JP2014002252A - Imaging optical system and imaging device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wide-angle imaging optical system excellent in optical performance even though it is compact and thin in physical dimensions by two elements of a lens, and to provide an imaging device that employs the wide-angle imaging optical system.SOLUTION: An imaging optical system is configured to arrange in order from an object side: a first lens that has a diffraction surface formed on an object-side surface in a meniscus shape with a convex surface facing the object side and has negative refractive power; an aperture stop; and a second lens that has positive refractive power in a shape with a convex surface facing an image side. Preferably the imaging optical system satisfies the following conditional expressions (1) and (2). -2.2<f1/f<-1.5...(1) and 1.5<f2/f<2.0...(2), where f denotes a focal length of an entire lens system, and f1 and f2 denote respective focal lengths of the first and second lenses. The imaging device includes the imaging optical system and an imaging element that converts an optical image to be formed by the imaging optical system into an electric signal.

Description

  The present invention relates to a single-focus wide-angle imaging optical system used in an imaging apparatus equipped with a solid-state imaging device, such as a monitoring camera and an in-vehicle camera, and an imaging apparatus using the same.

  Imaging optical systems used for surveillance cameras, in-vehicle cameras, and the like are required to have good imaging performance over the entire screen while ensuring a wide angle of view. In addition, since the mounting space is often limited, it is required to be small and lightweight.

  The following Patent Documents 1, 2, and 3 have been proposed as single-focus wide-angle imaging optical systems that may be able to meet these demands. However, the single focus lens described in Patent Documents 1 and 2 is a wide-angle imaging optical system in which the number of constituent lenses is reduced, and the size and weight are reduced. The request for angle of view could not be satisfied. In addition, the single focus lens described in Patent Document 3 that overcomes this problem can achieve a wide angle of view by using three lenses, but at present, further miniaturization is desired.

JP 1993-215964 JP 2003-140038 A JP 2003-195161 A

  The present invention provides a wide-angle imaging optical system having a high optical performance while having a small size and a thin thickness due to the two-lens configuration, and an imaging apparatus using the same.

  In order to solve the above problems, an imaging optical system of the present invention includes, in order from the object side, a first lens having a negative refractive power of a meniscus shape having a convex diffractive surface facing the object side, an aperture stop, and an image side And a second lens having a positive refracting power and having a convex surface facing the surface.

  Preferably, the following conditional expressions (1) and (2) are satisfied.

−2.2 <f1 / f <−1.5 (1)
1.5 <f2 / f <2.0 (2)
However,
f: Focal length of the entire lens system f1: Focal length of the first lens f2: Focal length of the second lens In order to solve the above problems, an imaging apparatus according to the present invention includes any one of the imaging optical systems described above and the imaging thereof. And an imaging device that converts an optical image formed by the optical system into an electrical signal.

  According to the present invention, it is possible to provide a wide-angle imaging optical system having a high optical performance in which various aberrations are well corrected by a two-lens configuration. As a result, it is possible to realize a compact wide-angle imaging optical system suitable for mounting on a surveillance camera or a vehicle-mounted camera.

It is a figure which shows the basic composition of the imaging optical system of this embodiment. In this embodiment, it is a figure which shows the surface number provided with respect to the aperture | diaphragm | squeeze part of an imaging optical system, and each lens. FIG. 4 is an aberration diagram showing spherical aberration and astigmatism in Example 1. 6 is a diagram illustrating a configuration of an imaging optical system employed in Example 2. FIG. In Example 2, it is an aberrational figure which shows spherical aberration and astigmatism. 6 is a diagram illustrating a configuration of an imaging optical system employed in Example 3. FIG. In Example 3, it is an aberrational figure which shows spherical aberration and astigmatism. 1 is a diagram illustrating a basic configuration of an imaging apparatus according to an embodiment of the present invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 shows the lens configuration of the embodiment in an optical section. In these embodiments, in order from the object side, the first lens 110, the aperture stop 120, the second lens 130, the cover glass 140, an image sensor 150 such as a CCD (Charge Coupled Device) or a CMOS (Complementary Mental-Oxide Semiconductor device) is provided. This is a single-focus lens 100 having a two-lens configuration.

