to the small size of the angle α and the s-polarization of the incident light 22, at least 85% of the incident electromagnetic radiation is reflected at the surface 11 when there are no contaminant particles on the first surface 11. The light 24 reflected away from the light source 20 also has an angle α with the surface 11 of the flat glass.

[0035] For the inspection of the entire surface 11 of the flat glass 10, only one device according to the invention may be provided, or two or more devices according to the invention may be arranged side by side, so as to inspect the flat glass over the entire width of the flat glass.

[0036] The two lasers 20 are further arranged such that, in each case, light passes alongside a deflector 30. This deflector 30 works in conjunction with a line scan camera 40, which is attached to a support plate 17 above the first surface 11 of the flat glass. The camera's visible beam 41 is reflected by the deflector 30, for example, at a 90° angle, such that the light path then propagates parallel to and in a plane with the beams 22 of the two light sources 20, and also strikes the surface 11 of the flat glass in the illuminated linear region. In other words, the camera 40 observes the linear region 15 of the first surface 11, where the object-side end 42 of the camera 40's light path is in a plane with the two beams 22 of the light sources 20. Thus, as Figure 4 As shown, there is also an angle α between the object-side end 42 and the surface 11. Alternatively, the angle between the object-side end 42 of the camera 40 and the surface 11 can differ from angle α by less than 5°. By placing the light source 20 and the camera 40 at the same or approximately the same angle, the negative effects caused by changes in the position of the surface 11 of the flat glass 10 at the camera focus and the irradiation intensity of the laser 20 are largely prevented.

[0037] If one or more contamination particles are present in the linear region 15 illuminated by the first surface 11, the contamination particles will reflect the laser back to the camera 40. The electromagnetic radiation reflected back to the camera by the contamination particles is perceived by the camera as a bright spot and detected by at least one pixel of the camera line as brightness information (intensity of the reflected light).

[0038] Data processing device 50 connected to camera 40 (see Figure 2The data processing unit 50 receives intensity data detected by the camera, and transmits this intensity data, along with the assignment of each pixel, to the line scan camera. Furthermore, the data processing unit 50 knows the position of the illuminated radial region 15 on the first surface 11 of the flat glass 10, and thereby calculates the position of dirt particles belonging to the detected bright area of ​​the line scan camera. For example, the detected intensity at each camera pixel can be in the range of 0 to 255. For example, if the intensity at a pixel is greater than or equal to a first intensity threshold 21, the data processing unit 50 infers that the particle is located at the associated position within the illuminated linear region. Different intensities can be assigned to particle diameters of different sizes, as illustrated in the table below.

[0039] strength Particle diameter 21-80 2 µm 81-140 4 µm 141-200 6 µm 201-255 8 µm

[0040] In another possibility for evaluating the intensity values ​​obtained from the reflected electromagnetic radiation, it can be done alternatively or additionally in such a way that if adjacent pixels have predetermined intensity values ​​(e.g., within the range described above), it is assumed that the particles extend beyond two pixels. Then, for example, the determined particle diameters of these adjacent pixels can be added together. Similarly, in direction R, if an intensity value higher than a first intensity threshold is determined at adjacent pixels, the determined particle diameters can be added together. Other evaluations of the determined intensity values ​​of the reflected electromagnetic radiation are also conceivable. In this case, the scanning speed of camera 40 is adapted to the speed at which the flat glass strip moves in direction R, such that after detection of one line for the camera, the flat glass strip has moved exactly in direction R by the width of the exposed linear region detected by the line scan camera, so that in the next detection, the next linear region immediately adjacent to the previous linear region is detected.

[0041] In an alternative embodiment not shown, the laser 20 is not positioned next to the deflecting mirror 30, but rather behind it. The light from the laser 20 passes through the deflecting mirror 30 and reaches the first surface 11. For this purpose, the deflecting mirror is designed to be partially transparent.

