JP2017126041A - Imaging lens for infrared radiation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an imaging lens for infrared radiation that is more inexpensive than the conventional one.SOLUTION: An imaging lens for infrared radiation 10 includes, in order from an object side, a first lens L1 that is formed of a silicon having a minimum transmittance of an infrared ray with a wavelength of 8 μm or more and 13 μm or less of 40% or more when having a thickness of 1 mm, a second lens L2 that is formed of chalcogenide glass, and a third lens that is formed of chalcogenide glass. The silicon having a minimum transmittance of an infrared ray with a wavelength of 8 μm or more and 13 μm or less of 40% or more when having a thickness of 1 mm can be obtained inexpensively by controlling the oxygen concentration in a CZ method.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an infrared imaging lens that forms an image of a subject on an imaging surface of an image sensor using infrared rays.

  In recent years, surveillance cameras and vehicle-mounted cameras that photograph subjects such as the surrounding environment with so-called far infrared rays (infrared rays having a wavelength of about 3 μm to 15 μm) have become widespread. As a matter of course, a lens made of a material having a high infrared transmittance is used as an infrared imaging lens mounted on these cameras. For example, the infrared imaging lenses of Patent Documents 1 to 5 use Ge (germanium), chalcogenide glass, or ZnS (zinc sulfide). In addition, since sapphire and the like have a high transmittance with respect to infrared rays, they can be used for infrared imaging lenses.

JP 2014-109638 A JP 2012-173561 A JP 2012-037697 A JP 2012-173562 A JP 2010-039243 A

  Since the imaging lens for infrared rays must have a high transmittance with respect to infrared rays, a relatively small number of glass materials can be used, but these glass materials are all expensive. In particular, when a crystalline glass material is used, an aspherical surface can only be formed by grinding, so that the infrared imaging lens becomes very expensive. The cost of such infrared imaging lenses is one of the reasons for delaying the spread of surveillance cameras and in-vehicle cameras that capture the surrounding environment with infrared rays. There is a need to sell.

  Of the glass materials often used for infrared imaging lenses, chalcogenide glass is relatively inexpensive, and can easily obtain desired optical performance by forming an aspherical surface by molding. For this reason, chalcogenide glass is frequently used in recent infrared imaging lenses such as the infrared imaging lenses of Patent Documents 1 to 3. However, although chalcogenide glass is relatively inexpensive among glass materials that have been used for infrared imaging lenses in the past, a lens for imaging a subject with visible light (hereinafter, imaging for visible light). Compared with the glass material used for the lens), it is still very expensive.

For the visible light imaging lens, for example, inexpensive glass materials such as resin and synthetic quartz (SiO ₂ ) are used, but infrared rays (particularly far infrared rays) are very easily absorbed by these glass materials. For example, synthetic quartz can be used for an infrared imaging lens in terms of weather resistance and temperature dependency, but has a drawback of strongly absorbing infrared rays in the vicinity of 9 μm. For this reason, when synthetic quartz is used for an infrared imaging lens, it is difficult to obtain a desired image due to insufficient photographing light quantity. Therefore, as can be seen from the fact that it is not used in Patent Documents 1 to 5 and the like, the conventional infrared imaging lens does not use synthetic quartz.

  The reason why synthetic quartz absorbs infrared rays around 9 μm is because it contains oxygen in addition to silicon (Si). For this reason, if synthetic silicon is a silicon crystal or the like that does not contain oxygen, absorption of infrared rays in the vicinity of 9 μm can be suppressed, so that it can be used as a glass material for infrared imaging lenses. However, silicon crystals that do not contain oxygen are usually manufactured by the expensive FZ method (Floating Zone method), which is rather expensive. For this reason, conventional infrared imaging lenses do not use silicon crystals or the like as a glass material.

  In addition to the FZ method, for example, a CZ method (Czochralski method) is known as a typical method for producing a silicon crystal. Although the CZ method can obtain silicon crystals at a lower cost than the FZ method, it has a drawback that it cannot contain pure silicon crystals because it contains a large amount of impurities such as oxygen. However, in recent years, the amount of impurities such as oxygen can be controlled even by the CZ method, and the concentration of oxygen contained (hereinafter referred to as oxygen concentration) is lowered and far-infrared absorption is maintained while being inexpensive. It is now possible to obtain silicon crystals that suppress the above. Therefore, it is desired to provide an infrared imaging lens at a lower cost by using a silicon crystal that suppresses far-infrared absorption.

  In addition, since the conventional infrared imaging lens cannot use silicon, an infrared imaging lens using silicon is not known. In addition, since the performance of the lens varies greatly depending on the glass material used, simply replacing the conventional expensive infrared glass material with silicon cannot provide the image of the image quality required for a surveillance camera or an in-vehicle camera.

  It is an object of the present invention to provide an infrared imaging lens that is cheaper than the prior art by using silicon having an oxygen concentration that is at least low enough to be used with infrared rays and suppressing absorption of infrared rays around 9 μm. To do.

