Optical line sensor and surface inspection device

Abstract

The present invention provides an optical line sensor and a surface inspection device which make it possible to suppress a decrease in amount of light caused by a connection part between a plurality of light-receiving lens arrays. A plurality of light-receiving lenses 311 are disposed in a line along a main scanning direction and transmit light from an illuminated subject S. The working distance W.D. of the light-receiving lenses 311 is 75 mm≥W.D.≥40 mm. When the radius of the light-receiving lenses 311 is defined as Xsla and the radius of field of view of the light-receiving lenses 311 is defined as R0, a degree m of overlap represented by m=R0 / Xsla is m≥7. The plurality of light-receiving lenses 311 constitute a plurality of light-receiving lens arrays 31 which each comprise two or more light-receiving lenses 311. When the amount of received light at a light-receiving element 321 facing a connection part 312 of the plurality of light-receiving lens arrays 31 which are disposed along one row is defined as Ld, and the amount of received light at a light-receiving element 321 facing a light-receiving lens 311 at a part other than the connection part 312 is defined as L, the expression Ld / L×100%≥90% is satisfied.

Description

Optical line sensor and surface inspection device

[0001] The present invention relates to an optical line sensor that reads an object that is relatively moved in a sub-scanning direction with a reading line that extends in a main scanning direction, and to a surface inspection device that uses the same.

[0002] A typical optical line sensor includes a light source that illuminates an object such as a paper sheet, and multiple light-receiving elements that receive light from the object illuminated by the light source. Multiple light-receiving lenses are arranged between the object and the multiple light-receiving elements, and light that passes through each light-receiving lens is received by each light-receiving element. To read an image of an object, the object is illuminated with light from the light source as it is transported along the sub-scanning direction, and multiple light-receiving elements receive light from the object along a reading line extending in the main scanning direction, thereby obtaining multiple lines of data as image data of the object.

[0003] For example, the image sensor unit illustrated in Patent Document 1 below employs a configuration in which multiple short light-receiving lens arrays (imaging element arrays) are connected in the main scanning direction. Each light-receiving lens array is composed of multiple light-receiving lenses. Each light-receiving lens array is arranged in a line along the main scanning direction, and the ends of adjacent light-receiving lens arrays are connected to each other.

[0004] JP 2014-140157 A

[0005] In a configuration in which multiple light receiving lens arrays are connected in the main scanning direction as described above, there is a problem that reading accuracy decreases at the connection points of adjacent light receiving lens arrays. To solve this problem, Patent Document 1 proposes a configuration in which the connection points of multiple sensor substrates are shifted so that they do not overlap with the connection points of multiple light receiving lens arrays in the main scanning direction. However, even if the connection points between multiple light receiving lens arrays are shifted, there is a risk of missing pixels occurring due to a decrease in light intensity at the connection points.

[0006] The present invention has been made in view of the above-mentioned circumstances, and aims to provide an optical line sensor and a surface inspection device that can suppress a decrease in the amount of light due to the connection portion of the light receiving lens array.

[0007] (1) The optical line sensor according to the present invention is an optical line sensor that reads an object that is relatively moved along the sub-scanning direction with a reading line extending in the main scanning direction, and includes a plurality of light-receiving lenses and a plurality of light-receiving elements. The plurality of light-receiving lenses are arranged in a line along the main scanning direction and transmit light from an illuminated object. The plurality of light-receiving elements are arranged in a line along the main scanning direction and receive light that has transmitted through the plurality of light-receiving lenses. The working distance W.D. of the light-receiving lenses is 75 mm ≥ W.D. ≥ 40 mm. The radius of the light-receiving lenses is X ｓｌａ , the field radius of the light receiving lens is R ０ When m = R ０ / X ｓｌａ The degree of overlap m, expressed as the above formula, satisfies m≧7. The plurality of light-receiving lenses constitute a plurality of light-receiving lens arrays, each consisting of two or more light-receiving lenses. When the plurality of light-receiving lens arrays are arranged in a row, the amount of light received by the light-receiving elements facing the connection portions of the light-receiving lens arrays is Ld, and the amount of light received by the light-receiving elements facing the light-receiving lenses other than the connection portions is L, and the relationship Ld / L×100%≧90% is satisfied.

