Infrared optical system and infrared imaging device

Abstract

An infrared optical system 10 includes a plurality of lenses L made of chalcogenide glass. The total center-thickness of the plurality of lenses L is 7 mm or less. Among the plurality of lenses L, a first lens L1 closest to the object side is formed in a positive meniscus shape that is convex toward the object side on the near axis. Among the plurality of lenses L, an N-th lens Ln closest to the image-side does not have a shape that is convex on both surfaces on the near axis, but has a shape such that at least one of the object-side surface and the image-side surface has an extreme value at a location other than on the optical axis Ax.

Description

Infrared optical system and infrared imaging device

[0001] The present invention relates to an infrared optical system and an infrared imaging device.

[0002] Infrared optical systems, which generally use infrared light (infrared rays) with wavelengths of 3 to 5 μm and 8 to 14 μm, are capable of detecting radiant heat and are therefore used as thermal cameras for non-contact body temperature measurement. They are also used for surveillance, security, and inspection applications because they are less susceptible to disturbances (fog, haze, darkness, backlight, etc.) that visible light optical systems are not susceptible to, and can measure temperature. However, despite their versatility, infrared optical systems have not been widely adopted due to their higher cost compared to visible light optical systems. One of the main reasons for this high cost is the high cost of lenses. Another issue is that optical systems, including lenses, are large due to poor processability, making it difficult to reduce their height.

[0003] Known lens materials used in infrared optical systems include Ge, Si, ZnSe, chalcogenide glass, and ZnS. Among these, Ge, Si, and ZnSe are difficult to process, so they are often used to form spherical lenses by polishing, and are not suitable for asphericization. Creating an aspheric shape using these materials requires machining such as cutting, photolithography, and other processes, which increases costs.

[0004] In this regard, chalcogenide glass and ZnS have a relatively high material cost, but they can be molded and aspherical shapes can be produced relatively inexpensively. Chalcogenide glass in particular is expected to further reduce costs and is becoming increasingly popular (see, for example, Patent Documents 1 to 5).

[0005] JP 2010-113191 A JP 2011-128538 A JP 2011-237669 A Japanese Patent No. 5566780 A Japanese Patent No. 6149410 A

[0006] However, chalcogenide glass absorbs infrared light with wavelengths of 11 μm or more, so its transmittance decreases when it is thick. The technologies described in Patent Documents 1 to 4 do not incorporate the concept of thinness, and issues with transmittance arise due to the inclusion of a diffractive surface, etc. The technology described in Patent Document 5 has a biconvex lens on the sensor side. Therefore, in terms of performance when thinned, the design does not fully utilize the processability of chalcogenide glass, and reduction in height is limited.

[0007] The present invention has been made in view of the above circumstances, and has an object to provide an infrared optical system that can be made low-profile and low-cost, and an infrared imaging device including the same.

[0008] In order to achieve the above object, the present invention provides an infrared optical system comprising a plurality of lenses made of chalcogenide glass, the plurality of lenses having a total center thickness of 7 mm or less, a first lens of the plurality of lenses closest to the object side being formed in a positive meniscus shape convex toward the object side on a paraxial line, and an Nth lens of the plurality of lenses closest to the image side being not biconvex on a paraxial line, and having a shape in which at least one of the object side and image side surfaces has an extreme value other than on the optical axis.

[0009] According to the present invention, it is possible to achieve a low profile and low cost.

[0010] 1 is a schematic cross-sectional view of an infrared imaging device according to an embodiment; FIG. 2 is a graph showing the relationship between the thickness of a chalcogenide glass lens and the internal transmittance of infrared light, with the horizontal axis representing the wavelength of infrared light; FIG. 3 is a graph showing the relationship between the thickness of a chalcogenide glass lens and the internal transmittance of infrared light, with the horizontal axis representing the lens thickness; FIG. 4 is a ray diagram and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof; FIG. 5 is a ray diagram and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof; FIG. 6 is a ray diagram and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof; FIG. 7 is a ray diagram and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof; FIG. 8 is a ray diagram and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof; FIG. 9 is a ray diagram and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof; 1 is a ray diagram and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 2 is a ray diagram and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 3 is a ray diagram and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 4 is a ray diagram and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 5 is a ray diagram and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 6 is a ray diagram and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 7 is a ray diagram and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 8 is a ray diagram and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 9 is a ray diagram and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof.14A to 14C are ray diagrams and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 14B are ray diagrams and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 14C are ray diagrams and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 14D are ray diagrams and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 14E are ray diagrams and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 14F are ray diagrams and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 14G are ray diagrams and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 14G are ray diagrams and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 14H are ray diagrams and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 14H are ray diagrams and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 14H are ray diagrams and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 14H are ray diagrams and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 14H are ray diagrams and MTF curves for explaining the optical performance, etc., of an infrared optical system according to an embodiment or a comparative example thereof. 14H are ray diagrams and MTF

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0012] [Overall Configuration of Infrared Imaging Device] Fig. 1 is a schematic cross-sectional view of an infrared imaging device 100 according to this embodiment. The infrared imaging device 100 is an infrared camera that captures and visualizes light rays including infrared rays, and is used, for example, as a night vision camera or a thermometer. Specifically, as shown in Fig. 1, the infrared imaging device 100 includes an infrared optical system 10 and a sensor unit 50.

