Optical system, imaging device, and imaging system

Abstract

An optical system (2) that forms an image at a first image formation position (P1) by first light (L11) in a visible range, and forms an image at a second image formation position (P2) by second light (L12) in a far-infrared range is provided with a lens group (3) and an optical division element (21). The lens group has an optical axis extending from the front on which the first and the second light are incident to the rear from which the first and the second light are emitted, the focal length of the first light, and the focal length of the second light. The optical division element is disposed at the rear of the lens group, divides the first and the second light from the lens group from each other, guides the first light to the first image formation position, and guides the second light to the second image formation position. The lens group comprises lens elements that transmit the first and the second light such that the first image formation position corresponds to the focal length of the first light, and separately from the first image formation position, the second image formation position corresponds to the focal length of the second light.

Description

[Technical Field]

[0001] The present disclosure relates to an optical system, an imaging device, and an imaging system that perform imaging in the visible range and the far-infrared range. [Background technology]

[0002] Patent Document 1 discloses a uniaxial lens module for a thermal imaging camera, which aims to enable the thermal imaging camera to simultaneously capture visible and far-infrared light to obtain vivid images. The lens module includes an objective lens facing the subject, a beam separator located behind the objective lens, which reflects far-infrared light and transmits visible light from the light passing through the objective lens, a visible light imaging lens located behind the beam separator, and a far-infrared imaging lens. The visible light imaging lens forms an image using visible light from the beam separator onto a CCD sensor located behind it. The far-infrared imaging lens forms an image using far-infrared light from the beam separator onto a far-infrared detector, which converts the optical image into a thermal image signal and outputs it. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-80976 Summary of the Invention [Problem to be solved by the invention]

[0004] The present disclosure provides an optical system, an imaging device, and an imaging system that can easily achieve both imaging in the visible range and imaging in the far-infrared range. [Means for solving the problem]

[0005] The optical system in the present disclosure forms an image at a first image position using first light having a wavelength in the visible range and forms an image at a second image position using second light having a wavelength in the far-infrared range. The optical system includes a lens group and a light branching element. The lens group has an optical axis extending from a front where the first and second light enters to a rear where the first and second light exit, a focal length of the first light, and a focal length of the second light. The light branching element is disposed behind the lens group and branches the first and second light from the lens group, directing the first light to the first image position and directing the second light to a second image position. The lens group is composed of lens elements that transmit the first and second light so that the first image position corresponds to the focal length of the first light and the second image position corresponds to the focal length of the second light, separate from the first image position.

[0006] An imaging device according to the present disclosure includes the optical system described above, a first imaging unit, and a second imaging unit. The first imaging unit is disposed at a first imaging position and captures an image using a first light. The second imaging unit is disposed at a second imaging position and captures an image using a second light.

[0007] An imaging system according to the present disclosure includes the imaging device described above and a control unit that analyzes an image captured by the imaging device. [Effects of the Invention]

[0008] The optical system and imaging device of the present disclosure can easily perform imaging in both the visible range and the far-infrared range. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram illustrating an imaging device and an imaging system according to a first embodiment of the present disclosure. [Figure 2] FIG. 1 is a diagram showing the configuration of an optical system according to a first embodiment. [Figure 3] FIG. 1 is a diagram illustrating an example of an optical path of visible light in the back focus of an optical system; [Figure 4] FIG. 1 is a diagram illustrating an example of an optical path of far-infrared light in the back focus of an optical system; [Figure 5] A diagram illustrating data on various lens materials in an optical system. [Figure 6] Graph showing simulation results of the imaging performance of the optical system of the first embodiment. [Figure 7] Graph illustrating the MTF of visible light in the optical system of embodiment 1. [Figure 8] Graph illustrating the MTF of far-infrared light in the optical system of embodiment 1. [Figure 9] 1 is a diagram showing a first numerical example of the optical system according to the first embodiment. [Figure 10] 1 is a diagram showing a second numerical example of the optical system according to the first embodiment; [Figure 11] 1 is a diagram showing a third numerical example of the optical system according to the first embodiment; [Figure 12] FIG. 10 is a diagram showing the configuration of an optical system according to a second embodiment. [Figure 13] Graph showing simulation results of the imaging performance of the optical system of the second embodiment. [Figure 14] 1 is a diagram showing a first numerical example of the optical system according to the second embodiment. [Figure 15] 1 is a diagram showing a second numerical example of the optical system according to the second embodiment; [Figure 16] 1 is a diagram showing a third numerical example of the optical system according to the second embodiment; [Figure 17] FIG. 10 is a diagram showing the configuration of an optical system according to a third embodiment. [Figure 18] Graph showing simulation results of the imaging performance of the optical system of the third embodiment. [Figure 19] 1 is a diagram showing a first numerical example of the optical system according to the third embodiment. [Figure 20] 1 is a diagram showing a second numerical example of the optical system according to the third embodiment; [Figure 21] 1 is a diagram showing a third numerical example of the optical system according to the third embodiment. [Figure 22] FIG. 10 is a diagram showing the configuration of an optical system according to a fourth embodiment. [Figure 23] Graph showing the results of a simulation of the imaging performance of the optical system of the fourth embodiment. [Figure 24] 1 is a diagram showing a first numerical example of the optical system according to the fourth embodiment. [Figure 25]1 is a diagram showing a second numerical example of the optical system according to the fourth embodiment; [Figure 26] 1 is a diagram showing a third numerical example of the optical system according to the fourth embodiment. [Figure 27] FIG. 1 is a diagram showing a modified example of the optical system of the first embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. However, more detailed explanation than necessary may be omitted. For example, detailed explanation of well-known matters or redundant explanation of substantially the same configuration may be omitted. This is to avoid unnecessary redundancy in the following explanation and to facilitate understanding by those skilled in the art.