  The infrared imaging lens of the present invention comprises, in order from the object side, a first lens formed of silicon having a minimum infrared transmittance of 40% or more at a wavelength of 8 μm or more and 13 μm or less when the thickness is 1 mm, and a chalcogenide glass. A second lens formed, and a third lens formed of chalcogenide glass.

  It is preferable that the object side surface of the first lens is a spherical surface and the image side surface is a spherical surface.

  The second lens preferably has an aspheric surface on the object side and an aspheric surface on the image side.

  The third lens preferably has an aspheric surface on the object side and an aspheric surface on the image side.

  It is preferable to have a stop between the first lens and the second lens.

  The object side surface of the second lens is preferably a diffractive surface.

When the radius of curvature of the object side surface of the first lens is R1, and the radius of curvature of the image side surface of the first lens is R2,
(Formula 1) 1.05 ≦ R2 / R1 ≦ 1.37
It is preferable to satisfy.

When the focal length of the first lens is f1, and the distance from the image side surface of the first lens to the object side surface of the second lens is Δ,
(Formula 2) 2.7 ≦ f1 / Δ ≦ 5.8
It is preferable to satisfy.

The focal length of the entire system is f, the absolute value of the focal length of the second lens is | f2 |, the distance from the image side surface of the second lens to the object side surface of the third lens is D5, and the center thickness of the third lens Is D6,
(Expression 3) 0.75 ≦ (| f2 | / f) × (D5 / D6) ≦ 5.42
It is preferable to satisfy.

When the center thickness of the first lens is D1 and the center thickness of the third lens is D6,
(Formula 4) 0.8 ≦ D6 / D1 ≦ 3.0
It is preferable to satisfy.

When the center thickness of the first lens is D1,
(Formula 5) 1.0 mm ≦ D1 ≦ 2.5 mm
It is preferable to satisfy.

  The present invention provides an infrared imaging lens at a lower cost than before by using a lens made of silicon having a minimum infrared transmittance of 40% or more at a wavelength of 8 μm or more and 13 μm or less when the thickness is 1 mm. be able to.

It is sectional drawing of the imaging lens for infrared rays. It is a graph which shows the transmittance | permeability of 1 mm thick silicon. It is a graph which shows the transmittance | permeability of 1 mm thick silicon which gave antireflection coating. 1 is a cross-sectional view of an infrared imaging lens of Example 1. FIG. 2 is a graph showing (A) spherical aberration, (B) astigmatism, and (C) distortion in Example 1. FIG. 4 is a graph showing MTF with respect to the spatial frequency of Example 1. 6 is a cross-sectional view of an infrared imaging lens of Example 2. FIG. It is a graph which shows (A) spherical aberration, (B) astigmatism, and (C) distortion of Example 2. 6 is a graph showing MTF with respect to the spatial frequency of Example 2. 6 is a cross-sectional view of an infrared imaging lens of Example 3. FIG. 10 is a graph showing (A) spherical aberration, (B) astigmatism, and (C) distortion in Example 3. 10 is a graph showing MTF with respect to the spatial frequency of Example 3. 6 is a cross-sectional view of an infrared imaging lens of Example 4. FIG. It is a graph which shows (A) spherical aberration, (B) astigmatism, and (C) distortion of Example 4. It is a graph which shows MTF with respect to the spatial frequency of Example 4. 6 is a cross-sectional view of an infrared imaging lens of Example 5. FIG. 10 is a graph showing (A) spherical aberration, (B) astigmatism, and (C) distortion in Example 5. 10 is a graph showing MTF with respect to the spatial frequency of Example 5. 6 is a cross-sectional view of an infrared imaging lens of Example 6. FIG. It is a graph which shows (A) spherical aberration, (B) astigmatism, and (C) distortion of Example 6. 10 is a graph showing MTF with respect to the spatial frequency of Example 6. 7 is a cross-sectional view of an infrared imaging lens of Example 7. FIG. 10 is a graph showing (A) spherical aberration, (B) astigmatism, and (C) distortion in Example 7. 10 is a graph showing MTF with respect to spatial frequency in Example 7. 9 is a cross-sectional view of an infrared imaging lens of Example 8. FIG. 10 is a graph showing (A) spherical aberration, (B) astigmatism, and (C) distortion in Example 8. 10 is a graph showing MTF with respect to the spatial frequency of Example 8. 10 is a cross-sectional view of an infrared imaging lens of Example 9. FIG. 10 is a graph showing (A) spherical aberration, (B) astigmatism, and (C) distortion in Example 9. It is a graph which shows MTF with respect to the spatial frequency of Example 9. FIG. 10 is a cross-sectional view of an infrared imaging lens of Example 10. FIG. It is a graph which shows (A) spherical aberration, (B) astigmatism, and (C) distortion of Example 10. It is a graph which shows MTF with respect to the spatial frequency of Example 10. FIG. 10 is a sectional view of an infrared imaging lens of Example 11. FIG. It is a graph which shows (A) spherical aberration, (B) astigmatism, and (C) distortion of Example 11. 14 is a graph showing MTF with respect to the spatial frequency of Example 11.