[0008] With this configuration, by separating the light receiving lens from the object so that the working distance W.D. of the light receiving lens satisfies 75 mm≧W.D.≧40 mm and by setting the degree of overlap m between the multiple light receiving lenses to m≧7, it is possible to keep the reduction rate of the amount of light received by the light receiving element facing the connection portion to less than 10% relative to the amount of light received by the light receiving element facing the light receiving lens other than at the connection portion of the light receiving lens array. This makes it possible to suppress the reduction in light amount due to the connection portion of the multiple light receiving lens arrays.

[0009] (2) The modulation transfer function at the connection point of the plurality of light receiving lens arrays when arranged in a row is called MTF. ｄ , when the modulation transfer function of the light receiving lens other than the connection portion is MTF, MTFｄ It is preferable to satisfy the condition / MTF×100%≧90%.

[0010] With this configuration, it is possible to suppress a decrease in the modulation transfer function at the connection points of a plurality of light receiving lens arrays.

[0011] (3) The light receiving lens array is an erecting equal-magnification multi-lens array.

[0012] With this configuration, when a plurality of refractive index squared distribution lenses, particularly an erect equal-magnification multi-lens array, is used as the light-receiving lens array, it is possible to suppress a decrease in the amount of light due to the connection parts of the light-receiving lens array. An example of an erect equal-magnification multi-lens array is a SELFOC lens array (SELFOC is a registered trademark manufactured by Nippon Sheet Glass Co., Ltd.).

[0013] (4) A surface inspection device according to the present invention inspects the surface of an object using the optical line sensor.

[0014] With this configuration, when a plurality of erect equal-magnification multi-lens arrays are used as a light-receiving lens array, it is possible to suppress a decrease in the amount of light due to the connection portions, and to inspect the surface of the object with high accuracy.

[0015] The light receiving lens array described above is not limited to a configuration in which the lenses are arranged in one row, but may be an array in which the lenses are arranged in two or more rows. In this case, the degree of overlap m is larger than in a configuration in which the lenses are arranged in one row, so that the reduction in the amount of light at the joint can be further suppressed.

[0016] According to the present invention, it is possible to suppress a decrease in the amount of light due to connections between a plurality of light receiving lens arrays.

[0017] 1 is a cross-sectional view schematically illustrating the overall configuration of an optical line sensor in one embodiment; FIG. 2 is an exploded perspective view of an optical line sensor in one embodiment; FIG. 3 is a schematic diagram illustrating the configuration of a portion of the optical line sensor of FIG. 1; FIG. 4 is a schematic diagram for explaining the relationship between the working distance W.D. of a light-receiving lens and the field of view diameter; FIG. 5 is a diagram illustrating an example of a light-receiving lens array, where (A) shows the output value of each light-receiving element and (B) shows the modulation transfer function of the light-receiving lens array; and FIG. 6 is a diagram illustrating a comparative example of a light-receiving lens array, where (A) shows the output value of each light-receiving element and (B) shows the modulation transfer function of the light-receiving lens array.

[0018] 1. Overall Configuration of Optical Line Sensor First, the overall configuration of optical line sensor 1 according to one embodiment of the present invention will be described using Figures 1 to 3. Figure 1 is a cross-sectional view that schematically shows the overall configuration of optical line sensor 1 according to one embodiment. Also, Figure 2 is an exploded perspective view of optical line sensor 1 according to one embodiment. Figure 3 is a schematic diagram showing the configuration of a portion of optical line sensor 1 shown in Figure 1. Optical line sensor 1 obtains image information by reading an object S transported in the sub-scanning direction with a reading line L that extends in the main scanning direction. Hereinafter, the main scanning direction will be referred to as the X direction, the sub-scanning direction (the direction in which object S is transported) as the Y direction, and the direction perpendicular to the X and Y directions as the Z direction.