[0011] The applicant provides the accompanying drawings and the following description to enable those skilled in the art to fully understand the present disclosure, and does not intend for them to limit the subject matter described in the claims.

[0012] (Embodiment 1) Hereinafter, a first embodiment of the present disclosure will be described with reference to the drawings. In this embodiment, an imaging device and its optical system that can capture images in both the visible range and the far-infrared range will be described.

[0013] 1. Imaging device 1 is a diagram illustrating an image capturing device 1 and an image capturing system 20 according to this embodiment. The image capturing system 20 of this embodiment includes the image capturing device 1 and a control unit 15.

[0014] In this embodiment, the imaging device 1 includes an optical system 2, a visible light imaging sensor 11, and a far-infrared imaging sensor 12, as shown in Fig. 1 for example. The imaging device 1 of this embodiment is a camera device that performs imaging in the visible range, i.e., visible imaging, and imaging in the far-infrared range, i.e., far-infrared imaging, coaxially using the optical system 2. For example, the visible range is a wavelength range of 400 nm to 750 nm, and the far-infrared range is a wavelength range of 3 µm to 20 µm. For example, a part of the visible range and a part of the far-infrared range are targets for visible imaging and far-infrared imaging by the imaging device 1. For example, the far-infrared range may be 7 µm to 12 µm.

[0015] The imaging device 1 of this embodiment can be applied to various applications, such as combining visible imaging with thermal imaging using far-infrared imaging, motion sensors, or night vision. For example, in this system 20, image analysis can be applied, such as measuring the temperature of a subject 10 such as a person and performing personal recognition on the same subject 10. The imaging device 1 of this system 20 can be incorporated into various electronic devices such as mobile terminals, mounted on moving objects such as drones or vehicles, or installed as a surveillance camera. In these various application examples, miniaturization of the imaging device 1 is useful.

[0016] In the imaging device 1 of this embodiment, the optical system 2 collects light L10 incident from the subject 10, and guides visible light L11 in the incident light L10 to the visible light imaging sensor 11, and guides far-infrared light L12 in the incident light L10 to the far-infrared imaging sensor 12. Such an optical system 2 can provide a compact device configuration with only one lens barrel in the imaging device 1. The optical system 2 of this embodiment has a configuration that allows for further miniaturization. The configuration of the optical system 2 will be described later.

[0017] The visible light imaging sensor 11 is an imaging element, such as a CCD or CMOS image sensor, made of a material such as amorphous silicon that has light sensitivity in the visible range. The visible light imaging sensor 11 has an imaging surface on which a plurality of pixels are arranged at a predetermined pitch. The pixel pitch of the visible light imaging sensor 11 is, for example, approximately 3 μm. The visible light imaging sensor 11 captures an image formed on the imaging surface by visible light L11 incident via the optical system 2, and generates an image signal representing the captured image in the visible range, i.e., a visible image Im1. The visible light imaging sensor 11 is an example of a first imaging unit in this embodiment.