  As shown in FIG. 1, the infrared imaging lens 10 is a lens that forms an image of a subject on the imaging surface S <b> 10 of the image sensor 11 with far infrared rays. The infrared imaging lens 10 is a three-lens lens system having three lenses of a first lens L1, a second lens L2, and a third lens in order from the object side along the optical axis Z1. . In addition, the infrared imaging lens 10 includes a diaphragm S3 between the first lens L1 and the second lens L2. Since the image sensor 11 protects the imaging surface S10 with the cover glass CG, the infrared imaging lens 10 forms an image of the subject on the imaging surface S10 via the cover glass CG.

  The first lens L1 is formed of silicon having a minimum infrared transmittance of 40% or more at a wavelength of 8 μm or more and 13 μm or less when the thickness is 1 mm. In the case of 1 mm thickness, the condition that the minimum transmittance of infrared rays having a wavelength of 8 μm or more and 13 μm or less is 40% or more is that the concentration of oxygen contained in silicon is low (for example, compared with general silicon produced by the conventional CZ method) Satisfied when oxygen concentration is low). Hereinafter, in order to distinguish from conventional silicon having a high oxygen concentration, the glass material of the first lens L1 is referred to as low-oxygen silicon for convenience, and conventional silicon having a relatively high oxygen concentration is referred to as high-oxygen silicon.

  As indicated by a broken line in FIG. 2, high-oxygen silicon having a thickness of 1 mm has a remarkable absorption of infrared rays having a wavelength of about 9 μm. For this reason, when viewed in the wavelength band of 8 μm or more and 13 μm or less, the transmittance of high-oxygen silicon is the lowest near the wavelength of about 9 μm, and the minimum transmittance is less than 40%. On the other hand, as shown by a solid line in FIG. 2, 1 mm-thick low-oxygen silicon has a lower oxygen concentration than high-oxygen silicon, so that infrared transmittance near a wavelength of about 9 μm is increased, and a wavelength of 8 μm to 13 μm. The minimum infrared transmittance is 40% or more. The extent to which low-oxygen silicon can suppress infrared absorption (particularly, absorption of infrared rays in the vicinity of a wavelength of 9 μm) with respect to high-oxygen silicon depends on the oxygen concentration of the low-oxygen silicon, but is at least from 8 μm to 13 μm. If the minimum transmittance is 40% or more, the infrared imaging lens 10 can be used.

  Low oxygen silicon can be obtained at low cost by controlling the oxygen concentration in, for example, the CZ method. Low-oxygen silicon can be obtained by the more expensive FZ method, but it is usually produced by the cheapest method having the same characteristics. Therefore, low-oxygen silicon is less expensive than conventional infrared glass materials.

  As shown in FIG. 3, when the same antireflection coating is applied to low oxygen silicon (solid line) and high oxygen silicon (broken line), the infrared transmittance (see FIG. 2) as each glass material is applied. Both improve the transmittance as a whole. For this reason, if antireflection coating is applied, the minimum transmittance of infrared rays having a wavelength of 8 μm or more and 13 μm or less can be made 40% or more even with high oxygen silicon (broken line). However, the wavelength band in which the transmittance is improved by the antireflection coating and the degree of improvement in the transmittance depend on the performance of the antireflection coating. In addition, there is no change in the characteristics of the glass material itself that the absorption of infrared rays in the vicinity of a wavelength of about 9 μm is significant.

  For this reason, the condition that “the minimum transmittance of infrared rays having a wavelength of 8 μm or more and 13 μm or less is 40% or more” is a condition regarding the characteristics of the glass material itself of the first lens L1, and does not include the performance of the antireflection coating. However, this does not mean that when the infrared imaging lens 10 is actually constructed, the first lens L1 should not be provided with an antireflection coating. The first lens L1 is naturally provided with an antireflection coating on low-oxygen silicon. To form. Specifically, in the infrared imaging lens 10, an antireflection coating is applied to the object-side surface S1 and the image-side surface S2 of the first lens L1, or one of these surfaces. For this reason, the first lens L1 has, for example, a transmittance characteristic indicated by a solid line in FIG.

  The low oxygen silicon forming the first lens L1 is a crystal, and is used by cutting out from an ingot or the like and grinding it to maintain a low oxygen saturation state. For this reason, the first lens L1 cannot be aspherically formed by heat and pressure such as molding, and when the object-side surface S1 or the image-side surface S2 of the first lens L1 is aspherical, it is ground. Therefore, it is necessary to form an aspherical surface. However, if the aspherical surface is formed by grinding, the cost of the first lens L1 increases, and as a result, the infrared imaging lens 10 cannot be manufactured at low cost. Accordingly, in the infrared imaging lens 10, the first lens L1 is a spherical lens. That is, in the first lens L1, the object-side surface S1 is a spherical surface, and the image-side surface S2 is a spherical surface.

  The first lens L1 also serves as a protective member (for example, a cover glass) for the infrared imaging lens 10. That is, the infrared imaging lens 10 is used in a harsh environment such as a surveillance camera or an in-vehicle camera, but there is no need to protect the infrared imaging lens 10 by placing a protective member before the first lens L1.