[0019] Examples of the object S include thin objects such as printed paper sheets and films, but are not limited thereto. A thick object may also be used. The optical line sensor 1 irradiates the object S with light and receives the light reflected from the object S at a focal point 51 on an inspection surface (focal plane) 50 using the light-receiving element array 32. The optical line sensor 1 can also be used to inspect the surface of the object S, in which case the optical line sensor 1 constitutes a surface inspection device. However, the optical line sensor 1 is not limited to a configuration in which the light-receiving element array 32 receives the light reflected from the object S. The optical line sensor 1 may also be configured to receive the light transmitted through the object S. Furthermore, instead of conveying the object S in the Y direction, the optical line sensor 1 may be moved relative to a stationary object S, thereby achieving sub-scanning along the Y direction. In other words, any configuration is acceptable as long as the object S is moved relative to the optical line sensor 1 in the Y direction.

[0020] The optical line sensor 1 includes an illumination unit 2, a light-receiving unit 3, and a pair of holding members 4. The illumination unit 2 is elongated along the X direction and irradiates light onto the object S along an illumination optical axis A1 perpendicular to the X direction. The light-receiving unit 3 is elongated along the X direction and receives light emitted from the illumination unit 2 and reflected by the object S along a light-receiving optical axis A2 perpendicular to the X direction, and performs photoelectric conversion to output an electrical signal. The pair of holding members 4 are separably attached to both ends of the light-receiving unit 3 in the X direction. The light-receiving optical axis A2 is, for example, perpendicular to the object S and parallel to the Z direction. On the other hand, the illumination optical axis A1 is, for example, inclined with respect to the direction perpendicular to the object S (Z direction). The inclination angle of the illumination optical axis A1 with respect to the Z direction is not particularly limited, but is preferably within a range of 10 to 80 degrees, more preferably within a range of 20 to 70 degrees, and even more preferably within a range of 30 to 60 degrees.

[0021] Here, two illumination units 2 are provided, each disposed symmetrically with respect to the light-receiving optical axis A2, but this is not limiting, and for example, a configuration in which only one illumination unit 2 is provided may also be provided. Below, the configuration and assembly structure of one illumination unit 2 in the optical line sensor 1 provided with two illumination units 2 will be described, but the other illumination unit 2 has the same configuration and assembly structure.

[0022] The lighting unit 2 includes a plurality of LEDs 21, an LED board 22, a condensing lens 23, and a lighting housing 24. The lighting unit 2 is a light source unit that irradiates light toward the target object S and includes a plurality of LEDs 21 arranged in a line along the main scanning direction. Each LED 21 is an example of a light source and emits light along an irradiation optical axis A1 that is parallel to one another. The LED board 22 has an elongated shape along the X direction, and the plurality of LEDs 21 are mounted in an array along the X direction and are energized. In other words, the LED board 22 constitutes an irradiation board on which the light sources are mounted and energized. The condensing lens 23 condenses and emits light incident from each LED 21. The lighting housing 24 has an elongated shape along the X direction and integrally holds the LED board 22 and the condensing lens 23 in a predetermined position. The irradiation optical axis A1 is the optical axis of the LED 21 or the condensing lens 23. Furthermore, since the LEDs 21 generate a lot of heat along with the light and therefore require heat dissipation, the LED substrate 22 and the lighting housing 24 are made of a material with high thermal conductivity. For example, it is preferable that the LED substrate 22 and the lighting housing 24 be made of an aluminum alloy in terms of specific gravity, rigidity, and cost.

[0023] Furthermore, when reading a fast-moving object S, the time required to read one line (i.e., exposure time) must be shortened, necessitating increased illumination intensity. Increasing the current flowing through each LED 21 increases illumination intensity, but also increases the heat generated by each LED 21. In this case, it is recommended to provide a fin-shaped heat dissipation section (not shown) on the lighting housing 24 to enhance heat dissipation. The heat dissipation section may be integral with the lighting housing or may be attached separately. The multiple fins provided on the heat dissipation section are preferably shaped and arranged to extend vertically upward, so that air heated by natural convection can easily move vertically upward. It is even more preferable to provide a fan to generate forced convection around the fins.

