JP2019008271A - Optical system and imaging apparatus having the same - Google Patents

Abstract

To provide an optical system capable of easily achieving high optical performance with a simple lens structure even in an infrared region.SOLUTION: An optical system includes, arranged sequentially from an object side to an image side: a first lens having positive refractive power in a meniscus shape with a convex side facing the object side; and a second lens having positive refractive power in a meniscus shape with a convex side facing the object side. The first lens includes a silicon material. The second lens includes a silicon material or a germanium material. A focal distance f of an entire system and a distance D in an optical axis of the first lens and the second lens are respectively set appropriately.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical system, and is suitable for an imaging device such as a surveillance camera or an in-vehicle camera that obtains an infrared image using infrared rays having a wavelength of about 8 μm to 13 μm, for example.

  Using an optical system (infrared optical system) for light in the infrared region (wavelength 8 μm to 13 μm), the temperature distribution of the human body that cannot be obtained in the visible wavelength region (wavelength 0.4 μm to 0.7 μm) It is possible to detect and visualize thermal information such as. The material of the lens constituting the infrared optical system includes a material that transmits light in the infrared region (infrared material), such as germanium (Ge), gallium arsenide (GAAS), chalcogenide, zinc selenide (ZnSe), and zinc sulfide. (ZnS), silicon (Si), and the like.

  Patent Document 1 discloses an optical system composed of two lenses made of germanium or two lenses made of silicon. Patent Document 2 discloses an optical system composed of two lenses made of silicon. In addition, there is disclosed an optical system composed of two lenses composed of a lens made of chalcogenide or zinc selenide and a lens made of any one of silicon, gallium arsenide, germanium and chalcogenide. Patent Document 3 discloses an optical system including three lenses including one lens made of silicon and two lenses made of chalcogenide.

JP 2000-75203 A US Patent No. 9007683 JP 2017-90786 A

  Chalcogenides, zinc selenide, and zinc sulfide have a large effect on the human body. In order to construct an optical system with high resolving power using zinc selenide, zinc sulfide or the like having a low refractive index and high dispersion, many lenses are used for correcting various aberrations such as chromatic aberration and curvature of field. Is required. Further, since germanium is a rare metal, it is difficult to construct an optical system with only lenses made of germanium.

  An object of the present invention is to provide an optical system in which high optical performance can be easily obtained in the infrared region.

The optical system of the present invention is arranged in order from the object side to the image side, a first lens having a meniscus positive refractive power with a convex surface facing the object side, and a meniscus positive refraction with the convex surface facing the object side. Having a second lens of force,
The first lens is made of a silicon material,
The second lens is made of a silicon material or a germanium material,
When the focal length of the entire system is f and the distance on the optical axis between the first lens and the second lens is D,
0.90 <D / f <2.00
It is characterized by satisfying the following conditional expression.

  According to the present invention, it is possible to obtain an optical system that has a simple lens configuration and can easily obtain high optical performance in the infrared region.

Lens sectional view of Example 1 MTF and longitudinal aberration diagram of Example 1 Lens sectional view of Example 2 MTF and longitudinal aberration diagram of Example 2 Lens sectional view of Example 3 MTF and longitudinal aberration diagram of Example 3 Lens sectional view of Example 4 MTF and longitudinal aberration diagram of Example 4 Lens sectional view of Example 5 MTF and longitudinal aberration diagram of Example 5 Lens sectional view of Example 6 MTF and longitudinal aberration diagram of Example 6 Lens sectional view of Example 7 MTF and longitudinal aberration diagram of Example 7 Lens cross section of Example 8 MTF and longitudinal aberration diagram of Example 8 Lens sectional view of Example 9 MTF and longitudinal aberration diagram of Example 9 Lens sectional view of Example 10 MTF and longitudinal aberration diagram of Example 10 Schematic diagram of main parts of an imaging apparatus according to an embodiment

  The configuration of the optical system according to each embodiment of the present invention will be described. The optical system of each embodiment is an infrared optical system that forms an infrared image using infrared rays having a wavelength of 8 μm to 13 μm. The optical system of each embodiment is arranged in order from the object side to the image side, the first lens having a positive meniscus power having a convex surface facing the object side, and the positive meniscus shape having a convex surface facing the object side. A second lens having refractive power is included. The imaging apparatus of the present invention includes an optical system and an imaging element (infrared sensor) that receives an image formed by the optical system.

  FIG. 1 is a lens cross-sectional view of an optical system according to Example 1 of the present invention. 2A and 2B are an MTF diagram and a longitudinal aberration diagram, respectively, relating to the optical system of Example 1. FIG.

  FIG. 3 is a lens sectional view of Example 2 of the optical system of the present invention. 4A and 4B are an MTF diagram and a longitudinal aberration diagram, respectively, relating to the optical system of Example 2. FIG.

  FIG. 5 is a lens sectional view of Example 3 of the optical system according to the present invention. 6A and 6B are an MTF diagram and a longitudinal aberration diagram, respectively, relating to the optical system of Example 3. FIG.