[0013] The infrared optical system 10 is a single-focus optical system for forming a subject image on the imaging surface (projection surface) I of the infrared imaging element 51, and is housed in a lens barrel (lens frame) 41. The lens barrel 41 has an opening OP through which light from the object side is incident. The infrared optical system 10 includes at least two lenses L (two in this embodiment). The configuration of the infrared optical system 10 will be described in detail below.

[0014] The sensor unit 50 includes an infrared imaging element 51 that captures the subject image formed by the infrared optical system 10. The infrared imaging element 51 is an imaging element (solid-state imaging element) sensitive to infrared light, such as a thermal microbolometer. The infrared imaging element 51 is fixed in a state where it is positioned relative to the optical axis Ax of the infrared optical system 10. The infrared imaging element 51 has a conversion unit as an imaging surface I, around which a signal processing circuit (not shown) is formed. Pixels, i.e., conversion elements, are arranged two-dimensionally in the conversion unit. Note that the infrared imaging element 51 is not limited to a thermal microbolometer, and may incorporate a quantum or other thermal imaging element.

[0015] [Specific Configuration of Infrared Optical System] Next, the configuration of the infrared optical system 10 will be described in more detail with reference to FIGS. 2A to 15. FIG. 2A is a graph showing the relationship between the thickness of a chalcogenide glass lens and the internal transmittance of infrared light, with the horizontal axis representing the infrared wavelength. FIG. 2B is a graph showing the relationship between the thickness of a chalcogenide glass lens and the internal transmittance of infrared light, with the horizontal axis representing the lens thickness. FIGS. 3A to 14 are ray diagrams and MTF curves for explaining the optical performance of the infrared optical system 10 or its comparative example, and FIG. 15 is a legend for the MTF curves in FIGS. 3A to 14. In the optical systems of FIGS. 3A to 14, the HFOV is 23°, and the F-number is F / 1 only in FIGS. 3A to 3C, and F / 1.2 in FIGS. 4 to 14. In addition, the following specifications of each infrared optical system 10 are shown in FIGS. 3A to 14. T: central thickness of each lens L [mm] Tsum: total central thickness of multiple lenses L [mm] TTL: total optical length from the object side surface of the first lens L1 to the imaging plane I [mm] BFL: back focus [mm] φ: effective optical diameter [mm] Note that, for ease of understanding, in Figures 3A to 14, the optical systems of comparative examples may also be described using the same symbols as those of the infrared optical system 10 of this embodiment.

[0016] 1, the infrared optical system 10 of this embodiment is substantially composed of, in order from the object side, an aperture stop AP and multiple lenses L (two lenses, a first lens L1 and a second lens L2 in this embodiment). However, the position of the aperture stop AP is not limited to being closer to the object side than the first lens L1.

[0017] Each lens L is a chalcogenide glass lens that transmits infrared light. As shown in Figures 2A and 2B, in the case of chalcogenide glass lenses, when the total center thickness (thickness along the optical axis Ax) of multiple lenses L is 10 to 20 mm, the internal transmittance at wavelengths of 12 to 14 μm decreases significantly, and the average internal transmittance at wavelengths of 8 to 14 μm is low. On the other hand, when the total center thickness of multiple lenses L is small (thin), the internal transmittance improves. For example, when the total center thickness is 7 mm or less, the average internal transmittance at wavelengths of 8 to 14 μm can be 90% or more. Furthermore, costs can be reduced by reducing the amount of material used and shortening the manufacturing takt time. Furthermore, when the total center thickness of multiple lenses L is 5 mm or less, an internal transmittance of 80% or more can be ensured over the entire wavelength range of 8 to 14 μm, and further cost reductions are possible. Hereinafter, an optical system in which the total center thickness of multiple lenses is 7 mm or less may be referred to as a "thin optical system," and an optical system in which the total center thickness exceeds 7 mm may be referred to as a "thick optical system."