[0024] The light-receiving unit 3 includes a light-receiving lens array 31, a light-receiving element array 32, a light-receiving substrate 33, and a light-receiving housing 34. The light-receiving lens array 31 forms an image of light that is irradiated from the illumination unit 2 and reflected by the object S along a light-receiving optical axis A2. The light-receiving element array 32 receives the light that has been formed into an image by the light-receiving lens array 31, performs photoelectric conversion, and outputs an electrical signal. The light-receiving substrate 33 has an elongated shape along the X direction, and is equipped with the light-receiving element array 32 and is electrically connected to it. The light-receiving housing 34 has an elongated shape along the X direction, and integrally holds the light-receiving lens array 31 and the light-receiving substrate 33 in a predetermined position.

[0025] As shown in Fig. 2, protrusions 25 having a track-like cross section are provided as fitting portions at both ends (end faces) of the lighting unit 2 in the X direction. The shape of the protrusions 25 is not limited to a track shape, and may be any shape other than circular (point symmetric). The protrusions 25 may be provided integrally with the lighting unit 2, or may be provided as separate parts. Preferably, if the protrusions 25 are provided so that they can move in the X direction to change the amount of protrusion, the lighting unit 2 can be removed without removing the holding member 4.

[0026] Plate-shaped holding members 4 are attached to both ends of the lighting unit 2 in the X direction with fasteners such as screws. The light-receiving unit 3 and holding members 4 may be positioned via positioning pins (not shown), for example. Each holding member 4 is provided with a hole 41 as a fitting portion into which the protrusion 25 fits. The shape of the hole 41 corresponds to the shape of the protrusion 25, and by fitting the protrusion 25 into the hole 41, the lighting unit 2 is fixed to the holding member 4 so as not to be misaligned.

[0027] 2. Configuration of the Light Receiving System As shown in Figure 3, the light receiving element array 32 has a plurality of light receiving elements 321 arranged in a line along the X direction, and light from the illuminated object S is received by each of the light receiving elements 321 on a reading line L extending in the X direction.

[0028] As the light-receiving lens array 31, for example, a rod lens array such as a SELFOC lens array (registered trademark: Nippon Sheet Glass) is mainly used. Specifically, as shown in Fig. 3, the light-receiving lens array 31 includes a plurality of light-receiving lenses 311, each of which is associated with a respective light-receiving element 321. Therefore, light from the object S that passes through each light-receiving lens 311 is received by the corresponding light-receiving element 321. Each light-receiving lens 311 is an erecting, equal-magnification lens, and the plurality of light-receiving lenses 311 are arranged in a line along the main scanning direction to form an erecting, equal-magnification multi-lens array.

[0029] Light passing through the light-receiving lens array 31 is received by the light-receiving surface 32A of each light-receiving element 321 of the light-receiving element array 32, and a signal corresponding to the amount of received light is output from each light-receiving element 321. As the object S is transported in the Y direction along the inspection surface 50, light from the object S is continuously received by the light-receiving element array 32, and an image of the object S is obtained based on the output signal from the light-receiving element array 32. In this way, the object S transported in the Y direction is read by the light-receiving element array 32 extending in the X direction, along a reading line L formed by the light-receiving surfaces 32A of the light-receiving element array 32.

[0030] 3, in this embodiment, a plurality of light receiving lens arrays 31 are provided. Specifically, a plurality of light receiving lens arrays 31, each consisting of two or more light receiving lenses 311, are arranged in a line in the X direction. Ends of adjacent light receiving lens arrays 31 are connected to each other, and a connection portion 312 is formed between the ends of adjacent light receiving lens arrays 31. The connection portion 312 is made of, for example, an adhesive, and is an area where no light receiving lenses 311 exist.

