Inspection system and inspection method

Abstract

To achieve a surface inspection using an object-side telecentric optical system that can obtain a wide field of view, reduce stray light, achieve a high signal-to-noise ratio, and achieve high resolution.SOLUTION: An inspection system comprises an image sensor 210 for imaging a workpiece W, an object-side telecentric optical system 1, a plate-type beam splitter 2, and a light source for irradiating a workpiece with light, in which the object-side telecentric optical system is set so as to satisfy the following formula, {n*t*(EFL" / Z')2} / Wfn0≤Px, where n is the refractive index of the beam splitter, t is its thickness, EFL" is the composite focal length of the lens group that is on the image sensor side of the beam splitter among the lenses that make up the object-side telecentric optical system, Z' is the distance to the virtual image of the workpiece as viewed from the lens group, Wfn0 is the effective F-value of the object-side telecentric optical system, and Px is the pixel size of the image sensor.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to an inspection system and an inspection method for irradiating a workpiece with light and capturing an image of its appearance, scratches, defects, etc., and in particular to an inspection system and an inspection method that use an object-side telecentric optical system. [Background technology]

[0002] For example, a known inspection system for surface inspection uses a coaxial illumination device that reflects light emitted from a light source in a direction different from the observation direction by a beam splitter in the same direction as the observation direction of a camera to illuminate a workpiece to be observed, as shown in Patent Document 1. In this inspection system, the lens and aperture of the coaxial illumination device form an object-side telecentric optical system, and are configured so that the light emitted from the light source can be collimated and irradiated onto the workpiece. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. WO2020 / 095843 Summary of the Invention [Problem to be solved by the invention]

[0004] Incidentally, the inspection system of the above-mentioned Patent Document 1 uses a cube-type beam splitter formed by combining two prisms into a cube shape. As a result, as shown in Figure 3, part of the light irradiated from the light source to the beam splitter is reflected by the outer surface of the beam splitter and the surface where the two prisms are combined, and goes toward the image sensor of the camera instead of going toward the workpiece, causing so-called stray light.

[0005] In such an inspection system, if the magnification of the object-side telecentric optical system is reduced to obtain a wide field of view, the illuminance on the work surface decreases in proportion to the square of the magnification of the optical system, resulting in a problem that the signal-to-noise ratio on the image sensor decreases to the point where the noise components caused by stray light in the cube-type beam splitter cannot be ignored, making it impossible to guarantee inspection accuracy. For example, if the magnification is 1 and the irradiance is 1W / mm 2 If the workpiece is a completely scattering object with a reflectance of 100% and the effective F-number of the optical system is 2.0 (NA = 0.25), ignoring the energy loss in each optical system, the energy that can be captured by the imaging optical system is 0.25 2 = 0.0625, so only about 6% of the signal component (S) reflected from the workpiece reaches the image sensor at the irradiance on the workpiece surface. If the magnification of the object-side telecentric optical system becomes even smaller (for example, 0.5x), only about 2% of the signal component (S) reflected from the workpiece reaches the image sensor. If the stray light generated by the beam splitter is 1%, the noise component (N) due to the stray light will be equivalent to half the signal component (S) due to the reflected light, resulting in a significant drop in the S / N ratio.

[0006] In order to reduce the effects of such stray light, it is possible to use a plate-type beam splitter using thin flat glass as shown in Figure 4. By using a plate-type beam splitter, it is possible to reduce stray light and achieve a high S / N ratio at low cost compared to using a cube-type beam splitter. However, on the other hand, when using a plate-type beam splitter, another problem occurs in that the resolution decreases due to the effects of astigmatism, which reduces the inspection accuracy.

[0007] The present invention has been made to solve all of these problems at once, and its main objective is to obtain a wide field of view, reduce stray light, achieve a high S / N ratio, and achieve high resolution in surface inspection using an object-side telecentric optical system. [Means for solving the problem]

[0008] That is, the inspection system of the present invention comprises an image sensor that images a workpiece to be imaged, an object side telecentric optical system that is arranged between the workpiece and the image sensor, a plate-type beam splitter that is arranged diagonally on the optical axis of the object side telecentric optical system, and a light source that is arranged to irradiate the beam splitter from a direction different from the optical axis and to irradiate the workpiece with the light reflected by the beam splitter, and the refractive index of the beam splitter is n, the thickness is t, the composite focal length of a lens group that is located on the image sensor side of the beam splitter among the lenses that constitute the object side telecentric optical system is EFL", the distance to a virtual image of the workpiece as viewed from the lens group is Z', and the effective F value of the object side telecentric optical system is W. fnо The object-side telecentric optical system is set so as to satisfy the following formula (1), where Px is the pixel size of the image sensor. {n·t·(EFL” / Z') 2} / W fnо ≦Px (1)

[0009] In such an inspection system, by using a plate-type beam splitter, stray light reaching the imaging element can be reduced compared to the case of using a cube-type beam splitter, and even if the magnification of the object-side telecentric optical system is reduced to obtain a wide field of view, a decrease in the S / N ratio can be suppressed and a high S / N ratio can be achieved. Furthermore, by setting the optical constants of the object-side telecentric optical system to satisfy formula (1), high resolution can be achieved with almost no effect of astigmatism, even when a plate-type beam splitter is used.