  FIG. 7 is a lens sectional view of Example 4 of the optical system according to the present invention. 8A and 8B are an MTF diagram and a longitudinal aberration diagram, respectively, according to the optical system of Example 4. FIG.

  FIG. 9 is a lens sectional view of Example 5 of the optical system according to the present invention. 10A and 10B are an MTF diagram and a longitudinal aberration diagram, respectively, relating to the optical system of Example 5. FIG.

  FIG. 11 is a lens sectional view of Example 6 of the optical system according to the present invention. 12A and 12B are an MTF diagram and a longitudinal aberration diagram, respectively, relating to the optical system of Example 6. FIG.

  FIG. 13 is a lens sectional view of Example 7 of the optical system according to the present invention. 14A and 14B are an MTF diagram and a longitudinal aberration diagram, respectively, according to the optical system of Example 7. FIG.

  FIG. 15 is a lens cross section of Example 8 of the optical system according to the present invention. FIGS. 16A and 16B are an MTF diagram and a longitudinal aberration diagram, respectively, according to the optical system of Example 8. FIGS.

  FIG. 17 is a lens cross section of Example 9 of the optical system according to the present invention. 18A and 18B are an MTF diagram and a longitudinal aberration diagram, respectively, according to the optical system of Example 9. FIG.

  FIG. 19 is a lens cross-sectional view of Example 10 of the optical system according to the present invention. FIGS. 20A and 20B are an MTF diagram and a longitudinal aberration diagram according to the optical system of Example 10, respectively.

  FIG. 21 is a schematic diagram of a main part of the imaging apparatus of the present invention.

  In the lens cross-sectional view, the left side is the object side (front), and the right side is the image side (rear). L0 is an optical system. L11, L21, L31, L41, L51, L61, L71, L81, L91, and L101 are each a first lens. L12, L22, L32, L42, L52, L62, L72, L82, L92, and L102 are second lenses. L63, L73, L83, L93, and L103 are each a third lens. L104 is a fourth lens. S1, S2, S3, S4, S5, S6, S7, S8, S9, and S10 are each an aperture stop (stop).

  CG1, CG2, CG3, CG4, CG5, CG6, CG7, CG8, CG9, and CG10 are cover glasses. Each of IM1, IM2, IM3, IM4, IM5, IM6, IM7, IM8, IM9, and IM10 is an infrared sensor (imaging device).

  In the spherical aberration diagram, the vertical axis represents the pupil position. The solid line indicates spherical aberration with a wavelength of 13 μm, and the dotted line indicates a spherical aberration with a wavelength of 8 μm. In the astigmatism diagram, the vertical axis represents the image height. S1 is a sagittal image plane having a wavelength of 13 μm, and T1 is a meridional image plane having a wavelength of 13 μm. S2 is a sagittal image plane having a wavelength of 8 μm, and T2 is a meridional image plane having a wavelength of 8 μm. In the distortion aberration, the vertical axis represents the image height. Distortion is shown for a wavelength of 13 μm.

  2. Description of the Related Art Conventionally, in applications such as in-vehicle cameras and surveillance cameras, there has been a demand for an infrared optical system with a bright F number, a small entire system, and high resolution. As infrared materials, germanium (Ge) and silicon (Si) have higher refractive index and lower dispersion than other infrared materials, making it easy to construct an infrared optical system with high resolution with a small number of lenses. It is.

  In the infrared optical system, it is difficult to obtain high imaging performance unless various aberrations such as chromatic aberration, spherical aberration, and field curvature are corrected well, as in the visible optical system. Become. In order to obtain high imaging performance in an infrared optical system, it is important to appropriately set the lens configuration, for example, the number of lenses, lens materials, refractive power arrangement, and the like.

  The optical system of the present invention is arranged in order from the object side to the image side, a first lens having a meniscus positive refractive power with a convex surface facing the object side, and a meniscus positive refraction with the convex surface facing the object side. Having a second lens of force. The first lens is made of a silicon material, and the second lens is made of a silicon material or a germanium material.

  In addition, the optical system of the present invention includes a first lens and a second lens, and the second lens is made of a germanium material.

  In addition, the optical system of the present invention includes a first lens, a second lens, and a third lens having a positive refractive power with a convex surface facing the object side, arranged in order from the object side to the image side. The lens is made of silicon material or germanium material.

  In addition, the optical system of the present invention includes a first lens, a second lens, a third lens having a positive refractive power with a convex surface facing the object side, and a convex surface facing the object side. A fourth lens having a positive refractive power directed toward the lens. The fourth lens is made of a silicon material or a germanium material.

  In addition, the silicon and germanium used in each embodiment may contain some impurities.

  In the infrared optical system L0 of the present invention, the following configuration is adopted in order to improve the imaging performance.