[0031] Similarly, in this embodiment, a plurality of light receiving element arrays 32 are also provided. Specifically, a plurality of light receiving element arrays 32, each consisting of two or more light receiving elements 321, are arranged in a line in the X direction. Ends of adjacent light receiving element arrays 32 are connected to each other, and a connection portion 322 is formed between the ends of adjacent light receiving element arrays 32. The connection portion 322 is made of, for example, an adhesive, and is an area where no light receiving elements 321 exist.

[0032] 3, the connection portions 312 of the light receiving lens array 31 and the connection portions 322 of the light receiving element array 32 are arranged so as not to overlap when viewed in the Z direction. In other words, the connection portions 322 of the light receiving element array 32 are not located opposite the connection portions 312 of the light receiving lens array 31 in the Z direction, and the connection portions 312 of the light receiving lens array 31 are not located opposite the connection portions 322 of the light receiving element array 32 in the Z direction. However, the present invention is not limited to this configuration, and the connection portions 312 of the light receiving lens array 31 and the connection portions 322 of the light receiving element array 32 may be arranged so as to overlap when viewed in the Z direction.

[0033] 3. Specific Configuration of the Light-Receiving Lens FIG. 4 is a schematic diagram illustrating the relationship between the working distance W.D. of the light-receiving lens 311 and the field diameter. In FIG. 4, the field diameter of one of the light-receiving lenses 311 constituting the light-receiving lens array 31 is shown in association with each of the inspection surfaces 1 to 4 located at different working distances W.D. Specifically, the field diameters (field diameters) corresponding to each of the inspection surfaces 1 to 4 when the light-receiving lens 311 is viewed along the optical axis are shown coaxially with the light-receiving lens 311 and are also clearly shown in association with the working distance W.D. of each of the inspection surfaces 1 to 4. The working distance W.D. refers to the distance between the end face of the light-receiving lens 311 and the inspection surface.

[0034] In FIG. 4, the radius of the light receiving lens 311 is X ｓｌａ , the field radius of the light receiving lens 311 is R ０ When R is set as R, the value of the degree of overlap m expressed by the following formula (1) is shown corresponding to each of the inspection surfaces 1 to 4. That is, m=7 for inspection surface 1, m=4.5 for inspection surface 2, m=2.5 for inspection surface 3, and m=1 for inspection surface 4. The degree of overlap m is an index that indicates the rate at which the fields of view of adjacent light-receiving lenses 311 overlap, and the greater the working distance W.D., the greater the degree of overlap m. If the degree of overlap m is large, a lens system that is brighter than a single light-receiving lens 311 can be realized. m=R ０ / X ｓｌａ ...(Formula 1)

[0035] The refractive index distribution constant is A １／２ , the lens length of the light receiving lens 311 is Z ０ In this case, the field of view radius R ０ is expressed by the following formula (2): ０ is the refractive index distribution constant A １／２ and the lens length Z of the light receiving lens 311 ０ Therefore, sec(A １／２ ・Z ０ / 2) >> 1, then the field of view radius R ０ The radius X of the light receiving lens 311 ｓｌａ The overlapping degree m can be increased from several times to several tens of times. ０ ∝X ｓｌａsec (A １／２ ・Z ０ / 2) ... (Formula 2)

[0036] When focusing on any given light receiving lens 311, the greater the degree of overlap m, the more light from the group of light receiving lenses 311 surrounding that light receiving lens 311 is shared at the image forming point. As a result, the brightness of the light that has passed through each light receiving lens 311 is added to the brightness of the light that has passed through the surrounding group of light receiving lenses 311, resulting in greater brightness at the image forming point of each light receiving lens 311. In other words, the substantial F-number (effective F-number) becomes smaller than the F-number of each light receiving lens 311.