  In the imaging optical system embodying the present invention, the two lenses are, in order from the object side, a first lens 110 having a negative refractive power, an aperture stop 120, and a second lens 130 having a positive refractive power. It is arranged. With a wide-angle lens, it is necessary to shorten the focal length in order to obtain a wide angle of view, but the back focus must be longer than the focal length due to mechanical limitations. Therefore, a lens having a negative refractive power is arranged in the front, once incident light is diverged, and then condensed by a lens having a positive rear refractive power, so that the principal point of the lens system is moved backward. It is possible to ensure a long back focus compared to the focal length. Specifically, light is diverged by a negative first lens and condensed by a positive second lens. By disposing a negative lens on the object side, various aberrations can be favorably corrected while obtaining a negative refractive power sufficient to place the principal point behind. By disposing the second lens having a strong refractive power after the aperture stop, it is possible to reduce the incident angle on the image plane and correct aberrations satisfactorily.

  In the imaging optical system, the first lens 110 is a meniscus lens having a convex surface facing the object side, and the second lens 130 has a convex surface facing the image side. Since the first lens has a meniscus shape with a convex surface facing the object side, the incident angle of off-axis rays with respect to the first surface can be kept small, and the occurrence of aberration can be suppressed. Since the second lens has a convex surface on the image side and an aspheric surface on the image side, the incident angle on the image surface can be reduced. In addition, since the diffractive surface is provided on the object side surface of the first lens, chromatic aberration generated in the first lens is corrected, and a good image can be obtained.

  In the imaging optical system 100, the light incident from the object side OBJS is the object side R1 surface 1, the image surface side R2 surface 2, the surface 3 of the aperture stop 120 of the first lens 110, and the object side R3 surface 4 of the second lens 130. The image side R4 surface 5, the object side R5 surface 6 and the image surface side R6 surface 7 of the cover glass 140 are sequentially passed through and condensed on the image sensor 150.

The imaging optical system embodying the present invention is preferably configured to satisfy conditional expressions (1) and (2).
−2.2 <f1 / f <−1.5 (1)
1.5 <f2 / f <2.0 (2)
Here, f is the focal length of the entire lens system, f1 is the focal length of the first lens, and f2 is the focal length of the second lens.

  If the upper limit of (1) is exceeded, the negative refractive power increases, and the correction of chromatic aberration of magnification becomes easy, but the curvature of the first lens image side surface becomes too small, making it difficult to manufacture. When the lower limit is exceeded, the effective diameter increases because the curvature of the side surface of the first lens object decreases, and it becomes difficult to reduce the size of the lens system, and the negative refractive power necessary to obtain a wide angle of view is obtained. Disappear. Since the aberration correction of the second lens, particularly on the image side surface, is largely performed, if the upper limit of (2) is exceeded, the positive refractive power becomes too small and it becomes difficult to correct the aberration. On the other hand, if the lower limit is exceeded, the curvature of the side surface of the second lens image becomes too small, making manufacture difficult.

  Examples 1 to 3 according to specific numerical values of the imaging optical system are shown below. In the numerical examples 1 to 3, the focal length, F number, angle of view, image height, total lens length, and back focus (Bf) are as shown in Table 1 below. Similarly, in the numerical examples 1 to 3, the numerical data of the conditional expressions (1) and (2) are the values described in Table 2 below.

  For the aspherical shape of the lens described in the following numerical examples, when the direction from the object side to the image plane side is positive, the depth Z from the tangent plane to the surface vertex is h. The height of the ray, c is the reciprocal of the central radius of curvature, and k, A, B, C, and D are coefficients. Here, k represents a conic coefficient, A represents a fourth-order aspheric coefficient, B represents a sixth-order aspheric coefficient, C represents an eighth-order aspheric coefficient, and D represents a tenth-order aspheric coefficient. .