[0042] To determine the contamination on the second surface 12, the camera, deflector, and light source are arranged in a mirror-inverted manner below the flat glass, illuminating a linear area at an angle α to the second surface 12. The camera's light path propagates along the second surface 12. The camera's light path also propagates at an angle α to the second surface 12 at the end of the object.

[0043] By means of the apparatus or method according to the invention, contamination can be targeted on one side of a flat glass or other foil-type transparent object.

Claims

1. An apparatus for inspecting a foil-type transparent object (10), the foil-type transparent object having a first surface (11) on the top side of the transparent object and a second surface (12) on the bottom side of the transparent object, The device includes a camera (40) and at least one light source (20), wherein the light source (20) is configured such that electromagnetic radiation emitted by the light source illuminates a linear region (15) of the first surface (11) of the object from above or a linear region (15) of the second surface (12) of the object from below, wherein the illumination is performed at a predetermined angle (α) to the respective illuminated surface, wherein the camera (40) is arranged to detect the intensity of electromagnetic radiation reflected back from at least a portion of the linear region (15), wherein, The predetermined angle (α) is less than or equal to 15°, and the electromagnetic radiation emitted by the light source (20) is mainly linearly polarized and s-polarized. The characteristic feature is that the optical path of the camera (40) in the object-side end adjacent to the linear region irradiated by the corresponding surface forms an angle of less than 20° with the optical path (22) of the electromagnetic radiation emitted by the light source (20).

2. The apparatus according to claim 1, characterized in that, The object-side end of the optical path of the camera (40) is defined at one end by a deflecting mirror (30) and at the opposite end by the illumination surface of the object.

3. The apparatus according to claim 2, characterized in that, The at least one light source (20) is arranged next to the deflector (30).

4. The apparatus according to claim 2 or 3, characterized in that, The deflector is partially transparent, and when viewed from the transparent object (10), the at least one light source is arranged behind the deflector.

5. The apparatus according to claim 1 or 2, characterized in that, The light source (20) is designed as a laser with linear optics and emits electromagnetic radiation.

6. The apparatus according to claim 5, characterized in that, The light source (20) emits electromagnetic radiation with wavelengths in the visible light range.

7. The apparatus according to claim 1 or 2, characterized in that, The camera (40) is configured to detect the intensity of the reflected electromagnetic radiation pixel by pixel.

8. The apparatus according to claim 1 or 2, characterized in that, A data processing unit (50) is provided, which is connected to the camera (40) and the determined intensity data can be sent to the data processing unit, wherein the data processing unit (50) is configured to determine the position and / or size of particles on the irradiated surface of the transparent object (10) based on the intensity data.

9. A method for inspecting a foil-type transparent object (10) using an apparatus, the foil-type transparent object having a first surface (11) on the top side of the transparent object (10) and a second surface (12) on the bottom side of the transparent object, The device includes a camera (40) and at least one light source (20), wherein a linear region (15) of the first surface (11) of the object is irradiated from above or from below by electromagnetic radiation emitted by the light source (20), wherein the irradiation is performed at a predetermined angle (α) to the respective irradiated surface, wherein the intensity of the electromagnetic radiation reflected back from at least a portion of the linear region (15) is detected by the camera (40), wherein the predetermined angle (α) is less than or equal to 15°, and the electromagnetic radiation emitted by the light source (20) is primarily linearly polarized and s-polarized, wherein the degree of contamination of the irradiated surface is determined based on the intensity detected by the camera (40). Its features are, The camera (40) is aligned such that the optical path of the camera (40) at the end of the object side adjacent to the linear region of the corresponding surface being illuminated forms an angle of less than 20° with the optical path (22) of the electromagnetic radiation emitted by the light source (20).

10. The method according to claim 9, characterized in that, The camera detects the intensity of the reflected electromagnetic radiation pixel by pixel.

11. The method according to claim 9 or 10, characterized in that, The position and / or size of the particles on the irradiated surface of the transparent object (10) are determined by a data processing unit (50) connected to the camera (40) and receiving data of a determined intensity, based on the determined intensity.

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