[0037] A conventional gradient index lens array such as a SELFOC lens ("SELFOC" is a registered trademark) has a short focal length, and therefore a short working distance W.D., and a small degree of overlap m. While FIG. 4 shows a case where the light-receiving lenses 311 are arranged in a single row along the main scanning direction, multiple rows of such light-receiving lenses 311 may be arranged in the sub-scanning direction, resulting in a stack of multiple light-receiving lenses 311. Even when the light-receiving lenses 311 are arranged in multiple rows (e.g., two or three rows), the degree of overlap m can be calculated by taking into account the distance between each light-receiving lens 311.

[0038] For example, if the light receiving lenses 311 are arranged in two rows (two-tiered stack), the light receiving lenses 311 in the first row and the light receiving lenses 311 in the second row are offset in the main scanning direction by the radius of the light receiving lenses 311, so the light receiving element array 32 is arranged in the center of the first and second rows in the sub-scanning direction. Therefore, when focusing on any given light receiving lens 311, the contribution rate to brightness of the group of light receiving lenses 311 around that light receiving lens 311 becomes more complex than in the case of a single row. In Figure 4, to simplify the calculation, a case will be described in which the light receiving lenses 311 are arranged in a single row, but the greater the number of rows of light receiving lenses 311, the greater the degree of overlap m and the smaller the effective F-number.

[0039] 4, in inspection plane 4 where m = 1, the fields of view of adjacent light-receiving lenses 311 do not overlap, and therefore only the brightness of light transmitted through a single light-receiving lens 311 is measured. In inspection plane 3 where m = 2.5, m > 1, but the contribution of the brightness between adjacent light-receiving lenses 311 is small. In inspection plane 2 where m = 4.5, m > 1, and although adjacent light-receiving lenses 311 contribute to the brightness between themselves, this contribution is only slight, and is not sufficient.

[0040] In contrast, on the inspection surface 1 where m = 7, adjacent light-receiving lenses 311 contribute to the brightness of each other and further contribute to the brightness of the adjacent light-receiving lenses 311. Therefore, in this embodiment, the working distance W.D. of the light-receiving lenses 311 is set so that m ≥ 7. Specifically, the working distance W.D. of each light-receiving lens 311 is 75 mm ≥ W.D. ≥ 40 mm.

[0041] When the working distance W.D. is 40 mm, the resolution of each light receiving element 321 that receives light transmitted through each light receiving lens 311 is approximately 600 dpi. When the working distance W.D. is 75 mm, the resolution is approximately 300 dpi, and from the viewpoint of the diffraction limit and S / N of each light receiving lens 311, it is preferable that the working distance W.D. be 75 mm or less. The degree of overlap m is preferably m≧10, more preferably m≧15, and even more preferably m≧20.

[0042] When the light source is an LED, the illumination system must be spaced at a distance equal to or greater than that of the light-receiving system in order to ensure the required light density. Currently available LEDs are approaching their limits in external quantum efficiency, making it difficult to further improve the light density. Therefore, taking into account the limitations on the distance of the illumination system, the working distance W.D. was set to 75 mm or less.

[0043] However, when an LD (Laser Diode) is used as the light source, the light density can be improved compared to when an LED is used. In this case, it is possible to increase the working distance (WD), but this is unrealistic because it involves a significant increase in cost.

[0044] The reason for setting the working distance W.D. to 75 mm or less will be further explained. First, the upper limit of the working distance W.D. is determined by the relationship between the diffraction limit and the numerical aperture NA, and since the numerical aperture NA decreases as the working distance W.D. increases, the working distance W.D. must be 75 mm or less to ensure a resolution of 300 dpi.

[0045] Furthermore, the upper limit of the working distance W.D. can be determined from the diffraction limit. To achieve 300 dpi using Abbe's diffraction limit formula, the focal length f≦75 mm must be satisfied. In the following formula (3), which represents the resolution of Abbe's lens system, the resolution is worse on the long wavelength side than on the short wavelength side (the diffraction limit is larger). Therefore, if the upper limit of the visible range is λ=650 nm, when f=75 mm, the diffraction limit is approximately 81.25 μm. On the other hand, at 300 dpi, one pixel is 84.7 μm, and the resolution in this case is greater than 300 dpi. d=λ / 2NA=λ / 2n sin θ (Formula 3) Here, λ is the wavelength, n is the refractive index of air, d is the resolution, and θ is the maximum angle of the incident light beam relative to the optical axis.