  The phase distribution φ of the diffractive surface described in the following numerical examples is expressed by Equation 2 where λ0 is the design wavelength, C1, C2, C3, and C4 are the phase coefficients of the diffractive surface, and h is the height of the light beam. It is represented by Here, C1 represents a second-order phase coefficient, C2 represents a fourth-order phase coefficient, C3 represents a sixth-order phase coefficient, and C4 represents an eighth-order phase coefficient.

The basic configuration of the lens system in Embodiment 1 is shown in FIG. 2, each numerical data (setting value) is shown in Table 3, Table 4, and Table 5, and aberration diagrams showing spherical aberration and astigmatism are shown in FIG. Each is shown.
As shown in FIG. 2, the first lens has a meniscus shape with a convex surface facing the object side, and the second lens arranged on the image side of the aperture stop has a biconvex shape. Each of the first lens and the second lens has an aspheric surface on both sides.

  Further, as shown in the figure, the distance between the R1 surface 1 and the R2 surface 2 which is the thickness of the first lens is D1, the distance between the R2 surface 2 of the first lens and the surface 3 of the aperture portion is D2, and the surface of the aperture portion 3 is the distance between the R3 surface 4 of the second lens and D3, the distance between the R3 surface 4 and the R4 surface 5 which is the thickness of the second lens is D4, the R4 surface 5 of the second lens and the R5 surface 6 of the cover glass 140. Is D5, the distance between the R5 surface 6 and the R6 surface 7 that is the thickness of the cover glass 140 is D6, and the distance between the R6 surface 7 of the cover glass 140 and the imaging surface 150 is D7.

Table 3 shows the stop corresponding to each surface number of the imaging optical system in Example 1, the radius of curvature R, the interval D, the refractive index Nd, and the dispersion value νd of each lens. The symbol * in the table represents an aspheric surface, and # represents a diffraction surface (the same applies to the following examples). Table 4 shows the aspheric coefficient of the predetermined surface. Table 5 shows the diffraction surface coefficients of the predetermined surface.
<Numerical Example 1>

  3A and 3B show spherical aberration and FIG. 3B shows astigmatism in Example 1, respectively. The vertical axis in FIG. 3B represents the image height on the imaging plane 150. In FIG. 3B, the broken line T in the light beam having a wavelength of 587.6 nm is the value of the tangential image plane, and the solid line S is the sagittal image plane. Each value is shown. As can be seen from FIG. 3, according to the first embodiment, the spherical optical and astigmatism aberrations are corrected well, and an imaging optical system with excellent imaging performance can be obtained.

The basic configuration of the lens system in the second embodiment is shown in FIG. 4, each numerical data (setting value) is shown in Table 6, Table 7, and Table 8, and the aberration diagram showing spherical aberration and astigmatism is shown in FIG. Each is shown.
As shown in FIG. 4, the first lens 110 has a meniscus shape with a convex surface facing the object side, and the second lens 130 arranged on the image side of the aperture stop 120 has a biconvex shape. Each of the first lens and the second lens has an aspheric surface on both sides.

Table 6 shows the stop corresponding to each surface number of the imaging optical system in Example 2, the radius of curvature R, the interval D, the refractive index Nd, and the dispersion value νd of each lens. Table 7 shows the aspheric coefficient of the predetermined surface. Table 8 shows the diffraction surface coefficients of the predetermined surface.
<Numerical Example 2>

  5A and 5B, in Example 2, FIG. 5A shows spherical aberration, and FIG. 5B shows astigmatism, respectively. The vertical axis in FIG. 5B represents the image height on the imaging plane 150. As can be seen from FIG. 5, according to the second embodiment, spherical and astigmatism aberrations are satisfactorily corrected, and an imaging optical system having excellent imaging performance can be obtained.