[0018] The far-infrared imaging sensor 12 is an imaging element having light-receiving sensitivity in the far-infrared region, such as a bolometer, a thermopile, or an SOI diode. The far-infrared imaging sensor 12 has an imaging surface on which a plurality of pixels are arranged at a predetermined pitch. The pixel pitch of the far-infrared imaging sensor 12 is, for example, 10 μm to 300 μm. The far-infrared imaging sensor 12 captures an image formed on the imaging surface by far-infrared light L12 incident via the optical system 2, and generates an image signal representing the captured image in the far-infrared region, i.e., a far-infrared image Im2. The far-infrared imaging sensor 12 is an example of a second imaging unit in this embodiment.

[0019] According to the imaging device 1 configured as above, the visible image Im1 and the far-infrared image Im2 are output as coaxial imaging results using the optical system 2. Therefore, according to the imaging device 1 of this embodiment, it is possible to suppress image misalignment of the subject 10 between the visible image Im1 and the far-infrared image Im2, for example, and obtain an image output that is easy to analyze in various applications that combine visible imaging and far-infrared imaging.

[0020] In this system 20, the control unit 15 receives image signals from the imaging device 1 and performs various image analyses based on the images Im1 and Im2 represented by the received image signals. The control unit 15 includes, for example, a CPU or MPU that executes programs stored in an internal memory to realize various functions. The control unit 15 may also include dedicated hardware circuits designed to realize desired functions. The control unit 15 may also include a CPU, MPU, GPU, DSP, FPGA, ASIC, or the like.

[0021] For example, the control unit 15 of the present system 20 performs personal recognition of the subject 10 based on the visible image Im1 captured by the visible imaging sensor 11 in the imaging device 1, and recognizes the temperature of the subject 10 based on the far-infrared image Im2 captured by the far-infrared imaging sensor 12. Furthermore, the control unit 15 associates the recognition results from the visible image Im1 and the far-infrared image Im2 based on, for example, the position of the subject 10 in each image Im1, Im2, and manages them as analysis result information. According to the present system 20, the optical system 2 of the imaging device 1 can suppress image misalignment of the same subject 10 between the visible image Im1 and the far-infrared image Im2, so the control unit 15 can easily manage the information as described above.

[0022] In this embodiment, as described above, the imaging device 1 can be miniaturized, and an optical system 2 capable of forming an image with high accuracy using both visible light L11 and far-infrared light L12 is provided. The configuration of the optical system 2 of this embodiment will be described below.

[0023] 2. Optical system 2 is a diagram showing the configuration of the optical system 2 according to the first embodiment. The optical system 2 includes a lens group 3 having an optical axis Z0 onto which externally incident light L10 is incident, a light branching element 21 that branches the incident light L10 from the lens group 3 into visible light L11 and far-infrared light L12, and a far-infrared transmission filter 22. Hereinafter, the direction of the optical axis Z0 of the lens group 3 in the optical system 2 will be referred to as the Z direction, and two directions perpendicular to the Z direction will be referred to as the X and Y directions. In addition, in the Z direction, the object side facing outward from the optical system 2 will be referred to as the -Z side or front, and the opposite image plane side will be referred to as the +Z side or rear.

[0024] The optical system 2 has, on the +Z side, i.e., rear side, an imaging position P1 where an image is formed by visible light L11 and an imaging position P2 where an image is formed by far-infrared light L12. The imaging surface of the visible light imaging sensor 11 is disposed at the imaging position P1 where the visible light L11 is formed by the optical system 2. The imaging surface of the far-infrared imaging sensor 12 is disposed at the imaging position P2 where the far-infrared light L12 is formed. In the optical system 2, a light branching element 21 is disposed between the lens group 3 and each imaging sensor 11, 12. A far-infrared transmitting filter 22 is disposed between the light branching element 21 and the far-infrared imaging sensor 12.

[0025] The optical system 2 of this embodiment is configured so that, for example, no lens element with refractive power is provided between the light branching element 21 and each of the imaging sensors 11, 12. This configuration allows for miniaturization in terms of the overall length of the optical system 2, the number of parts, etc., and reduces the lens alignment process after light branching, thereby reducing costs. In this embodiment, to achieve both visible light imaging and far-infrared imaging in such a compact configuration, the lens group 3 on the -Z side (i.e., forward) of the light branching element 21 forms an imaging optical system for each of the visible light L11 and far-infrared light L12 contained in the incident light L10. In other words, each imaging position P1, P2 is set corresponding to the focal length of the lens group 3.