A first lens having a meniscus positive refractive power and having a convex surface facing the object side and a second lens having a meniscus positive refractive power having a convex surface facing the object side are arranged in order from the object side to the image side. . The first lens is made of a silicon material, and the second lens is made of a silicon material or a germanium material. Let f be the focal length of the entire system, and D be the distance on the optical axis between the first lens and the second lens. At this time,
0.90 <D / f <2.00 (1)
This satisfies the conditional expression

  Conditional expression (1) corrects various aberrations such as chromatic aberration, spherical aberration, curvature of field and the like while maintaining the lens thickness of the first lens made of silicon as a thin material in each embodiment, and achieves high optical performance. It is for obtaining the optical system for infrared which has. When the upper limit value or lower limit value of conditional expression (1) is exceeded, the correction balance between curvature of field and spherical aberration is lost, and the optical performance deteriorates.

More preferably, the numerical range of conditional expression (1) is set as follows.
0.90 <D / f <1.65 (1a)

  In the infrared optical system in each embodiment, the first lens is a meniscus positive lens having a convex surface facing the object side, and the second lens is a meniscus positive lens having a convex surface facing the object side. doing. If the second lens is a meniscus positive lens with the concave surface facing the object surface, it is difficult to correct astigmatism due to the effect of the concave surface. In each embodiment, the first lens and the second lens are configured as described above. According to this, an infrared optical system having a bright F number and high resolving power can be easily obtained.

  The lens configuration of each example will be described. A general imaging lens has a lens configuration that satisfies the expression (1X) in order to correct chromatic aberration. In the formula (1X), f1 is the focal length of the first lens, f2 is the focal length of the second lens, fi is the focal length of the i-th lens, ν1 is the dispersion value (Abbe number) of the material of the first lens, and ν2 is the first The dispersion value of the material of the two lenses, νi, the dispersion value of the material of the i-th lens.

  Usually, since the dispersion value of the lens material is positive, it is preferable to make the focal length of one lens negative in order to reduce chromatic aberration. Therefore, in many cases, the lens configuration for correcting chromatic aberration is a lens configuration combining a positive lens and a negative lens.

  Further, in order to correct the curvature of field, a lens configuration that satisfies the following formula (2X) that reduces the Petzval sum is required. Since the Petzval sum correlates with the curvature of field, the curvature of field can be reduced by reducing the Petzval sum.

  In formula (2X), f1 is the focal length of the first lens, f2 is the focal length of the second lens, fi is the focal length of the i-th lens, n1 is the refractive index of the material of the first lens, and n2 is the material of the second lens. N i represents the refractive index of the material of the i-th lens.

  Since the refractive index of a general lens material is positive, it is preferable to make the focal length of one lens negative in order to reduce the Petzval sum. Therefore, in many cases, the lens configuration for correcting curvature of field is a lens configuration in which a positive lens and a negative lens are combined.

  However, when the refractive index n1, n2, ni of the lens material is very large and the dispersion values ν1, ν2, νi of the lens material are very large, it is not always necessary to use a combination of a positive lens and a negative lens. There is.

  Table 21 shows a refractive index N10 and a dispersion value ν10 of a typical material that transmits infrared rays. In Table 21, the refractive index N10 is a refractive index at a wavelength of 10 μm, and the dispersion value ν10 is a numerical value defined by the formula (3X) described later.

  In general, the larger the dispersion value, the smaller the change (dispersion) in the refractive index due to wavelength. In the formula (3X), N8 is a refractive index at a wavelength of 8 μm, and N12 is a refractive index at a wavelength of 12 μm. Since the numerical values differ slightly depending on the glass material manufacturer of each company, it is described here as an approximate value. As shown in Table 21, in the infrared, zinc selenide (ZnSe) and zinc sulfide (ZnS) have a refractive index of about 2 and a dispersion value of about 20-60. Chalcogenide has a refractive index of about 2.5 and a dispersion of 109.

  This chalcogenide has a lower dispersion than zinc selenide (ZnSe) or zinc sulfide (ZnS), but when selected as the material of the first lens, chromatic aberration is greatly generated, and good imaging performance can be obtained. difficult.

  For correction of chromatic aberration and correction of field curvature, a combination of a positive lens and a negative lens is common, but in the case of germanium (Ge) or silicon (Si), the refractive index is very large (high), The variance is also very small. Therefore, as is clear from the equations (1X) and (2X), chromatic aberration and Petzval sum are reduced in each of the first lens and the second lens.

  That is, when germanium (Ge) or silicon (Si) is used, it is not always necessary to use a combination of a positive lens and a negative lens in order to correct chromatic aberration and curvature of field. Imaging performance is obtained. Further, since germanium (Ge) and silicon (Si) have little influence on the human body, even when the lens is damaged, there is little influence on the human body and the environment.

  In order to obtain thermal information with high accuracy in an infrared camera, the F number of an infrared optical system is often set to 1.5 or less. In order to obtain good imaging performance with an F-number bright infrared optical system, it is important to correct spherical aberration proportional to the pupil diameter of the lens. This makes it useful to share and correct the amount of spherical aberration. Therefore, when the material of the infrared optical system is germanium (Ge) or silicon (Si), the combination of the positive lens and the positive lens is improved. .