  The basic configuration of the lens system according to Embodiment 3 is shown in FIG. 6, each numerical data (setting value) is shown in Table 9, Table 10, and Table 11, and aberration diagrams showing spherical aberration and astigmatism are shown in FIG. Each is shown.

  As shown in FIG. 6, the first lens 110 has a meniscus shape with a convex surface facing the object side, and the second lens 130 disposed on the image side of the aperture stop 120 has a biconvex shape. Each of the first lens and the second lens has an aspheric surface on both sides.

Table 9 shows the stop corresponding to each surface number of the imaging optical system in Example 3, the radius of curvature R, the interval D, the refractive index Nd, and the dispersion value νd of each lens. Table 10 shows the aspheric coefficient of the predetermined surface. Table 11 shows the diffraction surface coefficients of the predetermined surface.
<Numerical Example 3>

  7A and 7B, in Example 3, FIG. 7A shows spherical aberration, and FIG. 7B shows astigmatism. The vertical axis in FIG. 7B represents the image height at the imaging plane 150. As can be seen from FIG. 7, according to the third embodiment, the spherical optical and astigmatism aberrations are favorably corrected, and an imaging optical system excellent in imaging performance can be obtained.

  Although the wide-angle imaging optical system of the present embodiment has been described above, the present invention is not limited to these examples, and various modifications can be made without departing from the scope of the invention. For example, in the above embodiment, the cover glass 140 may be provided with an infrared removing filter, or an infrared cut coat may be applied to the surface of the cover glass 140. Further, infrared coating may be applied to other lens surfaces and filters such as a low-pass filter.

  According to the wide-angle imaging optical system of the present embodiment, it is suitable for an imaging system using an imaging device, particularly a surveillance camera, an in-vehicle camera, and the like, and is a small-sized, thin and high-optical performance wide-angle imaging optical system, and An imaging device including the wide-angle imaging optical system can be realized.

  FIG. 8 shows a cross-sectional view of an embodiment of an imaging apparatus 200 using the imaging optical system 100 according to the present invention. The imaging optical system 100 and the imaging element 210 such as a CCD (Charge Coupled Device) or a CMOS (Complementary Mental-Oxide Semiconductor device) are defined and held by the housing 220. At this time, the imaging surface 150 of the imaging optical system 100 is disposed so as to coincide with the light receiving surface of the imaging element 210.

  The subject image captured by the imaging optical system 100 and formed on the light receiving surface of the imaging element 210 is converted into an electrical signal by the photoelectric conversion function of the imaging element 210 and output from the imaging apparatus 200 as an image signal.

  Since the imaging optical system 100 as described above has a small number of components, is small and lightweight, and can be compact in mounting space, it is suitable for imaging apparatuses for various applications. In addition, since it is a wide-angle imaging optical system, it can reduce the occurrence of distortion, and can form a subject image with high optical performance on the light receiving surface of the imaging device 210, and output an image signal with excellent visibility. It is possible to realize an imaging device having a high advantage in a monitoring camera, a vehicle-mounted camera, or the like.

DESCRIPTION OF SYMBOLS 100,100A-100C ... Imaging optical system 110 ... 1st lens 120 ... Aperture stop part 130 ... 2nd lens 140 ... Cover glass 150 ... Imaging surface 200 ... Imaging Device 210... Image sensor 220.

Claims (3)

  In order from the object side, a first lens having a negative refractive power having a meniscus shape with a convex surface facing the object side and a diffractive surface formed on the side surface of the object, an aperture stop, and positive refraction having a shape with the convex surface facing the image side An imaging optical system in which a second lens having power is disposed. The imaging optical system according to claim 1, wherein the following conditional expressions (1) and (2) are satisfied.
−2.2 <f1 / f <−1.5 (1)
1.5 <f2 / f <2.0 (2)
However,
f: focal length of the entire lens system f1: focal length of the first lens f2: focal length of the second lens   An imaging apparatus comprising: the imaging optical system according to claim 1; and an imaging element that converts an optical image formed by the imaging optical system into an electrical signal.