[0026] The lens group 3 in the optical system 2 is made of a lens material that is optically transparent in both the visible and far-infrared regions. Due to the wavelength dependency of the refractive index of the lens material, the focal length of the lens group 3 can vary depending on the wavelength of the light to be imaged. The lens group 3 functions as an imaging optical system by applying a refractive power according to the focal length to incident light L10 from outside. After passing through the lens group 3, the incident light L10 includes visible light L11 and far-infrared light L12 and enters the light branching element 21. The visible light L11 and far-infrared light L12 are examples of first and second light, respectively, in this embodiment.

[0027] In the optical system 2 of the first embodiment, the lens group 3 includes two lens elements 31 and 32 and an aperture stop 30. In the lens group 3, a first lens element 31 and a second lens element 32 are arranged along the optical axis Z0 in order from the front. In the first embodiment, as an example of the lens material, the first lens element 31 is made of chalcohalide A (CHA) and the second lens element 32 is made of zinc sulfide (ZnS) (see FIG. 5). Note that FIG. 5 illustrates three types of chalcohalide glasses having different compositions, which are referred to as chalcohalides A, B, and C, respectively. The lens material of the lens group 3 is not limited to the above, and may be various materials that transmit the wavelength band of visible light L11 used in visible imaging in the visible range and the wavelength band of far-infrared light L12 used in far-infrared imaging in the far-infrared range. For example, the wavelength band transmitted by the lens material may be 0.4 μm to 12 μm.

[0028] The diaphragm 30 is, for example, an aperture diaphragm, and limits the amount of visible light L11 and far-infrared light L12 in the incident light L10. In the first embodiment, the diaphragm 30 is disposed between the first and second lens elements 31 and 32. The diaphragm 30 may be disposed at any position in the lens group 3, and is not necessarily provided in the optical system 2.

[0029] 2, the optical branching element 21 is configured to transmit visible light L11 and emit it to the +Z side, while reflecting far-infrared light L12 and emit it to the +X side, for incident light L10 from the -Z side. For example, the optical branching element 21 has a specific wavelength band (i.e., transmission band) that selectively transmits light, and is configured by setting the wavelength band of visible light L11 as the transmission band in advance in a bandpass filter with optical properties that reflects light outside the transmission band.

[0030] 2, the far-infrared transmission filter 22 is disposed on the +X side of the optical branching element 21 and selectively transmits the far-infrared light L12. The far-infrared transmission filter 22 is configured with various filter elements, such as a band-pass filter whose transmission band is set in advance to the wavelength band of the far-infrared light L12.

[0031] In the optical system 2 of this embodiment as described above, the lens group 3 as an imaging optical system is optically designed in consideration of the back focus that the visible light L11 and the far-infrared light L12 pass through the optical branching element 21 and the like behind the lens group 3 and reach the respective imaging positions P1 and P2.

[0032] 2-1.Back focus The back focus in the optical system 2 of this embodiment will be described with reference to Figures 3 and 4. Hereinafter, the focal length of the visible light L11 by the lens group 3 will be referred to as "Fvis," and the focal length of the far-infrared light L12 will be referred to as "Fir."

[0033] 3 illustrates the optical path of visible light L11 from a rear position P3 of the lens group 3. The rear position P3 is a position on the optical axis Z0 of the rearmost lens surface of the lens group 3. In the optical path of FIG. 3, visible light L11 on the optical axis Z0 of the lens group 3 is emitted from the rear position P3 in the +Z direction and enters the light branching element 21. The incident visible light L11 then passes through the light branching element 21, exits from the light branching element 21 in the +Z direction, and reaches the imaging position P1. The air-equivalent optical path length of this optical path, i.e., the air-equivalent length Lvis, corresponds to the focal length Fvis of the visible light L11 through the lens group 3.

[0034] For example, when the optical system 2 is focused at infinity, the image position P1 is located at the focal point of the lens group 3, and therefore the difference between the air-equivalent length Lvis and the focal length Fvis is expressed as the distance difference between the principal point and the rear position P3 of the lens group 3. Furthermore, when the optical system 2 is focused at a finite subject distance, the shorter the subject distance, the farther the image position P1 of visible light L11 is from the focal point of the lens group 3, and the air-equivalent length Lvis of the back focus increases accordingly.

