JP4480379B2 - Imaging lens - Google Patents

Description

  The present invention relates to an inverted Ernostar imaging lens having a four-group four-lens configuration, and more particularly to an imaging lens that is mounted on a mounting machine that mounts electronic components on a substrate and is used in a camera for recognizing the components.

In the process of mounting electronic components on the board, in order to accurately position each electronic component at a predetermined position, an image of the electronic component sucked by the suction nozzle is taken by a recognition camera, and the obtained image data is calculated. By processing and finely adjusting the position of the suction nozzle, a slight positional deviation of the electronic component is corrected.
As an imaging lens used for this recognition camera, it is necessary to enhance the image side telecentricity in order to avoid the phenomenon that the image suddenly becomes dark around the screen, that is, the so-called shading phenomenon. In many cases, the reverse Ernostar lens is used. For the same reason, this inverted Ernostar type lens is also widely used in applications such as digital cameras (see, for example, Patent Document 1, Patent Document 2, and Patent Document 3).

Japanese Patent Laid-Open No. 5-40220 JP-A-9-258100 JP 2002-90620 A

By the way, the lens developed for use in the above-described conventional digital camera or the like has been improved to satisfy predetermined optical performance while paying attention to image side telecentricity. Therefore, it is conceivable to apply these lenses to a recognition camera mounted on a mounting machine for mounting electronic components and the like.
However, even if the inverse Elnostar type lens satisfies a predetermined requirement in terms of performance, there is a problem that the number of lenses is four, which makes the cost higher than an inexpensive triplet type lens.

  The present invention has been made in view of the above circumstances, and its object is to provide a three-lens configuration in which various aberrations are well corrected while being a four-group four-lens inverted Ernostar lens. It is an object of the present invention to provide an imaging lens that is kept at the same price as a triplet type lens.

The imaging lens of the present invention includes, in order from the object side to the image plane side, a plano-convex first lens having a positive refractive power and a convex surface facing the object side, an aperture stop having a predetermined aperture, and a negative aperture A biconcave second lens having a refractive power of 2 and a concave surface facing the object side and the image side, a meniscus third lens having a positive refractive power and a concave surface facing the object side, and a positive refractive power And a biconvex fourth lens having a convex surface facing the object side and the image side, and the following conditional expression
R8 = -R9,
However, it is characterized in that R8: radius of curvature of the convex surface on the object side in the fourth lens and R9: radius of curvature of the convex surface on the image side in the fourth lens are satisfied.
According to this configuration, the subject light emitted from the subject (object) passes through the first lens, is limited to a predetermined light flux by the aperture stop, and passes through the second lens, the third lens, and the fourth lens. After that, the light enters the imaging surface. Here, since the first lens is a plane on the image plane side, and the fourth lens is a symmetric biconvex lens having biconvex surfaces formed at the same radius of curvature, processing correspondingly. By sharing tools, processing setup, etc. can be reduced. Therefore, the processing time of the lens can be shortened, and the processing cost can be reduced.

Further, the imaging lens of the present invention includes, in order from the object side to the image plane side, a plano-convex first lens having a positive refractive power and a convex surface facing the object side, and an aperture stop having a predetermined aperture. A biconcave second lens having negative refractive power and having a concave surface facing the object side and the image surface side; a meniscus third lens having positive refractive power and having a concave surface facing the object side; It consists of a fourth lens of biconvex shape having its convex surface facing the object side and the image side has a refractive power, the following conditional expression
R 4 = -R5,
R8 = -R9,
Where R4: radius of curvature of the concave surface on the object side in the second lens, R5: radius of curvature of the concave surface on the image side in the second lens, R8: radius of curvature of the convex surface on the object side in the fourth lens, R9: fourth lens Satisfying the radius of curvature of the convex surface on the image plane side of the lens.
According to this configuration, the subject light emitted from the subject (object) passes through the first lens, is limited to a predetermined light flux by the aperture stop, and passes through the second lens, the third lens, and the fourth lens. After that, the light enters the imaging surface. Here, the first lens is a flat image-side surface, the second lens is a symmetrical biconcave lens having biconvex surfaces formed at the same radius of curvature, and the fourth lens is Since the biconvex lens is a symmetrical biconvex lens with the same radius of curvature, the machining setup can be further reduced by sharing the machining tool, thereby further reducing the lens machining time. The processing cost can be further reduced.