  For example, since chalcogenide glass is brittle and has low weather resistance, at least the most object-side lens made of chalcogenide glass is a conventional infrared imaging lens that uses a protective member in front of the infrared imaging lens. It is necessary to protect the imaging lens (in particular, the lens formed of the most object-side chalcogenide glass). The protective member is, for example, a parallel plate that does not have power as a lens. However, since the protective member must sufficiently transmit infrared rays necessary for photographing, it is necessary to use an expensive infrared glass material having good weather resistance and the like for the protective member. The point that an expensive material must be used for the protective member in this way is one of the causes of the cost increase of the conventional infrared imaging lens. On the other hand, in the infrared imaging lens 10, the first lens L 1 located closest to the object side is made of an inexpensive material such as low-oxygen silicon, and is made of an inexpensive material. It is not necessary to use an expensive protective member necessary for the lens. As a result, the infrared imaging lens 10 can be configured at a lower cost than the conventional infrared imaging lens.

  The second lens L2 and the third lens L3 are both lenses formed of chalcogenide glass. Chalcogenide glass is a glass containing, as a main component, at least one of a series of similar elements called chalcogen elements such as sulfur (S), selenium (Se), and tellurium (Te) instead of oxygen (O). It is. Chalcogenide glass is relatively inexpensive among various infrared glass materials. For this reason, the infrared imaging lens 10 has a low-cost configuration as the entire infrared imaging lens 10 by forming the second lens L2 of chalcogenide glass.

  Further, the chalcogenide glass can easily form an aspherical surface by molding. Therefore, in the infrared imaging lens 10, the second lens L2 and the third lens formed of chalcogenide glass are aspherical lenses, thereby correcting various aberrations of the infrared imaging lens 10, and the first lens L1. Even when formed of low oxygen silicon, the imaging performance required for the infrared imaging lens 10 as a whole is obtained. More specifically, in the second lens L2, the object-side surface S4 is an aspheric surface, and the image-side surface S5 is an aspheric surface. In the third lens L3, the object-side surface S6 is an aspheric surface, and the image-side surface S7 is an aspheric surface. Thus, by forming both surfaces of the second lens L2 as aspherical surfaces, excellent imaging performance can be achieved even when the first lens L1 is made of low-oxygen silicon and both surfaces of the first lens L1 are spherical surfaces. Is easy to obtain. Similarly, by forming both surfaces of the third lens L3 as aspherical surfaces, good imaging performance can be achieved even when the first lens L1 is made of low oxygen silicon and both surfaces of the first lens L1 are aspherical surfaces. Is easy to obtain. In particular, in the infrared imaging lens 10, the second lens L2 and the third lens are both aspherical on both sides, so that the first lens L1 is formed of low oxygen silicon and the both surfaces of the first lens L1. Even if a is an aspherical surface, good imaging performance is easily obtained.

  Furthermore, the object side surface S4 of the second lens L2 is a diffractive surface. This is for correcting chromatic aberration. When correcting chromatic aberration with a diffractive surface, it is possible to correct chromatic aberration better when the diffractive surface is as close to the stop S3 as possible. Therefore, in the infrared imaging lens 10, the object-side surface S4 closer to the stop S3 among the object-side surface S4 and the image-side surface S5 of the second lens L2 capable of forming a diffraction surface is used as the diffraction surface. ing.

  In addition, the infrared imaging lens 10 has the diaphragm S3 disposed between the first lens L1 and the second lens L2 in order to reduce the cost of the infrared imaging lens 10 and to correct aberrations. is there. If the stop S3 is disposed on the object side of the first lens L1 instead of disposing the stop S3 between the first lens L1 and the second lens L2, the second lens L2 and the third lens L3 increase in diameter. When the diameters of the second lens L2 and the third lens L3 are increased, the amount of chalcogenide glass that is the glass material of the second lens L2 and the third lens L3 increases, and the cost increases accordingly. Further, instead of disposing the diaphragm S3 between the first lens L1 and the second lens L2, if the diaphragm S3 is disposed between the second lens L2 and the third lens L3, the first lens L1 and the second lens L2 are arranged. It is more difficult to correct various aberrations than when the stop S3 is disposed between them. That is, it is easier to correct various aberrations more easily and better if the diaphragm S3 is disposed between the first lens L1 and the second lens L2 as in the infrared imaging lens 10.

The infrared imaging lens 10 is formed to satisfy the following five conditions. First, the infrared imaging lens 10 has a radius of curvature of the object-side surface S1 of the first lens L1 as R1 and a radius of curvature of the image-side surface S2 of the first lens L1 as R2.
(Formula 1) 1.05 ≦ R2 / R1 ≦ 1.37
Meet.

  When the value of R2 / R1 is below the lower limit of Equation 1, the second lens L2 and the third lens L3 are likely to have a large diameter. When the diameters of the second lens L2 and the third lens L3 are increased, the amount of chalcogenide glass that is the glass material of the second lens L2 and the third lens L3 is increased accordingly. This increases the cost of the infrared imaging lens 10.