  Further, in addition to the above-described aberration correction action, an infrared optical system in which aberration correction is satisfactorily obtained while securing high transmittance by making the first lens a meniscus shape and the second lens a meniscus shape is obtained. ing. The amount of spherical aberration is suppressed by gently converging the light flux by the meniscus refracting surfaces of the first lens and the second lens, and the lens thickness of the first lens made of silicon (Si) is reduced. The reduction in transmittance due to silicon is reduced.

  In the optical system of each embodiment, it is more preferable to satisfy one or more of the following conditional expressions. The focal length of the first lens is f1, and the focal length of the second lens is f2. Let the lens thickness of the first lens be t1. When the optical system according to the present invention has a third lens having a positive refractive power adjacent to the second lens on the image side of the second lens, the focal length of the third lens is f3. At this time, one or more of the following conditional expressions should be satisfied.

0.1 <f2 / f1 <3.0 (2)
1.0 <f1 / f <6.0 (3)
0.5 <f2 / f <6.0 (4)
0.001 <t1 / f1 <0.065 (5)
0.1 <f3 / f2 <2.0 (6)
0.1 <f3 / f <3.0 (7)

  Next, the technical meaning of each conditional expression described above will be described. Conditional expressions (2) to (4) are for setting the balance of the refractive powers of the first lens and the second lens appropriately and mainly reducing the occurrence of field curvature. A deviation from the numerical range of the conditional expressions (2) to (4) is not good because the correction of the field curvature is insufficient or excessive, and the imaging performance deteriorates. Conditional expression (5) relates to the ratio between the thickness of the first lens having a large lens aperture and the focal length of the first lens.

  Conditional expression (5) suppresses the generation amount of spherical aberration by gently converging the light flux between the first lens and the second lens, and suppresses the lens thickness of the first lens made of silicon (Si). This is to ensure high transmittance. If it is out of the numerical range of the conditional expression (5), the transmittance is lowered by the lens thickness while correcting the spherical aberration, which is not good.

  Conditional expressions (6) and (7) show the amount of aberration generated from each lens while appropriately setting the balance of the refractive power of the second lens and the third lens when the entire optical system is composed of three or more lenses. It is for mitigating. If the numerical ranges of conditional expressions (6) and (7) are not met, it will be difficult to converge the light beam gently with the first lens, the second lens, and the third lens, and the amount of aberration generated from each lens will increase. So not good. In particular, if the numerical values out of the conditional expressions (6) and (7) are deviated, a large amount of spherical aberration occurs from each lens, and the imaging performance deteriorates. More preferably, the numerical ranges of the conditional expressions (2) to (7) are set as follows.

0.2 <f2 / f1 <1.5 (2a)
1.5 <f1 / f <5.0 (3a)
0.7 <f2 / f <4.0 (4a)
0.01 <t1 / f1 <0.05 (5a)
0.15 <f3 / f2 <1.00 (6a)
0.5 <f3 / f <2.5 (7a)

  By increasing the F-number, it is difficult to correct high-order spherical aberration and field curvature with a small number of lenses. Therefore, it is preferable to adopt at least one aspherical shape for each lens shape.

Next, the lens configuration of the infrared optical system L0 in each embodiment will be described.
[Example 1]
FIG. 1 is a lens cross-sectional view of the optical system L0 according to the first embodiment. Example 1 is an optical system having a focal length of 25 mm and an F number of 1.0. The optical system L0 according to the first exemplary embodiment is arranged in order from the object to the image side, the first lens L11 having a positive refractive power made of a silicon (Si) material, an aperture stop S1, and a positive refractive power made of a germanium (Ge) material. The second lens L12. Hereinafter, in all of the embodiments, the material of germanium or silicon may be a single crystal or a polycrystalline material. Moreover, some impurities may be included.

  A light beam in the infrared wavelength region (8 to 13 μm) emitted from the object and guided by the optical system L0 passes through the cover glass CG1 and forms an image on the infrared sensor IM1. Incidentally, even when a cover glass is provided between the first lens L11 and the object, or even when the material of the cover glass CG1 of the infrared sensor IM1 is an infrared material other than germanium (Ge), the effect of this embodiment can be obtained. Table 1 shows numerical data of the optical system of Example 1. The unit of curvature radius and interval of each optical surface in Table 1 is mm.

  Table 2 shows aspherical shape data of the aspherical surface of the optical system L0 of Example 1. The aspherical shape is represented by the formula (4X) described later.

  FIG. 2A is an MTF diagram of the first embodiment. A typical infrared sensor has a pixel pitch of several tens of microns. Assuming that an infrared sensor having a pixel pitch of 17 μm is used as an example, the Nyquist frequency is about 30 lp / mm. In order to resolve the subject at this Nyquist frequency, an MTF value of 30% or more is empirically required. Point 11 in FIG. 2A is the MTF value at a frequency of 30 lp / mm in Example 1, and the value is 44%. It can be seen that the imaging performance is good because the MTF value is 30% or more at the Nyquist frequency of the infrared sensor IM1.