[0010] An embodiment in which the effect of the present invention is particularly remarkable is one in which the optical magnification of the object-side telecentric optical system is 0.5 or less.

[0011] The inspection method of the present invention is an inspection method using an inspection system including an image sensor that images a workpiece to be imaged, an object side telecentric optical system that is arranged between the workpiece and the image sensor, a beam splitter that is arranged obliquely on the optical axis of the object side telecentric optical system, and a light source that is arranged to irradiate the beam splitter from a direction different from the optical axis and to irradiate the workpiece with the light reflected by the beam splitter, wherein a plate type is used as the beam splitter, the refractive index of the beam splitter is n, the thickness is t, the composite focal length of a lens group that is located on the image sensor side of the beam splitter among the lenses that constitute the object side telecentric optical system is EFL", the distance to a virtual image of the workpiece as viewed from the lens group is Z', and the effective F value of the object side telecentric optical system is W. fnо The object-side telecentric optical system is set so as to satisfy the following formula (1), where Px is the pixel size of the image sensor: {n·t·(EFL” / Z') 2} / W fnо ≦Px (1)

[0012] Such an inspection method can achieve the same effects as the inspection system of the present invention. Effect of the Invention

[0013] According to the present invention configured in this manner, in surface inspection using an object-side telecentric optical system, a wide field of view can be obtained, stray light can be reduced, a high signal-to-noise ratio can be achieved, and high resolution can be achieved. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a diagram illustrating a configuration of an inspection system according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a diagram illustrating an optical system of the inspection system according to the embodiment. [Diagram 3] FIG. 13 is a diagram for explaining a state of light reflection when a cube-shaped beam splitter is used. [Figure 4] 1A and 1B are diagrams for explaining a manner in which light is reflected when a plate-type beam splitter is used. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] An inspection system 300 according to an embodiment of the present invention will be described below with reference to the drawings.

[0016] The inspection system 300 of this embodiment is for automatically inspecting a workpiece W, which is an object such as a product, by irradiating the workpiece W with light and capturing an image of scratches, characters, and the like on the surface of the workpiece W with a camera or the like. Specifically, as shown in Fig. 1, the inspection system 300 includes an imaging device 200 incorporating an imaging element 210 for capturing an image of the workpiece W, and a coaxial illumination device 100 disposed between the imaging element 210 and the workpiece W. The coaxial illumination device 100 is disposed on an imaging axis A1 connecting the imaging element 210 and the workpiece W, and is configured to be able to irradiate the workpiece W with inspection light from the same axis as the imaging axis A1, forming a so-called coaxial epi-illumination structure.

[0017] Specifically, the coaxial lighting device 100 includes an object-side telecentric optical system 1 arranged between the workpiece W and the image sensor 210, a beam splitter 2 arranged diagonally on the optical axis of the object-side telecentric optical system 1, a light source 3 that irradiates light toward the beam splitter 2, and a housing 4 that accommodates these.

[0018] The object-side telecentric optical system 1 is arranged so that its optical axis coincides with the imaging axis A1, and is configured so that the optical axis and the principal ray are parallel on the workpiece W side (object side). This object-side telecentric optical system 1 includes a plurality of lenses arranged along the optical axis direction and an aperture stop 13. The object-side telecentric optical system 1 includes, as a plurality of lenses, a front lens group 11 arranged on the workpiece W side of the aperture stop 13, and a rear lens group 12 arranged on the image sensor 210 side of the aperture stop 13. This aperture stop 13 is arranged near the focal position 11F of the front lens group 11. In the object-side telecentric optical system 1 of this embodiment, each lens is arranged so that its optical magnification is 0.5 or less.

[0019] The beam splitter 2 is a plate-type beam splitter that uses a thin flat glass. One surface of the face plate of the flat glass is coated with an optical thin film such as a dielectric multilayer film or a metal thin film, and the other surface of the face plate is coated with an anti-reflection film. The beam splitter 2 is disposed on the workpiece W side of the position of the aperture stop 13 in the housing 4, so that the surface of the face plate is inclined at 45° with respect to the optical axis of the object-side telecentric optical system 1.

[0020] The light source 3 emits light from a rectangular, for example, square, light-emitting surface, and emits uniform diffused light from the light-emitting surface. The light source 3 is formed, for example, by arranging a large number of LED chips in an array on a substrate. The light source 3 is fixed to the housing 4 so that its light-emitting surface faces the beam splitter 2 and the light-emitting surface and the aperture diaphragm 13 are in a conjugate positional relationship. In this embodiment, a variable diaphragm 14 capable of adjusting the aperture diameter is provided between the light source 3 and the beam splitter 2.