  On the other hand, when the value of R2 / R1 exceeds the upper limit of Equation 1, the curvature of field increases. Specifically, when the value of R2 / R1 exceeds the upper limit of Expression 1, the curvature of field caused by forming the first lens L1 with low-oxygen silicon and making both surfaces spherical will be described. It becomes difficult to correct by L2 and the 3rd lens L3.

  That is, Equation 1 favorably corrects the curvature of field caused by the glass material and shape of the first lens L1 by the second lens L2 and the third lens L3 while suppressing the cost of the infrared imaging lens 10. This is a condition for suppressing the range to be obtained. In addition, it is more preferable that R2 / R1 of Formula 1 satisfies 1.10 ≦ R2 / R1 ≦ 1.25.

Second, the infrared imaging lens 10 has a focal length f1 of the first lens L1, and a distance from the image side surface S2 of the first lens L1 to the object side surface S4 of the second lens L2 (of S2 and S4). When the center distance is Δ,
(Formula 2) 2.7 ≦ f1 / Δ ≦ 5.8
Meet.

  If the value of f1 / Δ is below the lower limit of Equation 2, the curvature of field tends to increase. Specifically, when the value of f1 / Δ is below the lower limit of Equation 2, the curvature of field caused by forming the first lens L1 with low-oxygen silicon and making both surfaces spherical can be obtained. L2 and the third lens L3 may not be sufficiently corrected.

  On the other hand, when the value of f1 / Δ exceeds the upper limit of Equation 2, spherical aberration increases. Specifically, when the value of f1 / Δ exceeds the upper limit of Expression 2, spherical aberration caused by forming the first lens L1 with low-oxygen silicon and making both surfaces spherical is represented by the second lens L2. In some cases, the third lens L3 cannot be corrected sufficiently. Further, if the value of f1 / Δ exceeds the upper limit of Equation 2, the second lens L2 and the third lens L3 are increased in diameter and the cost is increased.

  That is, Equation 2 favorably corrects the curvature of field and spherical aberration due to the glass material and shape of the first lens L1 by the second lens L2 and the third lens L3 while suppressing the cost of the infrared imaging lens 10. This is a condition for limiting the range to a possible range. In addition, it is more preferable that f1 / Δ in Formula 2 satisfies 3.0 ≦ f1 / Δ ≦ 4.0.

Third, the infrared imaging lens 10 has the focal length of the entire system as f, the absolute value of the focal length of the second lens L2 as | f2 |, and the third lens L3 from the image side surface S5 of the second lens L2. When the distance of the object side surface S6 (center distance between S5 and S6) is D5 and the center thickness of the third lens L3 (center distance between S6 and S7) is D6,
(Expression 3) 0.75 ≦ (| f2 | / f) × (D5 / D6) ≦ 5.60
Meet.

  When the value of (| f2 | / f) × (D5 / D6) is below the lower limit of Equation 3, the curvature of field increases. Specifically, when the value of (| f2 | / f) × (D5 / D6) is below the lower limit of Equation 3, the first lens L1 is made of low-oxygen silicon and both surfaces are made spherical. The resulting curvature of field may not be sufficiently corrected by the second lens L2 and the third lens L3. On the other hand, if the value of (| f2 | / f) × (D5 / D6) is below the lower limit of Equation 3, the diameter of the third lens L3 increases and the cost increases.

  On the other hand, if the value of (| f2 | / f) × (D5 / D6) exceeds the upper limit of Expression 3, the back focus of the infrared imaging lens 10 becomes too short and it is difficult to incorporate it into the imaging system. There is a case. Further, if the value of (| f2 | / f) × (D5 / D6) exceeds the upper limit of Expression 3, the third lens L3 is increased in diameter and the cost is increased.

  That is, Formula 3 suppresses the diameter of the third lens L3 to reduce the cost of the infrared imaging lens 10 and reduces the curvature of field due to the glass material and the shape of the first lens L1 to the second lens L2 and the third lens L3. This is a condition for keeping the back focus to such an extent that the lens L3 can be favorably corrected and the infrared imaging lens 10 can be easily incorporated into the imaging system. Note that (| f2 | / f) × (D5 / D6) in Expression 4 more preferably satisfies 1.00 ≦ (| f2 | / f) × (D5 / D6) ≦ 5.00.

Fourth, in the infrared imaging lens 10, the center thickness of the first lens L1 (center distance between S1 and S2) is D1, and the center thickness of the third lens L3 (center distance between S6 and S7) is D6. In addition,
(Formula 4) 0.8 ≦ D6 / D1 ≦ 3.0
Meet.

  When the value of D6 / D1 exceeds the upper limit of Equation 4, the spherical aberration increases. Specifically, when the value of D6 / D1 exceeds the upper limit of Expression 4, the spherical aberration caused by forming the first lens L1 with low-oxygen silicon and making both surfaces spherical is represented by the second lens L2. In some cases, the third lens L3 cannot be corrected sufficiently. On the other hand, if the value of D6 / D1 exceeds the upper limit of Expression 4, the first lens L1 becomes too thick and the workability deteriorates. Furthermore, if D6 / D1 exceeds the upper limit of Expression 4, the third lens L3 becomes thick and the volume increases, which increases the cost.