[0035] 4 illustrates the optical path of far-infrared light L12 from a rear position P3 of the lens group 3. In this optical path, when the far-infrared light L12 on the optical axis Z0 of the lens group 3 enters the light branching element 21 from the rear position P3, like the visible light L11, it is reflected from the light branching element 21 in the +X direction. The reflected far-infrared light L12 passes through the far-infrared transmission filter 22 and reaches the imaging position P2. The air-equivalent length Lir of this optical path and the focal length Fir of the far-infrared light L12 by the lens group 3 have the same corresponding relationship as in the case of the visible light L11 described above.

[0036] 3 and 4, the lens group 3 is configured so that the air-equivalent length Lvis of visible light L11 is shorter than the air-equivalent length Lir of far-infrared light L12. This makes it easy to obtain the performance of forming images with high accuracy in both the visible and far-infrared ranges in the above-described compact optical system 2 by utilizing the wavelength dependency of the lens material, etc.

[0037] 2-1-1. Optimizing back focus Furthermore, in the optical system 2 of Example 1 of this embodiment, the lens group 3 is configured so as to satisfy the following conditional expression (1). 0.13<(Lir-Lvis) / Fir<0.23 …(1)

[0038] In evaluating the above formula (1), the wavelength of visible light L11 is λvis = 587 nm, the wavelength of the d-line, which is generally used as the reference wavelength in the visible range. Furthermore, the wavelength of far-infrared light L12 is λir = 10 μm, which is within the so-called atmospheric window that is easily usable for various applications and is also the peak wavelength of light emitted in the far-infrared range by materials at room temperature. Hereinafter, the middle part of the above formula (1), (Lir - Lvis) / Fir, may be referred to as the factor Lf.

[0039] According to the optical system 2 that satisfies the above formula (1), it is possible to optimize the back focus in the above-mentioned compact configuration and improve the imaging performance in both the visible range and the far-infrared range. The understanding of the condition formula (1) will be explained using FIG. 5.

[0040] Fig. 5 is a diagram illustrating data on various lens materials. Fig. 5 shows the names, compositions, refractive indices nvis and nir at wavelengths λvis and λir, and a ratio Nr (described later) of the various lens materials that can be used in optical system 2.

[0041] The refractive index nvis of visible light L11 and the refractive index nir of far-infrared light L12 each have various values ​​for various lens materials, as shown in Figure 5. In contrast, the above formula (1) utilizes universal characteristics common to various wavelength-dependent lens materials, as found below, and is applicable to various optical systems that are not limited by the lens material or number of lenses.

[0042] First, regarding the difference in air-equivalent length (Lir-Lvis) in the factor Lf in the above equation (1), it is considered that the position of the principal point in the lens group 3 hardly changes, for example, depending on the wavelength. From this, the difference in air-equivalent length (Lir-Lvis) can be approximated by the difference between the focal length Fvis of visible light L11 and the focal length Fir of far-infrared light L12, as shown in the following equation (11): Lir-Lvis ≒ Fir-Fvis … (11)

[0043] Furthermore, the focal lengths Fvis and Fir can be approximated as in the following equation (12) based on the lens manufacturer's formula when the lens group 3 is considered to be a thin lens, where C is a constant determined by the curvature of the lens. Fvis≒C / (nvis-1) Fir≒C / (nir-1) …(12)

[0044] According to the above expressions (11) and (12), the factor Lf of the conditional expression (1) can be approximated as in the following expression (13) using the refractive indices nvis and nir for the visible light L11 and the far-infrared light L12. Lf≒(nvis-1) / (nir-1)-1 …(13)

[0045] In the right-hand side of the above equation (13), the refractive indices nvis and nir are included as a ratio Nr = (nvis-1) / (nir-1). The ratio Nr indicates the ratio between the deviation of the refractive index nir of far-infrared light L12 and the deviation of the refractive index nvis of visible light L11 from the refractive index n = 1 of a vacuum, and can be considered as a global physical property, such as the change in refractive index across the far-infrared and visible regions.

[0046] According to the ratio Nr of the refractive indices nvis and nir as expressed in the above formula (13), the variation in various lens materials is much smaller than the variation in the refractive indices nvis and nir themselves, as shown in Figure 5. In this way, the ratio Nr of the global physical property is almost the same for various lens materials, and it is therefore believed that conditional formula (1) based on the ratio Nr can be universally applied.