[0018] The first lens L1, which is closest to the object, is formed into a positive meniscus shape convex toward the object side near the optical axis Ax (paraxial). On the other hand, the Nth lens Ln (the second lens L2 in this embodiment), which is closest to the image side, is not biconvex in the paraxial direction. FIGS. 6A to 6C are diagrams illustrating the difference in optical performance depending on the paraxial shape of the second lens L2 in a thin optical system. Of these, the second lens L2 in FIG. 6A has a meniscus shape convex toward the object side, the second lens L2 in FIG. 6B has a convex surface toward the object side and a flat surface toward the image side, and the second lens L2 in FIG. 6C has convex surfaces on both sides. In all of FIGS. 6A to 6C, the first lens L1 has a positive meniscus shape convex toward the object side in the paraxial direction. These diagrams show that when the first lens L1 has a positive meniscus shape convex toward the object side, if the paraxial shape of the second lens L2 (the Nth lens Ln) is biconvex, the optimized design will result in a lens that is close to a flat lens. In this case, it can also be seen that a meniscus paraxial shape for the second lens L2 (Nth lens Ln) can suppress field curvature more effectively than a plano-convex shape, resulting in superior optical performance (on-axis performance). That is, by configuring the first lens L1 to have a positive meniscus shape convex toward the object side and the Nth lens Ln to be non-biconvex, optical performance can be improved. Consequently, it is possible to achieve both a thinner lens, resulting in a shorter TTL, and improved optical performance.

[0019] It is preferable that both the object-side and image-side surfaces of the first lens L1 be aspherical, which makes it possible to achieve both improved optical performance and a reduced height (shortened TTL), as shown in Figures 3A to 3C and 4.

[0020] The Nth lens Ln is formed so that at least one of its object-side and image-side surfaces has an extremum other than on the optical axis Ax. Figures 5A to 5C are diagrams illustrating the influence of the presence or absence of an extremum other than on the optical axis Ax on the second lens L2 (optical surface) in a thin optical system on optical performance. Of these, Figure 5A shows a case where both the object-side and image-side surfaces have extremums, Figure 5B shows a case where only one of the surfaces has an extremum, and Figure 5C shows a case where neither surface has an extremum. Herein, an "extremum" refers to a point on the aspheric surface where, in the curve of the cross-sectional shape of the lens within the optical effective diameter, the tangent plane of the aspheric vertex is a plane perpendicular to the optical axis Ax. Hereinafter, when simply referring to the "extremum" of a lens, it refers to a point other than on the optical axis Ax. These diagrams reveal that in a thin optical system, the second lens L2 (Nth lens Ln) having extremums has better optical performance, and that the optical performance is better when both surfaces have extremums than when only one of the object-side and image-side surfaces have extremums. That is, it is preferable that the Nth lens Ln be formed in a shape in which at least one of the object-side and image-side surfaces has an extreme value other than on the optical axis Ax. If the Nth lens Ln does not have an extreme value, it is difficult to achieve suitable image plane correction, and it is difficult for light rays at the maximum image height to pass through. In other words, in a lens L of a thin optical system, it is possible to adjust the field curvature by providing undulations on the optical axis Ax and in the peripheral portion other than on the optical axis Ax.

[0021] Furthermore, it is preferable that the infrared optical system 10 satisfy the following formula (1): TTL>2.5×Tsum (1) Figures 7A, 7B, 8A, and 8B are diagrams for explaining the effect of the sum of the center thicknesses Tsum on the optical performance of a thin optical system. As shown in these figures, the optical performance improves generally as the sum of the center thicknesses Tsum of the multiple lenses L becomes smaller, and better optical performance can be obtained within a range in which the relationship with the total optical length TTL satisfies the above formula (1).

[0022] Furthermore, it is preferable that the infrared optical system 10 satisfy the following formula (2): 2≦BFL / Tn≦5 (2), where Tn is the central thickness of the Nth lens Ln (the second lens L2 in this embodiment). FIGS. 9A, 9B, 10A, 10B, 11A, 11B, 12A, and 12B are diagrams illustrating the influence of back focus BFL on the optical performance of a thin optical system. As shown in these diagrams, in a thin optical system, better optical performance can be obtained when the back focus BFL is in the range of two to five times the central thickness Tn of the Nth lens Ln. Note that, as shown in FIGS. 13A to 13C, in a thick optical system, unlike a thin optical system, the optical performance improves as the back focus BFL decreases.

[0023] The number of lenses L in the infrared optical system 10 is not particularly limited as long as it is at least two. For example, as shown in Fig. 14, the infrared optical system 10 may have three lenses L (first lens L1 to third lens L3). In this case, the Nth lens Ln closest to the image side is the third lens L3.