  On the other hand, when D6 / D1 falls below the lower limit of Equation 4, the curvature of field increases. Specifically, when D6 / D1 falls below the lower limit of Expression 4, the curvature of field caused by forming the first lens L1 with low-oxygen silicon and making both surfaces spherical is represented by the second lens L2 and The third lens L3 may not be sufficiently corrected. Further, when D6 / D1 is below the lower limit of Equation 4, the transmittance of infrared rays is reduced because the first lens L1 is too thick. As a result, the captured image may become dark.

  That is, Equation 4 reduces the spherical aberration and curvature of field due to the glass material and shape of the first lens L1 while reducing the cost of the infrared imaging lens 10 by reducing the volume of the third lens L3, and the second lens L2. In addition, it is a condition for limiting to a range that can be satisfactorily corrected by the third lens L3 and ensuring the workability and infrared transmittance of the first lens L1. In addition, D6 / D1 of Formula 4 more preferably satisfies 0.8 ≦ D6 / D1 ≦ 2.5, and particularly preferably satisfies 1.0 ≦ D6 / D1 ≦ 2.0.

Fifth, the infrared imaging lens 10 has a center thickness (center distance between S1 and S2) of the first lens L1 as D1.
(Formula 5) 1.0 mm ≦ D1 ≦ 2.5 mm
Meet.

  When the center thickness D1 of the first lens L1 exceeds the upper limit of Expression 5, the infrared transmittance decreases because the first lens L1 is too thick. As a result, the captured image may become dark. On the other hand, when the center thickness D1 of the first lens L1 is less than the lower limit of Formula 5, the first lens L1 becomes too thin, and the workability deteriorates. That is, Formula 5 is a condition for ensuring the workability of the first lens L1 while ensuring the infrared transmittance of the first lens L1. In addition, it is more preferable that D1 of Formula 5 satisfies 1.0 ≦ D1 ≦ 2.0.

  When the first lens L1 is formed of low-oxygen silicon and is a spherical lens like the infrared imaging lens 10, the conditions of the above formulas 1 to 5 satisfy at least one of them. It is particularly preferable that all conditions of Formulas 1 to 5 are satisfied.

  As described above, the infrared imaging lens 10 includes the first lens L1 made of low-oxygen silicon and the second lens L2 and the third lens L3 made of chalcogenide glass. Less expensive. In addition, the imaging lens 10 for infrared rays is an aspherical lens in which the second lens L2 and the third lens L3 are formed of chalcogenide glass, and the diaphragm S3 is interposed between the first lens L1 and the second lens L3. Due to the arrangement and the conditions of Formulas 1 to 5 being satisfied, the disadvantages such as aberration caused by the glass material and the shape of the first lens L1 can be improved at low cost, and inexpensive and good imaging performance can be achieved. Have.

[Example]
Hereinafter, examples of the infrared imaging lens 10 will be described. FIG. 4 is a sectional view of the infrared imaging lens 10 according to the first embodiment. Surface numbers are indicated by Si (i = 1 to 10) in order from the object-side surface S1 of the first lens L1. S3 is a stop, S8 is an object side surface of the cover glass CG, S9 is an image side surface of the cover glass CG, and S10 is an imaging surface of the image sensor 11. The surface interval Di (i = 1 to 10, unit mm) is the interval from the surface Si to the surface Si + 1.

  The lens data of Example 1 are shown in Tables 1 to 3 below. Table 1 shows the surface number i of each surface Si of the infrared imaging lens 10 of Example 1, the radius of curvature Ri (i = 1 to 10, unit mm) of each surface Si, the refraction of infrared light having a surface interval Di and a wavelength of 10 μm. The ratio ni (i = 1 to 10) and the materials of the first lens L1, the second lens L2, and the third lens are shown. Further, the “*” mark attached to the surface number Si represents an aspheric surface, and the “#” mark represents a diffractive surface. All curved surfaces without “*” and “#” marks are spherical.

  The aspherical surface is represented by the following aspherical expression. In the aspherical expression of Equation 1, “Z” is the depth (mm) of the aspheric surface, “h” is the distance from the optical axis to the lens surface (mm), and “C” is the paraxial curvature (ie, the paraxial radius of curvature). Where R = mm (C = 1 / R), "K" is the conic constant, and "Ai" is the aspheric coefficient. Table 2 shows “K” and “Ai” of each aspheric surface of Example 1 (see Table 1 *).

  The diffractive surface is represented by the following optical path difference function φ of Formula 2. The optical path difference function φ is a distance “r” from the optical axis when the diffractive surface has a load amount of the optical path length difference, “r” is a distance (mm) from the optical axis, and “Cn” (n = 1 to 10). ) Is the diffraction surface coefficient. Table 3 shows non-zero diffraction surface coefficients Cn of the diffractive surface of Example 1 (see # 1 in Table 1).