  FIG. 2 (B) shows the longitudinal aberration of Example 1. FIG. As shown in FIG. 2B, it can be seen that spherical aberration, field curvature, and chromatic aberration are well corrected.

  From the optical conditions, it was shown that a combination of a positive lens and a positive lens using germanium (Ge) and silicon (Si) as materials is good for the infrared optical system. Germanium (Ge) is a rare metal. On the other hand, silicon (Si) can be easily obtained. From the viewpoint of transmittance, silicon (Si) has a large attenuation of transmittance with respect to thickness due to a chemical factor. Therefore, a lens configuration of a combination of a positive lens and a positive lens using only germanium (Ge) as a material tends to be difficult to manufacture.

  On the other hand, a combination of a positive lens and a positive lens using only silicon (Si) requires a predetermined thickness for correcting spherical aberration, and the transmittance decreases. Furthermore, the aperture of the lens is smaller on the image side than on the object side. For this reason, it is preferable to use silicon (Si) as the material of the first lens and germanium (Ge) as the material of the second lens.

  The optical system L0 according to the first embodiment includes the first lens having a positive refractive power made of silicon (Si) and the second lens having a positive refractive power made of germanium (Ge). Thus, an infrared optical system L0 having a balanced imaging performance and transmittance is configured.

[Example 2]
FIG. 3 is a lens cross-sectional view of the optical system L0 according to the second embodiment. Example 2 is an optical system having a focal length of 25 mm and an F number of 1.0. The optical system L0 of Example 2 is arranged in order from the object to the image side, and includes an aperture stop S2, a first lens L21 having a positive refractive power made of silicon (Si) material, and positive refraction made of germanium (Ge). The second lens L22 is composed of a force.

  By arranging the position of the aperture stop S2 on the object side, the outer diameter of the lens can be reduced, so that the entire system is further downsized. A light beam in the infrared wavelength region (8 to 13 μm) emitted from the object and guided by the optical system L0 passes through the cover glass CG2 and forms an image on the infrared sensor IM2. Table 3 shows numerical data of the optical system L0 of Example 2. The unit of the radius of curvature and spacing of each optical surface in Table 3 is mm. Table 4 shows aspherical shape data of the aspherical surface of the optical system L0 of Example 2. The aspherical shape is represented by the expression (4X) as in the first embodiment.

  FIG. 4A is an MTF diagram of the second embodiment. Point 21 in FIG. 4A is the MTF value at a frequency of 30 lp / mm in Example 2, and the value is 45%. Since the MTF value is 30% or more at the Nyquist frequency of the infrared sensor IM2, it can be seen that the imaging performance is good. FIG. 4B shows longitudinal aberrations of Example 2. As shown in FIG. 4B, it can be seen that spherical aberration, field curvature, and chromatic aberration are well corrected. Thus, in the optical system L0 of the present embodiment, the position of the aperture stop may be anywhere in the optical axis direction.

[Example 3]
FIG. 5 is a lens cross-sectional view of the optical system L0 according to the third embodiment. Example 3 is an optical system having a focal length of 25 mm and an F number of 1.0. The optical system L0 of Example 3 is arranged in order from the object to the image side. The first lens L31 having a positive refractive power made of a silicon (Si) material, the aperture stop S3, and the positive refractive power made of a germanium (Ge) material. The second lens L32.

  The back focus of Example 3 is shorter than that of Example 1. A light beam in the far-infrared wavelength region (8 to 13 μm) emitted from the object and guided by the optical system L0 passes through the cover glass CG3 and forms an image on the infrared sensor IM3. Table 5 shows numerical data of the optical system L0 of Example 3. The unit of the radius of curvature and spacing of each optical surface in Table 5 is mm. Table 6 shows aspherical shape data of the aspherical surface of the optical system L0 of Example 3. The aspherical shape is represented by the expression (4X) as in the first embodiment.

  FIG. 6A is an MTF diagram of the third embodiment. A point 31 in FIG. 6A is the MTF value at a frequency of 30 lp / mm in Example 3, and the value is 53%. Since the MTF value is 30% or more at the Nyquist frequency of the infrared sensor IM3, it can be seen that the imaging performance is good. FIG. 6B shows longitudinal aberrations of Example 3. As shown in FIG. 6B, it can be seen that spherical aberration, field curvature, and chromatic aberration are well corrected.

[Example 4]
FIG. 7 is a lens cross-sectional view of the optical system L0 according to the fourth embodiment. Example 4 is an optical system having a focal length of 40 mm and an F number of 1.0. The optical system L0 of Example 4 is arranged in order from the object to the image side. The first lens L41 having a positive refractive power made of a silicon (Si) material, the aperture stop S4, and the positive refractive power made of a germanium (Ge) material. The second lens L42.

  A light beam in the infrared wavelength region (8 to 13 μm) emitted from the object and guided by the optical system L0 passes through the cover glass CG4 and forms an image on the infrared sensor IM4. Table 7 shows numerical data of the optical system L0 of Example 4. The unit of curvature radius and interval of each optical surface in Table 7 is mm. Table 8 shows aspherical shape data of the aspherical surface of the optical system L0 of Example 4. The aspherical shape is represented by the (4X) equation as in the first embodiment.