[0047] The minimum value of the right-hand side of equation (13) in Figure 5 is 0.139, which is calculated using chalcohalide CHA. Equation (13) uses an approximation that considers optical system 2 as a thin lens. The left-hand side of equation (13) for actual optical system 2 tends to be larger than the ideal case (i.e., the right-hand side of equation (13)) as shown in the approximation. The numerical range of equation (1) was determined by taking into account practical variations due to various differences in optical systems, such as lens material and number of lenses.

[0048] 2-1-2.Simulation of imaging performance Numerical simulations of the optical system 2, which confirmed the effect of the imaging performance due to the above-described conditional formula (1), will be described with reference to FIGS.

[0049] Fig. 6 is a graph showing the results of a simulation of the imaging performance of the optical system 2 in embodiment 1. In Fig. 6, the horizontal axis represents the set value of the factor Lf (= (Lir - Lvis) / Fir) in conditional formula (1), and the vertical axis represents the evaluation value of the average MTF (modulation transfer function). As will be described later, the average MTF is the average value of the MTF for visible light L11 and the MTF for far-infrared light L12.

[0050] In the numerical simulation of Fig. 6, the factor Lf in the above formula (1) was set to various values ​​in the configuration of the optical system 2 in embodiment 1, and the optical design was optimized for each setting value. CODE V software from Synopsys was used for this simulation. Furthermore, the average MTF was used from the perspective of evaluating the imaging performance of both visible light L11 and far-infrared light L12 in the designed optical system 2.

[0051] Each plot point in the graph of Figure 6 corresponds to a different example using a different optical design in the optical system 2 of embodiment 1. Example 1 corresponds to plot point p11 where the factor Lf (=0.172) is within the range defined in conditional formula (1). Example 2 corresponds to plot point p12 where the factor Lf (=0.124) is below the lower limit of formula (1), and Example 3 corresponds to plot point p13 where the factor Lf (=0.260) is above the upper limit. The setting of the factor Lf was changed by optical design in which the air-equivalent length Lvis of the visible light L11 was changed while the focal length Fir and air-equivalent length Lir of the far-infrared light L12 were fixed.

[0052] The MTF for visible light L11 and the MTF for far-infrared light L12 for the average MTF of Example 1 are shown in Figures 7 and 8, respectively. In each of Figures 7 and 8, the horizontal axis represents the image height as a percentage, and the vertical axis represents the calculated value of each MTF. Also shown in each of Figures 7 and 8 are the MTF curves in the sagittal direction, the MTF curves in the tangential direction, and the boundary line of the diffraction limit.

[0053] For the MTF for visible light L11 shown in Figure 7, the wavelength λvis of the d-line and a high-frequency spatial frequency of 30 lp / mm (lp: line pairs) were used. For the MTF for far-infrared light L12 shown in Figure 8, the wavelength λir and a low-frequency spatial frequency of 10 lp / mm were used. These spatial frequencies were set taking into consideration the pixel pitch of each image sensor 11, 12. That is, since the pixel pitch of the far-infrared image sensor 12 is considered to be larger than the pixel pitch of the visible image sensor 11 due to physical limitations, the spatial frequency of the far-infrared MTF was set lower than the spatial frequency of the visible MTF.

[0054] The average MTF was calculated by averaging the MTF for visible light L11 and the MTF for far-infrared light L12 obtained as shown in Figures 7 and 8 for image heights of 0%, 30%, 60%, and 100% in the sagittal and tangential directions. It is believed that the larger this average MTF, the higher the imaging performance for visible light L11 and far-infrared light L12.

[0055] According to the graph of FIG. 6, the graph has a mountain-like shape in the numerical range of the following formula (1a) which includes the numerical range of conditional formula (1). 0.08<(Lir-Lvis) / Fir<0.28 …(1a) Within the range of the above formula (1a), the average MTF in the optical system 2 of Example 1, which satisfies the conditional formula (1), is significantly greater than the average MTF in Examples 2 and 3, which do not satisfy the conditional formula (1). As described above, it has been confirmed that the imaging performance of visible light L11 and far-infrared light L12 can be improved by the conditional formula (1).

[0056] 2-2. Numerical examples Numerical Examples 1 to 3 showing Examples 1 to 3 of the optical system 2 of the first embodiment as described above will be described with reference to FIGS.

[0057] Fig. 9 is a table showing Numerical Example 1 of the optical system 2 of Embodiment 1. The table of Fig. 9 includes surface data D11a, aspherical surface data D11b, and various data D11c of the optical system 2 of Example 1 of this embodiment.