  FIG. 5A shows spherical aberration for each infrared ray having a wavelength of 8 μm, a wavelength of 10 μm, and a wavelength of 12 μm in Example 1. FIG. 5B shows the astigmatism S in the sagittal (radical) direction and the astigmatism T in the tangential (meridional) direction of Example 1 for an infrared ray having a wavelength of 10 μm. FIG. 5C shows the distortion of Example 1 for an infrared ray having a wavelength of 10 μm. The infrared imaging lens 10 of Example 1 is a lens used for the image sensor 11 having a pitch of 12 μm and 384 pixels × 288 pixels, and thus has a maximum image height of 2.88 mm.

  FIG. 6 shows the MTF on the axis (image height 0.0 mm) with respect to the spatial frequency and the MTF (symbol T) in the tangential direction at the maximum image height 2.88 mm for the infrared imaging lens 10 of Example 1. The MTF (symbol R) in the radial direction is shown.

  Similarly to Example 1, FIGS. 7 to 36 and Tables 4 to 33 show cross-sectional views, various lens data, various aberrations, and MTFs of the infrared imaging lens 10 of Examples 2 to 11, respectively. However, the infrared imaging lens 10 of Example 2 and Examples 8 to 11 is a lens used for the image sensor 11 of 12 μm pitch and 384 pixels × 288 pixels, as in Example 1, and has a maximum image height. Is 2.88 mm. The infrared imaging lens 10 of Example 3, Example 4, and Example 7 is a lens used for the image sensor 11 of QVGA (320 pixels × 240 pixels) and 17 μm pitch, and the maximum image height is 3.40 mm. It is. The imaging lens 10 for infrared rays of Example 5 and Example 6 is a lens used for the image sensor 11 of 384 pixels × 288 pixels and a pitch of 17 μm, and the maximum image height is 4.08 mm.

  In Table 34 and Table 35 below, other performance such as F value (FNo) of Examples 1 to 11 and parameters for calculating each value of Formulas 1 to 5 are described in the lens data. None of the parameters and the values of Equations 1-5 are shown. “F” is a focal length of the entire system of the imaging lens 10 for infrared rays, “f1” is a focal length of the first lens L1, and “f2” is a focal length of the second lens. “Δ” is the distance between the image side surface S2 of the first lens L1 and the object side surface S4 of the second lens L2, that is, the sum of the surface distance D2 and the surface distance D3 of the lens data. As can be seen from Table 19, each of the infrared imaging lenses 10 of Examples 1 to 6 satisfies the conditions of Formulas 1 to 5. Further, as can be seen from Table 19 and the aberration diagrams and MTF graphs of each of the above examples, each of the infrared imaging lenses 10 of Examples 1 to 6 includes the first lens L1 formed of low oxygen silicon, and Despite the spherical surfaces on both sides, it has good imaging performance.

[Comparative example]
The conventional infrared imaging lens of each of the embodiments described in Patent Documents 1 to 5 is a lens that uses only a conventional expensive infrared glass material, and thus is simply compared with the infrared imaging lens 10 of the present invention. However, since it is an infrared imaging lens having a three-lens configuration similar to the infrared imaging lens 10 of the present invention, it will be described below as a comparative example.

  First, the infrared imaging lens of Example 1 (hereinafter referred to as Comparative Example 1) to Example 14 (hereinafter referred to as Comparative Example 14) of Patent Document 1 is a diaphragm between the first lens L1 and the second lens L2. Is an infrared imaging lens having a three-lens configuration. As shown in Tables 36 to 38, the first lens L1 and the second lens L2 are all made of germanium (Ge), and the third lens L3 is made of chalcogenide glass. And about the conditions of Formula 1-Formula 5, as shown with a bold letter and an underline, some conditions may be satisfied by chance, but the conditions of Formula 1-Formula 5 are not necessarily satisfied in all the Examples. From Patent Document 1, it can be seen that the infrared imaging lens is not configured to intentionally satisfy the conditions of Formulas 1 to 5.

  In the infrared imaging lens of Example 1 (hereinafter referred to as Comparative Example 15) to Example 5 (hereinafter referred to as Comparative Example 19) of Patent Document 2, a diaphragm is disposed between the first lens L1 and the second lens L2. This is a three-element infrared imaging lens. As shown in Table 39, the first lens L1 and the third lens L1 are formed of germanium (Ge), and the second lens L2 is formed of chalcogenide glass. And about the conditions of Formula 1-Formula 5, as shown with a bold letter and an underline, some conditions may be satisfied by chance, but the conditions of Formula 1-Formula 5 are not necessarily satisfied in all the Examples. Thus, Patent Document 2 also shows that the infrared imaging lens is not configured to intentionally satisfy the conditions of Formulas 1 to 5.