  FIG. 8A is an MTF diagram of the fourth embodiment. A point 41 in FIG. 8A is an MTF value at a frequency of 30 lp / mm in Example 2, and the value is 52%. Since the MTF value is 30% or more at the Nyquist frequency of the infrared sensor IM4, it can be seen that the imaging performance is good. FIG. 8B shows longitudinal aberrations of Example 4. As shown in FIG. 8B, it can be seen that spherical aberration, field curvature, and chromatic aberration are well corrected. The fourth embodiment is an optical system L0 having a telephoto focal length that is greater than that of the first embodiment.

[Example 5]
FIG. 9 is a lens cross-sectional view of the optical system L0 according to the fifth embodiment. Example 5 is an optical system having a focal length of 20 mm and an F number of 1.0. The optical system L0 of Example 5 is arranged in order from the object to the image side. The first lens L51 having a positive refractive power made of a silicon (Si) material, the aperture stop S5, and the positive refractive power made of a germanium (Ge) material. The second lens L52.

  In the fifth embodiment, the focal length is made wider than that in the first embodiment. A light beam in the infrared wavelength region (8 to 13 μm) emitted from the object and guided by the optical system L0 passes through the cover glass CG5 and forms an image on the infrared sensor IM5. Table 9 shows numerical data of the optical system L <b> 0 of Example 5. The unit of curvature radius and interval of each optical surface in Table 9 is mm. Table 10 shows aspherical shape data of the aspherical surface of the optical system L0 of Example 5. The aspherical shape is represented by the (4X) equation as in the first embodiment.

  FIG. 10A is an MTF diagram of Example 5. A point 51 in FIG. 10A is the MTF value at a frequency of 30 lp / mm in Example 5, and the value is 43%. Since the MTF value is 30% or more at the Nyquist frequency of the infrared sensor IM5, it can be seen that the imaging performance is good. FIG. 10B shows the longitudinal aberration of Example 5. As shown in FIG. 10B, it can be seen that spherical aberration, field curvature, and chromatic aberration are well corrected.

[Example 6]
FIG. 11 is a lens cross-sectional view of Example 6 of the optical system L0 of the present invention. Example 6 is an optical system having a focal length of 8 mm and an F number of 0.8.

  The optical system L0 of Example 6 includes the following lenses arranged in order from the object to the image side. First lens L61 having a positive refractive power made of silicon (Si) material, aperture stop S1, second lens L62 having a positive refractive power made of germanium (Ge) material, and third lens having a positive refractive power made of silicon material L13. Each of the first lens L61 to the third lens L63 has a meniscus shape with a convex surface facing the object side.

  A light beam in the infrared wavelength region (8 to 13 μm) emitted from the object and guided by the optical system L0 passes through the cover glass CG1 and forms an image on the infrared sensor IM1. Incidentally, the effect of this embodiment can be obtained when a cover glass is provided between the first lens L61 and the object, or when the material of the cover glass CG1 of the infrared sensor IM6 is an infrared material other than germanium (Ge).

  Table 11 shows numerical data of the optical system L0 of Example 6. The unit of the radius of curvature and the interval of each optical surface in Table 10 is mm. Table 12 shows aspherical shape data of the aspherical surface of the optical system L0 of Example 6. The aspherical shape is represented by the (4X) equation as in the first embodiment.

  In Table 11, the surface number indicates the order of the surface from the object side. ri represents the radius of curvature of the i-th optical surface, and di represents the distance between the i-th and (i + 1) -th. The materials of the first lens L61, the aperture stop S6, the second lens L62, the third lens L63, and the cover glass CG6 are also shown. The notations in Tables 1 and 2 are the same in the examples described later.

  FIG. 12A is an MTF diagram of the sixth embodiment. A typical infrared sensor has a pixel pitch of several tens of microns. Assuming that an infrared sensor having a pixel pitch of 12 μm is used as an example, the Nyquist frequency is about 42 lp / mm. In order to resolve the subject at this Nyquist frequency, an MTF value of 30% or more is empirically required. A point 11 in FIG. 12A is an MTF value at a frequency of 42 lp / mm in Example 6, and the value is 38%. It can be seen that since the MTF value is 30% or more at the Nyquist frequency of the infrared sensor IM6, the imaging performance is good.

  FIG. 12B shows longitudinal aberrations of Example 6. As shown in FIG. 12B, it can be seen that spherical aberration, field curvature, and chromatic aberration are well corrected.

  In Example 6, the diameter (effective diameter) of the germanium (Ge) lens is increased as the material of the second lens L62 in which the lens thickness is increased, but the material of the first lens L61 capable of reducing the thickness of the lens is silicon (Si). ) Is used. The third lens L63 having a small lens thickness and a small lens diameter is made of silicon (Si), thereby obtaining good optical performance while ensuring high transmittance.

[Example 7]
FIG. 13 is a lens cross-sectional view of Example 7 of the optical system L0 of the present invention. Example 7 is an optical system having a focal length of 18 mm and an F number of 0.8.