In the configuration satisfying the above conditional expressions R4 = -R5 and R8 = -R9 , the following conditional expressions
R 1 = -R7,
However, it is possible to adopt a configuration that satisfies R1: the radius of curvature of the convex surface on the object side in the first lens, and R7: the radius of curvature of the convex surface on the image side in the third lens.
According to this configuration, since the convex surface on the object side of the first lens and the convex surface on the image surface side of the third lens are formed with the same radius of curvature, they can be processed using the same tool. Therefore, the processing time can be further shortened, the processing setup can be further reduced, and the processing cost can be further reduced.

In each of the above-described configurations, it is possible to employ a configuration in which the aperture stop is disposed adjacent to the image-side surface of the first lens.
According to this configuration, when the aperture stop is held, it is only necessary to place it in contact with the plane of the first lens, and the structure of the lens holding frame and the like is simplified.

In each of the above configurations, a configuration in which the first lens, the third lens, and the fourth lens are formed of the same glass material can be employed.
According to this configuration, it is possible to reduce the overall cost by forming the three lenses from the same glass material.

  As described above, according to the imaging lens of the present invention, it is possible to obtain an inexpensive imaging lens in which various aberrations are favorably corrected although it is a four-group, four-element inverted Ernostar type lens. Specifically, an imaging lens with a brightness (F value) of about 2.9 and distortion aberration suppressed to within ± 0.1% can be obtained at the same price as an inexpensive triplet type lens with three lenses. It is done.

DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, exemplary embodiments of the invention will be described with reference to the accompanying drawings. 1 and 2 are a basic configuration diagram and an optical path diagram showing an embodiment of an imaging lens according to the present invention.
In the imaging lens according to this embodiment, as shown in FIG. 1, a plano-convex first lens 1 having a positive refractive power and a convex surface facing the object side from the object side to the image surface side, and a negative A birefringent second lens 2 having a refractive power of 2 and a concave surface facing the object side and the image surface side, a meniscus third lens 3 having a positive refractive power and a concave surface facing the object side, and a positive Biconvex fourth lenses having refractive power and having convex surfaces facing the object side and the image surface side are sequentially arranged. An aperture stop 5 having a predetermined aperture is arranged between the first lens 1 and the second lens 2 and adjacent to the first lens 1.

  In this arrangement, a glass plate 6 made of a parallel plate for dust prevention is arranged in front of the first lens 1 (object side), and a CCD or the like is behind the fourth lens 4 (image surface side). A glass plate 7 made of a parallel plate for protecting the image pickup device is disposed, and an image pickup surface (image surface) P of a CCD as the image pickup device is disposed behind the glass plate 7.

  Here, in the first lens 1, the aperture stop 5, and the second lens 4 to the fourth lens 4, as shown in FIG. 1, each surface is Si (i = 1 to 9), and the curvature of each surface Si. The radius is Ri (i = 1 to 9), the refractive index of the first lens 4 to the fourth lens 4 with respect to the d-line is Ni (i = 1 to 4), and the Abbe number is νi (i = 1 to 4). Further, each surface interval (thickness, air interval) on the main optical axis L from the first lens 1 to the imaging surface P is represented by Di (i = 1 to 9).

In the imaging lens having the above-described configuration, as shown in FIG. 2, a light beam (subject light) emitted from an object passes through the dust-proof glass plate 6 and then passes through the first lens 1 to the front surface (convex surface). ) Refracted at S1 and rear surface (plane) S2, limited to a predetermined light flux by the aperture stop 5, and refracted at the front surface (concave surface) S4 and rear surface (concave surface) S5 when passing through the second lens 2, Refracts at the front surface (concave surface) S6 and the rear surface (convex surface) S7 when passing through the three lenses 3.
When passing through the fourth lens 4, the light is refracted at the front surface (convex surface) S <b> 8 and the rear surface (convex surface) S <b> 9, and after passing through the glass plate 7, reaches the imaging surface P of the CCD.