[0024] [Technical Effects of the Present Embodiment] As described above, according to this embodiment, the total center thickness of the multiple chalcogenide glass lenses L is 7 mm or less. This allows for an average internal transmittance of 90% or more for wavelengths between 8 and 14 μm. Furthermore, costs can be reduced by reducing the amount of material used and shortening the manufacturing takt time. Furthermore, paraxially, the first lens L1 has a positive meniscus shape convex toward the object side, and the Nth lens Ln is not biconvex. This improves optical performance. Consequently, both a thinned lens (reduced TTL) and improved optical performance can be achieved. Furthermore, the Nth lens Ln is formed so that at least one of its object-side and image-side surfaces has an extremum other than on the optical axis Ax. This enables adjustment of field curvature, allowing both a thinned lens (reduced TTL) and improved optical performance to be achieved. As described above, this embodiment allows for a thin lens (reduced TTL) and improved optical performance to be achieved.

[0025] Furthermore, according to this embodiment, it is preferable that the total center thickness of the multiple lenses L is 5 mm or less. This ensures an internal transmittance of 80% or more over the entire wavelength range of 8 to 14 μm. This also enables further cost reductions by reducing the amount of material used and shortening the takt time during manufacturing.

[0026] Furthermore, according to this embodiment, it is preferable that both the object-side and image-side surfaces of the first lens L1 are aspherical, which makes it possible to achieve both improved optical performance and a reduced height (shortened TTL).

[0027] Furthermore, according to this embodiment, it is preferable that both the object-side and image-side surfaces of the Nth lens Ln have extrema other than those on the optical axis Ax, thereby improving optical performance compared to when only one of the object-side and image-side surfaces has an extrema.

[0028] Furthermore, according to this embodiment, it is preferable that the infrared optical system 10 satisfies the above formula (1). When the relationship between the sum of the center thicknesses Tsum of the multiple lenses L and the total optical length TTL satisfies the above formula (1), better optical performance can be obtained.

[0029] Furthermore, according to this embodiment, it is preferable that the infrared optical system 10 satisfies the above formula (2). When the relationship between the back focus BFL and the center thickness Tn of the Nth lens Ln satisfies the above formula (2), better optical performance can be obtained.

[0030] The above describes one embodiment of the present invention, but embodiments to which the present invention can be applied are not limited to the above-described embodiment and its variations, and can be modified as appropriate within the scope of the spirit of the present invention.

[0031] For example, in the above embodiment, the infrared optical system 10 is used in the infrared imaging device 100. However, the use of the infrared optical system according to the present invention is not particularly limited, and the infrared optical system can also be suitably applied to uses other than imaging, such as light projection, illumination, and light collection.

[0032] As described above, the present invention is useful for providing an infrared optical system that can be made thin and inexpensive, and an infrared imaging device including the same.

[0033] 100 Infrared imaging device 10 Infrared optical system 51 Infrared imaging element (imaging element) AP Aperture stop Ax Optical axis I Imaging surface L Lens L1 First lens L2 Second lens L3 Third lens Ln Nth lens T Lens center thickness Tn Nth lens center thickness Tsum Sum of center thicknesses of multiple lenses TTL Total optical length BFL Back focus

Claims

1. An infrared optical system comprising a plurality of lenses made of chalcogenide glass, the plurality of lenses having a total center thickness of 7 mm or less, a first lens of the plurality of lenses closest to the object side being formed into a positive meniscus shape convex toward the object side on the paraxial line, and an Nth lens of the plurality of lenses closest to the image side being not biconvex on the paraxial line, and having a shape in which at least one of the object side and image side surfaces has an extreme value other than on the optical axis.

2. The infrared optical system according to claim 1, wherein the total center thickness of the plurality of lenses is 5 mm or less.

3. The infrared optical system according to claim 1, wherein both the object side surface and the image side surface of the first lens are aspheric.

4. The infrared optical system according to claim 1, wherein the Nth lens has a shape in which both the object side surface and the image side surface have extreme values ​​other than on the optical axis.

5. The infrared optical system according to claim 1, which satisfies the following formula (1): TTL>2.5×Tsum (1) where TTL: total optical length from the object side of the first lens to the imaging surface Tsum: total value of center thicknesses of a plurality of lenses 6. The infrared optical system according to claim 1, which satisfies the following formula (2): 2≦BFL / Tn≦5 (2), where BFL is the back focus, and Tn is the central thickness of the Nth lens closest to the image side.

7. The infrared optical system according to claim 1, wherein the plurality of lenses consists of two lenses, the first lens and the Nth lens.

8. An infrared imaging device comprising: an infrared optical system according to any one of claims 1 to 7; and an imaging element for detecting an image formed by the infrared optical system.

Images