  The infrared imaging lens of Example 1 (hereinafter referred to as Comparative Example 20) to Example 5 (hereinafter referred to as Comparative Example 24) of Patent Document 3 is an infrared ray having a three-lens structure with the front lens diameter itself as a stop. Imaging lens. As shown in Table 40, the first lens L1, the second lens L2, and the third lens L3 are made of chalcogenide glass or zinc sulfide (ZnS). About the conditions of Formula 1 to Formula 5, as shown in bold and underline, some conditions may be met by chance, but since the conditions of Formula 1 to Formula 5 are not satisfied in all examples, Patent Document 3 also shows that the infrared imaging lens is not configured so as to intentionally satisfy the conditions of Expressions 1 to 5.

  In the infrared imaging lens of Example 1 (hereinafter referred to as Comparative Example 25) and Example 2 (hereinafter referred to as Comparative Example 26) of Patent Document 4, a diaphragm is disposed between the first lens L1 and the second lens L2. This is a three-lens imaging lens for infrared rays. As shown in Table 41, the first lens L1, the second lens L2, and the third lens L3 are all formed of germanium (Ge). About the conditions of Formula 1-Formula 5, as shown in bold and underline, some conditions may be satisfied by chance, but the conditions of Formula 1-Formula 5 are not necessarily satisfied in Example 1 and Example 2. From this, it is understood that Patent Document 4 also does not constitute an infrared imaging lens so as to intentionally satisfy the conditions of Expressions 1 to 5.

  Further, Example 1 (hereinafter referred to as Comparative Example 27) to Example 3 (hereinafter referred to as Comparative Example 29) of Patent Document 5 has a three-lens configuration in which a diaphragm is disposed between the first lens L1 and the second lens L2. This is an infrared imaging lens. As shown in Table 42, the first lens L1, the second lens L2, and the third lens L3 are all formed of germanium (Ge). About the conditions of Formula 1 to Formula 5, as shown in bold and underline, some conditions may be met by chance, but since the conditions of Formula 1 to Formula 5 are not satisfied in all examples, Patent Document 5 also shows that the infrared imaging lens is not configured so as to intentionally satisfy the conditions of Expressions 1 to 5.

  As described above, the conventional infrared imaging lens is not configured to satisfy the conditions of Formulas 1 to 5. Formula 1 to Formula 5 are like the infrared imaging lens 10 of the present invention. This is a special condition when the first lens L1 is formed of low-oxygen silicon and is a spherical lens in a two-lens lens system. The first lens L1, the second lens L2, and the third lens L3 have different conditions. This is because a conventional infrared imaging lens using a conventional expensive infrared glass material does not need to be configured to satisfy Equations 1 to 5.

  Note that various modifications can be made to the above-described embodiments and examples. For example, in addition to the infrared imaging lens 10 described in the above embodiment, the radius, refractive index, and other lens data are changed, and the shape, arrangement, and imaging performance of the infrared imaging lens 10 are the same as those of the infrared imaging lens 10. An imaging lens can be constructed.

DESCRIPTION OF SYMBOLS 10 Infrared imaging lens L1 1st lens L2 2nd lens L3 3rd lens S3 Diaphragm CG Cover glass S10 Imaging surface

Claims (11)

From the object side,
A first lens made of silicon having a minimum infrared transmittance of 40% or more at a wavelength of 8 μm or more and 13 μm or less when the thickness is 1 mm;
A second lens formed of chalcogenide glass;
A third lens formed of chalcogenide glass;
An infrared imaging lens.   2. The infrared imaging lens according to claim 1, wherein the object-side surface of the first lens is a spherical surface, and the image-side surface is a spherical surface.   The infrared imaging lens according to claim 1, wherein the second lens has an aspheric surface on the object side and an aspheric surface on the image side.   4. The infrared imaging lens according to claim 1, wherein the third lens has an aspheric object side surface and an aspheric image side surface. 5.   The infrared imaging lens according to claim 1, further comprising a diaphragm between the first lens and the second lens.   The infrared imaging lens according to claim 5, wherein the object side surface of the second lens is a diffractive surface. When the radius of curvature of the object side surface of the first lens is R1, and the radius of curvature of the image side surface of the first lens is R2,
(Formula 1) 1.05 ≦ R2 / R1 ≦ 1.37
The imaging lens for infrared rays of any one of Claims 1-6 which satisfy | fills. When the focal length of the first lens is f1, and the distance from the image side surface of the first lens to the object side surface of the second lens is Δ,
(Formula 2) 2.7 ≦ f1 / Δ ≦ 5.8
The infrared imaging lens according to claim 1, wherein: The focal length of the entire system is f, the absolute value of the focal length of the second lens is | f2 |, the distance from the image-side surface of the second lens to the object-side surface of the third lens is D5, and the third When the center thickness of the lens is D6,
(Expression 3) 0.75 ≦ (| f2 | / f) × (D5 / D6) ≦ 5.60
The infrared imaging lens according to claim 1, wherein: When the center thickness of the first lens is D1, and the center thickness of the third lens is D6,
(Formula 4) 0.8 ≦ D6 / D1 ≦ 3.0
The infrared imaging lens according to claim 1, wherein: When the center thickness of the first lens is D1,
(Formula 5) 1.0 mm ≦ D1 ≦ 2.5 mm
The imaging lens for infrared rays of any one of Claims 1-10 which satisfy | fills.

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