The first lens 1 is a plano-convex lens having a positive refractive power, which is made of a glass material and has a convex surface S1 facing the object side and a plane S2 facing the image surface side. Here, the surface S2 on the image plane side is a flat surface, so that processing is unnecessary, and cost is reduced.
The second lens 2 is a biconcave lens having a negative refractive power, which is made of a glass material and has a concave surface S4 facing the object side and a concave surface S5 facing the image surface side. Here, the radii of curvature R4 and R5 of both concave surfaces S4 and S5 are conditional expressions (1).
(1) R4 = −R5,
By forming the second lens 2 to have the same curvature radius so as to satisfy the above, it is possible to make the second lens 2 symmetrical and reduce the processing cost, and at the same time, it is possible to prevent erroneous assembly when the lens is assembled. In the equation (1), the minus sign of the radius of curvature R5 means that the curvature of the concave surface S5 is opposite to the curvature of the concave surface S4.

The fourth lens 4 is a biconvex lens having a positive refractive power and made of the same glass material as that of the first lens 1 and having a convex surface S8 facing the object side and a convex surface S9 facing the image surface side. is there. Here, the radii of curvature R8, R9 of both convex surfaces S8, S9 are conditional expressions (2).
(2) R8 = −R9,
If the fourth lens 4 is formed so as to have the same radius of curvature so as to satisfy the above, the processing cost can be reduced, and at the same time, erroneous assembly when the lens is assembled can be prevented. In the expression (2), the minus sign of the radius of curvature R9 means that the curvature of the convex surface S9 is opposite to the curvature of the convex surface S8.

The third lens 3 is a meniscus lens having a positive refractive power and made of the same glass material as that of the first lens 1 and having a concave surface S6 facing the object side and a convex surface S7 facing the image surface side. .
Here, the curvature radii R7 and R1 of the convex surface S7 on the image plane side of the third lens 3 and the convex surface S1 on the object side of the first lens 1 are defined by the conditional expression (3).
(3) R1 = −R7,
By forming so as to satisfy the same curvature radius, the processing cost can be further reduced. In the expression (3), the minus sign of the radius of curvature R7 means that the curvature of the convex surface S7 is opposite to the curvature of the convex surface S1.

  In the imaging lens having the above configuration, a configuration that satisfies only conditional expression (1), a configuration that satisfies only conditional expression (2), a configuration that satisfies only conditional expression (3), or conditional expression (1) , (2), (3) can be adopted to simultaneously satisfy the structure, and in any case, an inexpensive imaging lens in which various aberrations are well corrected is provided. Can do. Further, by forming the first lens 1, the third lens 3, and the fourth lens 4 from the same glass material, the overall cost can be further reduced, and a cheaper imaging lens can be provided.

As shown in FIG. 1, the aperture stop 5 is disposed adjacent to the plane S <b> 2 on the image plane side of the first lens 1. Thus, when the aperture stop 5 is held, it is only necessary to place it in contact with the plane S2 of the first lens 1, and the structure of the lens holding frame and the like is simplified.
Examples with specific numerical values of the imaging lens having the above-described configuration will be described below as Examples 1 to 4.

  Table 1 shows main specification specifications in the first embodiment, and Table 2 shows various numerical data (setting values). In addition, an aberration diagram regarding spherical aberration, astigmatism, and distortion in Example 1 is the result shown in FIG. In the astigmatism shown in FIG. 3, S represents the aberration on the sagittal plane, and M represents the aberration on the meridional plane.

  In Example 1 above, the angle of view (2ω) is 27.7 °, the brightness (F value) is 2.9, the distortion is within ± 0.1%, and other aberrations are well corrected. An inexpensive imaging lens can be obtained.

  Table 3 shows main specification specifications in the second embodiment, and Table 4 shows various numerical data (setting values). In addition, an aberration diagram regarding spherical aberration, astigmatism, and distortion in Example 2 is the result shown in FIG. In the astigmatism shown in FIG. 4, S represents the aberration on the sagittal plane, and M represents the aberration on the meridional plane.

  In Example 2 above, the angle of view (2ω) is 18.8 °, the brightness (F value) is 2.8, the distortion is within ± 0.1%, and other aberrations are well corrected. An inexpensive imaging lens can be obtained.