  The optical system L0 of Example 7 includes the following lenses arranged in order from the object to the image side. A first lens L71 having a positive refractive power made of a silicon (Si) material, an aperture stop S7, a second lens L72 having a positive refractive power made of a germanium (Ge) material, and a positive refractive power made of a germanium (Ge) material. The third lens L73 is configured. Each of the first lens L71 to the third lens L73 has a convex surface facing the object side.

  A light beam in the infrared wavelength range (8 to 13 μm) emitted from the object and guided by the optical system L0 passes through the cover glass CG7 and forms an image on the infrared sensor IM7. Table 12 shows numerical data of the optical system L <b> 0 of Example 7. The unit of the radius of curvature and the interval of each optical surface in Table 12 is mm. Table 13 shows aspherical shape data of the aspherical surface of the optical system L0 of Example 7. The aspherical shape is expressed by the expression (4X) as in the sixth embodiment.

  FIG. 14A is an MTF diagram of the seventh embodiment. A point 71 in FIG. 14A is an MTF value at a frequency of 42 lp / mm in Example 7, and the value is 39%. Since the MTF value is 30% or more at the Nyquist frequency of the infrared sensor IM2, it can be seen that the imaging performance is good. FIG. 14B shows longitudinal aberrations of Example 7. As shown in FIG. 14B, it can be seen that spherical aberration, field curvature, and chromatic aberration are well corrected. Thus, in the optical system L0 of the present invention, the position of the aperture stop may be anywhere.

[Example 8]
FIG. 15 is a lens cross-sectional view of Example 8 of the optical system L0 of the present invention. Example 8 is an optical system having a focal length of 18 mm and an F number of 0.8.

  The optical system L0 of Example 8 includes the following lenses arranged in order from the object to the image side. First lens L81 having a positive refractive power made of silicon (Si) material, aperture stop S8, second lens L82 having a positive refractive power made of silicon (Si) material, and having a positive refractive power made of silicon (Si) material It is composed of a third lens L83. Each of the first lens L81 to the third lens L83 has a convex surface facing the object side.

  The light beam in the far infrared wavelength region (8 to 13 μm) emitted from the object and guided by the optical system L0 passes through the cover glass CG8 and forms an image on the infrared sensor IM8. Table 15 shows numerical data of the optical system L <b> 0 of Example 8. The unit of curvature radius and interval of each optical surface in Table 15 is mm. Table 16 shows aspherical shape data of the aspherical surface of the optical system L0 of Example 8. The aspherical shape is represented by the expression (4X) as in the first embodiment.

  FIG. 16A is an MTF diagram of Example 8. FIG. A point 81 in FIG. 16A is the MTF value at a frequency of 42 lp / mm in Example 8, and the value is 40%. Since the MTF value is 30% or more at the Nyquist frequency of the infrared sensor IM3, it can be seen that the imaging performance is good. FIG. 16B shows longitudinal aberrations of Example 8. As shown in FIG. 16B, it can be seen that spherical aberration, field curvature, and chromatic aberration are well corrected.

[Example 9]
FIG. 17 is a lens sectional view of Example 9 of the optical system L0 of the present invention. Example 9 is an optical system having a focal length of 18 mm and an F number of 0.8.

  The optical system L0 of Example 9 includes the following lenses arranged in order from the object to the image side. First lens L91 having a positive refractive power made of silicon (Si) material, aperture stop S9, second lens L92 having a positive refractive power made of silicon (Si) material, positive refractive power made of germanium (Ge) material The third lens L93 is configured. Each of the first lens L91 to the third lens L93 has a convex surface facing the object side.

  A light beam in the infrared wavelength range (8 to 13 μm) emitted from the object and guided by the optical system L0 passes through the cover glass CG9 and forms an image on the infrared sensor IM9. Table 17 shows numerical data of the optical system L <b> 0 of Example 9. The unit of the radius of curvature and spacing of each optical surface in Table 17 is mm. Table 18 shows aspherical shape data of the aspherical surface of the optical system L0 of Example 9. The aspherical shape is represented by the (4X) equation as in Example 61.

  FIG. 18A is an MTF diagram of Example 9. A point 41 in FIG. 18A is an MTF value at a frequency of 42 lp / mm in Example 9, and the value is 36%. Since the MTF value is 30% or more at the Nyquist frequency of the infrared sensor IM4, it can be seen that the imaging performance is good. FIG. 18B shows longitudinal aberrations of Example 9. As shown in FIG. 18B, it can be seen that spherical aberration, field curvature, and chromatic aberration are well corrected.

[Example 10]
FIG. 19 is a lens cross-sectional view of Example 10 of the optical system L0 of the present invention. Example 10 is an optical system having a focal length of 18 mm and an F number of 0.8.