[0046] If the light intensity decreases by 10%, the number of electrons generated by the light receiving element 321 will also decrease by 10% due to the photoelectric effect. Since the square root of the number of generated electrons is the S / N ratio, the S / N ratio will decrease by about 5%. If the light intensity decreases by 30%, the S / N ratio will decrease by about 14%, which is nearly 10% lower than when the light intensity decreases by 10%.

[0047] Furthermore, assuming that the reciprocal of the degree of overlap m is the contribution rate of each light-receiving lens 311 in the light-receiving lens array 31 and that they contribute equally, when the working distance W.D. is 75 mm, the degree of overlap m is approximately 10, and its reciprocal is 1 / 10. If one light-receiving lens 311 is missing from the center of the light-receiving lens array 31, the light intensity will decrease by 1 / 10 x 100 = 10%, which corresponds to a 5% decrease in the S / N ratio mentioned above.

[0048] Similarly, when the degree of overlap m = 15, the light intensity is considered to decrease by 1 / 15, so the S / N ratio decreases by just over 3%. In other words, the greater the degree of overlap m, the greater the number of light receiving elements 321 at any focal position of the peripheral light receiving lens 311 that contribute to the amount of light received. However, when resolution is also taken into consideration, if the working distance W.D. is unnecessarily increased, the diffraction limit will increase, so it is preferable that the working distance W.D. be 75 mm or less.

[0049] In this embodiment, for the connection portion 312 between the ends of the adjacent light receiving lens arrays 31 described in FIG. 3, the amount of light received by the light receiving element 321 facing the connection portion 312 is set to L ｄ , when the amount of light received by the light receiving element 321 facing the light receiving lens 311 other than the connection portion 312 is L, L ｄ / L×100%≧90% is satisfied. That is, the reduction rate of the amount of light received by the light receiving element 321 facing the connection portion 312 can be made less than 10% with respect to the amount of light received by the light receiving element 321 facing the light receiving lens 311 other than the connection portion 312. This makes it possible to suppress the reduction in the amount of light at the connection portion 312 of the multiple light receiving lens array 31.

[0050] In addition, the amount of received light L ｄ When measuring the amount of received light L, a white reference medium is used as the object S. That is, the white reference medium is read by the optical line sensor 1 as the object S, and the amount of received light L is calculated based on the output values ​​from each light receiving element 321 at that time. ｄ and the amount of received light L is calculated.

[0051] In this way, increasing the working distance W.D. also increases the degree of overlap m, and the contribution to brightness by the surrounding light-receiving lenses 311 at the connection portions 312 between the ends of adjacent light-receiving lens arrays 31 increases. Therefore, even when a light-receiving element 321 faces the connection portion 312, the light-receiving lenses 311 around the connection portion 312 can compensate for the amount of light received by the light-receiving element 321 so that the amount of light received by the light-receiving element 321 does not decrease. In other words, by increasing the degree of overlap m and increasing the working distance W.D. at the same time, the contribution rate of the brightness of each light-receiving lens 311 to the entire lens system can be reduced, and the reduction in light amount due to the connection portion 312 can be suppressed.

[0052] That is, in this embodiment, m≧7 and 75 mm≧W.D.≧40 mm, and the degree of overlap m and working distance W.D. can be increased simultaneously compared to the conventional configuration. In the conventional configuration, m≦5 and W.D.≦20 mm, so by setting m≧7 and W.D.≧40 mm as in this embodiment, it is possible to suppress the reduction in light intensity due to the connection portion 312 more than in the conventional configuration.