[0058] The surface data D11a indicates the shape, radius of curvature, surface spacing, and material of each of the surfaces S1 to S11 arranged in order from the object side in the optical system 2, and also includes a remarks column. For example, surface number S2 is the object-side lens surface of the first lens element 31 and has an aspherical shape. Surface number S1 indicates an object such as subject 10 located at infinity (see the remarks column). In the surface data D11a of Fig. 9, surface number S7 is the object-side surface of the light branching element 21, and the surfaces after this surface indicate the surfaces through which the optical path of visible light L11 passes and the optical path of far-infrared light L12 passes, respectively (see Figs. 3 and 4).

[0059] The aspherical surface data D11b indicates various coefficients in the following equation (2) that define the shape of the aspherical surface for each of the surfaces S2 and S3 having an aspherical shape in the surface data D11a.

[0060]

number

[0061] In the above formula (2), h is the height in the radial direction, k is the conic constant, and An is the n-th order aspheric coefficient. In the second term on the right side of the above formula (2), for example, n is an even number between 4 and 20, and the sum is taken for each n. According to the above formula (2), the sag amount z at the height h in the radial direction on the target surface is defined rotationally symmetrically.

[0062] The various data D11c indicate the F-number of the diaphragm 30, the full angle of view, and the focal length Fir of the far-infrared light L12 in the optical system 2. The optical system 2 of this embodiment has a focal length Fvis of 5.8 mm for the visible light L11.

[0063] According to the above numerical example, the optical system 2 of Example 1 of Embodiment 1 has an air-equivalent length Lvis of visible light L11 of 6.550 mm and an air-equivalent length Lir of far-infrared light L12 of 7.721 mm at the back focus. As a result, the factor Lf of conditional formula (1) and the average MTF are obtained as shown by plot point p11 in FIG.

[0064] Fig. 10 is a table showing Numerical Example 2 of the optical system 2 of Embodiment 1. The table in Fig. 10 includes surface data D12a, aspherical surface data D12b, and various data D12c of the optical system 2 of Example 2 of this embodiment. Each piece of data D12a to D12c indicates information similar to each piece of data D11a to D11c of Example 1 for the optical system 2 of Example 2 of this embodiment.

[0065] In Example 2 of this embodiment, in order to extend the air-equivalent length Lvis for visible light L11 more than in Example 1, the surface spacing for surface number S8 in surface data D12a was changed from that in Figure 9, as shown in Figure 10, and the optical design, such as the surface shape, was optimized. In optical system 2 of Example 2 of Embodiment 1, the air-equivalent length Lir and focal length Fir for far-infrared light L12 are the same as in Example 1, and the air-equivalent length Lvis for visible light L11 is 6.879 mm. As a result, the factor Lf and average MTF shown at plot point p12 in Figure 6 are obtained.

[0066] Fig. 11 is a table showing Numerical Example 3 of the optical system 2 of Embodiment 1. The table in Fig. 11 includes surface data D13a, aspherical surface data D13b, and various data D13c of the optical system 2 of Example 3 of this embodiment. Each piece of data D13a to D13c indicates the same information as each piece of data D11a to D11c of Example 1 for the optical system 2 of Example 3 of this embodiment.

[0067] In Example 3 of this embodiment, contrary to Example 2, the air-equivalent length Lvis of visible light L11 was shortened compared to Example 1 to optimize the optical design. In the optical system 2 of Example 3 of Embodiment 1, the air-equivalent length Lir and focal length Fir of far-infrared light L12 are the same as in Example 1, and the air-equivalent length Lvis for visible light L11 is 5.950 mm. As a result, the factor Lf and average MTF shown in plot point p13 in FIG. 6 are obtained.

[0068] 6, the optical systems 2 of Examples 1 to 3 described above provide higher average MTF than when the factor Lf is 0, not only at plot point p11 in Example 1 but also at plot points p12 and p13 in Examples 2 and 3. Therefore, the optical systems 2 of Examples 1 to 3 of this embodiment can improve the imaging performance for both visible light L11 and far-infrared light L12 by making the air-equivalent length Lvis of visible light L11 shorter than the air-equivalent length Lir of far-infrared light L12.