  Table 5 shows main specification specifications in Example 3, and Table 6 shows various numerical data (setting values). In addition, an aberration diagram regarding spherical aberration, astigmatism, and distortion in Example 3 is the result shown in FIG. In the astigmatism shown in FIG. 5, S represents an aberration on the sagittal plane, and M represents an aberration on the meridional plane.

  In Example 3 above, the field angle (2ω) is 27.6 °, the brightness (F value) is 2.9, the distortion is within ± 0.1%, and other aberrations are corrected well. An inexpensive imaging lens can be obtained.

  Table 7 shows main specification specifications in Example 4, and Table 8 shows various numerical data (setting values). In addition, an aberration diagram regarding spherical aberration, astigmatism, and distortion in Example 4 is the result shown in FIG. In the astigmatism shown in FIG. 6, S represents an aberration on the sagittal plane, and M represents an aberration on the meridional plane.

  In Example 4 above, the angle of view (2ω) is 27.7 °, the brightness (F value) is 2.9, the distortion is within ± 0.1%, and other aberrations are well corrected. An inexpensive imaging lens can be obtained.

  As described above, the imaging lens of the present invention can be applied to a recognition camera mounted on a mounting machine that mounts electronic components or the like on a substrate, and has an angle of view of about 20 ° to 25 °. As long as it captures an image, it can be suitably used in other applications.

It is a basic lineblock diagram showing one embodiment of an imaging lens concerning the present invention. It is an optical path figure in the imaging lens shown in FIG. FIG. 4 is an aberration diagram of spherical aberration, astigmatism, and distortion in the imaging lens according to Example 1. FIG. 6 is an aberration diagram of spherical aberration, astigmatism, and distortion in the imaging lens according to Example 2. FIG. 6 is an aberration diagram of spherical aberration, astigmatism, and distortion in the imaging lens according to Example 3. FIG. 9 is an aberration diagram of spherical aberration, astigmatism, and distortion in the imaging lens according to Example 4.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 1st lens 2 2nd lens 3 3rd lens 4 4th lens 5 Aperture diaphragms 6 and 7 Glass plate P Imaging surface L Main optical axis

Claims (5)

In order from the object side to the image plane side,
A plano-convex first lens having positive refractive power and having a convex surface facing the object side;
An aperture stop having a predetermined aperture;
A biconcave second lens having negative refractive power and having a concave surface directed toward the object side and the image plane side;
A third meniscus lens having positive refractive power and a concave surface facing the object side;
A biconvex fourth lens having positive refractive power and having a convex surface facing the object side and the image surface side;
Made, an image pickup lens satisfies the following condition.
R8 = -R9,
Where R8: radius of curvature of convex surface on the object side in the fourth lens,
R9: radius of curvature of the convex surface on the image plane side in the fourth lens. In order from the object side to the image plane side,
A plano-convex first lens having positive refractive power and having a convex surface facing the object side;
An aperture stop having a predetermined aperture;
A biconcave second lens having negative refractive power and having a concave surface directed toward the object side and the image plane side;
A third meniscus lens having positive refractive power and a concave surface facing the object side;
A biconvex fourth lens having positive refractive power and having a convex surface facing the object side and the image surface side;
Made, an image pickup lens satisfies the following condition.
R 4 = -R5,
R8 = -R9,
Where R4: radius of curvature of concave surface on the object side in the second lens,
R5: radius of curvature of concave surface on the image plane side in the second lens,
R8: radius of curvature of the convex surface on the object side in the fourth lens,
R9: radius of curvature of the convex surface on the image plane side in the fourth lens. The imaging lens according to claim 2, wherein the following conditional expression is satisfied.
R 1 = -R7,
Where R1: radius of curvature of the convex surface on the object side in the first lens,
R7: radius of curvature of the convex surface on the image plane side in the third lens. The aperture stop is disposed adjacent to the image side surface of the first lens,
The imaging lens according to any one of claims 1 to 3 . The first lens, the third lens, and the fourth lens are formed of the same glass material.
Claims 1, characterized in that to the imaging lens according to 4 or.

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