[0021] In the inspection system 300 of this embodiment, as shown in FIG. 2, the refractive index of the beam splitter 2 is n, the thickness is t [mm], the composite focal length of the lens group (rear lens group 12) located on the imaging element 210 side from the beam splitter 2 among the multiple lenses constituting the object side telecentric optical system 1 is EFL″ [mm], the distance to the virtual image of the workpiece W as viewed from the lens group 12 is Z′ [mm], and the effective F value of the object side telecentric optical system 1 is W fnо The object-side telecentric optical system 1 is configured so as to satisfy the following formula (1), where the pixel size (size per pixel) of the image sensor 210 is Px [mm]. {n·t·(EFL” / Z') 2} / W fnо ≦Px (1) In FIG. 2, EFL' is the composite focal length of the lens group 11 (front lens group) located on the workpiece W side of the beam splitter 2, WD is the working distance of the coaxial lighting device 100, and 12F is the focal position of the rear lens group 12.

[0022] According to the inspection system 300 of the present embodiment configured as described above, by using a plate-type beam splitter 2, the amount of stray light reaching the image sensor 210 can be reduced compared to the case of using a cube-type beam splitter, and even if the magnification of the object-side telecentric optical system 1 is reduced to obtain a wide field of view, the decrease in the S / N ratio can be suppressed and a high S / N ratio can be realized. More specifically, when a cube-type beam splitter 2 is used, as shown in FIG. 3, a part of the irradiated light is reflected by the outer surface of the cube without traveling toward the workpiece, and reaches the image sensor 210 as stray light, thereby decreasing the S / N ratio. On the other hand, when a plate-type beam splitter 2 is used, as shown in FIG. 4, there is no reflecting surface that generates stray light as in the case of a cube-type beam splitter, so the S / N ratio can be improved without being affected by stray light. Then, by setting the optical constants of the object-side telecentric optical system 1 so as to satisfy the formula (1), even if a plate-type beam splitter 2 is used, a high resolution can be realized without being affected by astigmatism.

[0023] <Other embodiments> The present invention is not limited to the above-described embodiment. For example, in another embodiment, a plurality of apertures may be provided between the light source 3 and the beam splitter 2, and the plurality of apertures may have a double pinhole structure. In this way, it is possible to further reduce stray light generated in the beam splitter 2, and to further suppress the decrease in the signal-to-noise ratio when the object-side telecentric optical system 1 is set to a low magnification.

[0024] Furthermore, the present invention is not limited to the above-described embodiment, and it goes without saying that various modifications are possible without departing from the spirit and scope of the present invention. [Explanation of symbols]

[0025] 300 ···Inspection System 1. Object-side telecentric optical system 11 Front lens group 11F, 12F...focal position 12 Rear lens group 13 Aperture diaphragm 2. Beam splitter 3...Light source A1: Imaging axis 210 Imaging element W ···Work

Claims

1. An imaging element for imaging a workpiece that is an imaging target; an object side telecentric optical system disposed between the workpiece and the image sensor; a plate-type beam splitter disposed obliquely on the optical axis of the object-side telecentric optical system; a light source arranged to irradiate light onto the beam splitter from a direction different from the optical axis, and to irradiate the workpiece with the light reflected by the beam splitter; The refractive index of the beam splitter is n, its thickness is t, the composite focal length of the lens group located on the imaging element side of the beam splitter among the lenses constituting the object side telecentric optical system is EFL", the distance to the virtual image of the workpiece as viewed from the lens group is Z', and the effective F value of the object side telecentric optical system is W ｆｎо , the object-side telecentric optical system is set so as to satisfy the following formula (1), where Px is the pixel size of the image sensor. {n・t・(EFL” / Z’) ２ } / W ｆｎо ≦Px (1)

2. 2. The inspection system of claim 1, wherein the object-side telecentric optical system has an optical magnification of 0.5 or less.

3. An imaging element for imaging a workpiece that is an imaging target; an object side telecentric optical system disposed between the workpiece and the image sensor; a beam splitter disposed obliquely on an optical axis of the object side telecentric optical system; a light source arranged so that light is irradiated onto the beam splitter from a direction different from the optical axis and the light reflected by the beam splitter is irradiated onto the workpiece, The beam splitter is a plate type. The refractive index of the beam splitter is n, its thickness is t, the composite focal length of the lens group located on the imaging element side of the beam splitter among the lenses constituting the object side telecentric optical system is EFL", the distance to the virtual image of the workpiece as viewed from the lens group is Z', and the effective F value of the object side telecentric optical system is W ｆｎо The inspection method includes setting the object-side telecentric optical system so as to satisfy the following formula (1), where Px is a pixel size of the image sensor: {n・t・(EFL” / Z’) ２ } / W ｆｎо ≦Px (1)

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