[0053] 5 shows an example of the light-receiving lens array 31, where (A) shows the output value of each light-receiving element 321 and (B) shows the modulation transfer function of the light-receiving lens array 31. The modulation transfer function is a parameter for evaluating lens performance and is expressed by the amplitude of an AC waveform as shown in (B). The beat waveform shown in (B) is a moire pattern caused by interference between the Line & Space test chart and the pitch of the light-receiving element array 32.

[0054] In this example, the working distance W.D. was 50 mm, and the degree of overlap m≧7. In this case, at the connection 312 (the dashed line portion in FIG. 5 ) between the ends of adjacent light-receiving lens arrays 31, the reduction rate of the amount of light received by the light-receiving element 321 shown in (A) was less than 10%. Furthermore, at the connection 312 (the dashed line portion in FIG. 5 ), the modulation transfer function shown in (B) was larger than the modulation transfer function outside the connection 312, and no degradation of the modulation transfer function was observed.

[0055] The modulation transfer function at the connection portion 312 of the plurality of light receiving lens arrays 31 arranged in a line is defined as MTF. ｄ , when the modulation transfer function of the light receiving lens 311 other than the connection portion 312 is MTF, MTF ｄ It is preferable to satisfy the condition / MTF×100%≧90%. This makes it possible to suppress a decrease in the modulation transfer function at the connection portion 312 of the plurality of light receiving lens arrays 31.

[0056] FIG. 6 shows a comparative example of the light-receiving lens array 31, where (A) shows the output value of each light-receiving element 321 and (B) shows the modulation transfer function of the light-receiving lens array 31. In this comparative example, the working distance W.D. is 17 mm and the overlap degree m is less than 7. In this case, at the connection 312 (the dashed line portion in FIG. 6 ) between the ends of adjacent light-receiving lens arrays 31, the reduction rate of the amount of light received by the light-receiving element 321 shown in (A) was 30% or more. Furthermore, at the connection 312 (the dashed line portion in FIG. 6 ), the modulation transfer function shown in (B) was smaller than the modulation transfer function outside the connection 312, indicating degradation of the modulation transfer function.

[0057] REFERENCE SIGNS LIST 1 Optical line sensor 2 Illumination unit 3 Light receiving unit 21 LED 22 LED board 23 Condenser lens 24 Illumination housing 25 Protrusion 31 Light receiving lens array 32 Light receiving element array 33 Light receiving board 34 Light receiving housing 50 Inspection surface 311 Light receiving lens 312 Connection part 321 Light receiving element 322 Connection part

Claims

1. An optical line sensor that reads an object that is relatively moved along the sub-scanning direction with a reading line extending in the main scanning direction, comprising: a plurality of light-receiving lenses that are arranged in a line along the main scanning direction and transmit light from an illuminated object; and a plurality of light-receiving elements that are arranged in a line along the main scanning direction and receive light that has passed through the plurality of light-receiving lenses, wherein the working distance W.D. of the light-receiving lenses is 75 mm ≥ W.D. ≥ 40 mm, and the radius of the light-receiving lenses is X. ｓｌａ , the field radius of the light receiving lens is R ０ When m = R ０ / X ｓｌａ The degree of overlap m expressed by the following formula is m≧7, and the plurality of light receiving lenses constitute a plurality of light receiving lens arrays each consisting of two or more of the light receiving lenses, and when the plurality of light receiving lens arrays are arranged in a row, the amount of light received by the light receiving element facing the connection portion of the plurality of light receiving lens arrays is L ｄ When the amount of light received by the light receiving element facing the light receiving lens other than the connection portion is L, L ｄ An optical line sensor characterized by satisfying / L×100%≧90%.

2. The modulation transfer function at the connection point of the plurality of light receiving lens arrays when they are arranged in a row is called MTF. ｄ , when the modulation transfer function of the light receiving lens other than the connection portion is MTF, MTF ｄ 2. The optical line sensor according to claim 1, wherein / MTF×100%≧90% is satisfied.

3. The optical line sensor according to claim 1, wherein the light receiving lens array is an erecting equal magnification multi-lens array.

4. A surface inspection device that uses the optical line sensor according to claim 1 to inspect the surface of an object.

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