  The optical system L0 of Example 10 includes the following lenses arranged in order from the object to the image side. A first lens L101 having a positive refractive power made of a silicon (Si) material, an aperture stop S10, a second lens L102 having a positive refractive power made of a germanium (Ge) material, and a positive refractive power made of a silicon (Si) material. A third lens L103 is included. Further, the fourth lens L104 having a positive refractive power made of a silicon (Si) material is used. Each of the first lens L101 to the fourth lens L104 has a convex surface directed toward the object side.

  A light beam in the infrared wavelength range (8 to 13 μm) emitted from the object and guided by the optical system L0 passes through the cover glass CG10 and forms an image on the infrared sensor IM10. Table 19 shows numerical data of the optical system L0 of Example 10. The unit of curvature radius and interval of each optical surface in Table 19 is mm. Table 20 shows aspherical shape data of the aspherical surface of the optical system L0 of Example 10. The aspherical shape is represented by the (4X) equation as in the first embodiment.

  FIG. 20A is an MTF diagram of Example 10. A point 101 in FIG. 20A is the MTF value at a frequency of 50 lp / mm in Example 10, and the value is 34%. When the infrared sensor IM10 is a sensor having a pitch of 10 μm, it can be seen that the imaging performance is good because the MTF value is 30% or more at the Nyquist frequency of the sensor IM10. FIG. 20B shows longitudinal aberrations of Example 10. As shown in FIG. 20B, it can be seen that spherical aberration, field curvature, and chromatic aberration are well corrected. Thus, the effect of the invention is effective regardless of the number of lenses.

  Next, an embodiment of an infrared camcorder (video camera) (imaging device) using the optical system of each embodiment will be described with reference to FIG. In FIG. 21, reference numeral 13 denotes a camera body, and 11 denotes an imaging optical system constituted by any one of the optical systems described in the first to tenth embodiments. Reference numeral 12 denotes an imaging element (infrared sensor) such as a microbolometer that is built in the camera body and receives (photoelectrically converts) a subject image formed by the imaging optical system 11. As the infrared sensor, for example, a sensor formed using vanadium oxide or amorphous silicon is employed. This imaging apparatus can be applied to an in-vehicle camera, a surveillance camera, and the like.

  Tables 1 to 20 show numerical data relating to each specific lens of Examples 1 to 10. In each numerical data, the surface number i indicates the order counted from the object side, ri is the radius of curvature of the i-th optical surface (i-th surface or aperture stop), di is the i-th surface and the (i + 1) -th surface. The on-axis spacing between is shown. Moreover, the material of an optical member is shown. The aspheric coefficient is the X axis in the optical axis direction, the H axis in the direction perpendicular to the optical axis, the light traveling direction is positive, R is the paraxial radius of curvature, K is the cone coefficient, A, B, C, D, E, When F and G are aspheric coefficients, they are expressed by the following equation (4X).

“E−x” means 10 ^−x . Table 22 shows the relationship between the parameter values and the numerical data related to each conditional expression described above.

L11 1st lens L12 2nd lens Si Aperture stop CG1 Cover glass IM1 Image sensor

Claims (11)

A meniscus-shaped first lens having a positive refractive power facing the object side and a second lens having a meniscus-shaped positive refractive power facing the object side are arranged in order from the object side to the image side. And
The first lens is made of a silicon material,
The second lens is made of a silicon material or a germanium material,
When the focal length of the entire system is f and the distance on the optical axis between the first lens and the second lens is D,
0.90 <D / f <2.00
An optical system characterized by satisfying the following conditional expression: When the focal length of the first lens is f1, and the focal length of the second lens is f2,
0.1 <f2 / f1 <3.0
The optical system according to claim 1, wherein the following conditional expression is satisfied. When the focal length of the first lens is f1,
1.0 <f1 / f <6.0
The optical system according to claim 1, wherein the following conditional expression is satisfied. When the focal length of the second lens is f2,
0.5 <f2 / f <6.0
The optical system according to claim 1, wherein the following conditional expression is satisfied. When the focal length of the first lens is f1, and the lens thickness of the first lens is t1,
0.001 <t1 / f1 <0.065
The optical system according to claim 1, wherein the following conditional expression is satisfied.   The optical system according to any one of claims 1 to 5, wherein the optical system includes the first lens and the second lens, and the second lens is made of a germanium material.   The optical system includes the first lens, the second lens, and a third lens having a positive refractive power with a convex surface facing the object side, which are arranged in order from the object side to the image side. 6. The optical system according to claim 1, wherein the optical system is made of a silicon material or a germanium material.   The optical system includes, in order from the object side to the image side, the first lens, the second lens, a third lens having a positive refractive power with a convex surface facing the object side, and a positive lens with the convex surface facing the object side. The optical system according to claim 1, wherein the fourth lens is made of a silicon material or a germanium material. When the focal length of the second lens is f2, and the focal length of the third lens is f3,
0.1 <f3 / f2 <2.0
The optical system according to claim 7, wherein the following conditional expression is satisfied. When the focal length of the third lens is f3,
0.1 <f3 / f <3.0
The optical system according to claim 7, wherein the following conditional expression is satisfied.   An imaging apparatus comprising: the optical system according to claim 1; and an imaging element that photoelectrically converts an image formed by the optical system.