[0069] 3. Summary As described above, the optical system 2 in this embodiment forms an image at an image position P1, which is an example of a first image position, using visible light L11, which is an example of first light having a wavelength in the visible range. The optical system 2 forms an image at an image position P2, which is an example of a second image position, using far-infrared light L12, which is an example of second light having a wavelength in the far-infrared range. The optical system 2 includes a lens group 3 and a light branching element 2. The lens group 3 has an optical axis Z0 extending from a front where the visible light L11 and the far-infrared light L12 are incident to a rear where they exit, a focal length Fvis of the visible light L11, and a focal length Fir of the far-infrared light L12. The light branching element 21 is disposed behind the lens group 3 and branches the visible light L11 and the far-infrared light L12 from the lens group 3, and guides the visible light L11 to the image position P1 and the far-infrared light L12 to the image position P2. The lens group 3 is composed of lens elements 31 and 32 that transmit visible light L11 and far-infrared light L12 so that the imaging position P1 corresponds to the focal length Fvis of visible light L11, and the imaging position P2, separate from the imaging position P1, corresponds to the focal length Fir of far-infrared light L12.

[0070] According to the optical system 2 described above, the imaging position P1 of the visible light L11 and the imaging position P of the far-infrared light L12 are set corresponding to the focal lengths Fvis and Fir of the lens group 3, respectively, and the lens group 3 in front of the light branching element 2 constitutes both imaging optical systems. This simplifies the configuration of the back focus behind the light branching element 2 in the optical system 2, making it easier to achieve both visible light imaging and far-infrared imaging, for example by reducing the size of the imaging device 2.

[0071] In the optical system 2 of this embodiment, an air-equivalent length Lvis, which is an example of a first air-equivalent length that the visible light L11 travels from the rear end position of the lens group 3, i.e., the rear position P3, to the imaging position P1, is shorter than an air-equivalent length Lir, which is an example of a second air-equivalent length that the far-infrared light L12 travels from the rear position P3 to the imaging position P2. This makes it easier for the lens group 3 to ensure imaging performance for both the visible light L11 and the far-infrared light L12.

[0072] The optical system 2 in this embodiment may satisfy conditional expression (1) based on the air-equivalent length Lvis of the visible light L11 at the back focus, the air-equivalent length Lir of the far-infrared light L12, and the focal length Fir of the far-infrared light L12. This optimizes the back focus in the optical system 2, thereby improving the imaging performance of both the visible light L11 and the far-infrared light L12. Furthermore, the optical system 2 in this embodiment may satisfy the numerical range of expression (1a) above instead of conditional expression (1).

[0073] The optical system 2 in this embodiment may satisfy conditional formula (1) when the wavelength of the visible light L11 is λvis=587 nm and the wavelength of the far-infrared light L12 is λir=10 μm. These wavelengths λvis and λvis are useful for visible imaging and far-infrared imaging, respectively.

[0074] In the optical system 2 of this embodiment, no lens element is provided behind the light branching element 2, and the lens elements 31 and 32 of the lens group 3 are provided in front of the light branching element 2. This allows the optical system 2 to be made compact.

[0075] In the optical system 2 of this embodiment, the first lens element 31 in the lens group 3 is made of chalcohalide glass. This allows the optical system 2, which is optically transparent in the visible and far-infrared regions, to be provided using a material that is easy to handle for various applications. The lens element made of chalcohalide material is not limited to the first lens element 31. Furthermore, the first lens element 31 may be made of a lens material other than a chalcohalide material.

[0076] In the optical system 2 of this embodiment, the light branching element 21 has the optical property of transmitting visible light L11 and reflecting far-infrared light L12. This bends the optical path of the far-infrared light L12, which may be longer than the optical path of the visible light L11, at the back focus of the optical system 2 (see FIGS. 3 and 4), making it easier to make the optical system 2 compact.

[0077] In this embodiment, the imaging device 1 includes an optical system 2, a visible light imaging sensor 11 which is an example of a first imaging unit, and a far-infrared imaging sensor 12 which is an example of a second imaging unit. The visible light imaging sensor 11 is disposed at an imaging position P1 of visible light L11 formed by the optical system 2, and captures an image using the visible light L11. The far-infrared imaging sensor 12 is disposed at an imaging position P2 of far-infrared light L12 formed by the optical system 2, and captures an image using the far-infrared light L12. According to the imaging device 1 of this embodiment, the optical system 2 can reduce the size of a configuration that achieves both visible light imaging and far-infrared imaging, and improve the imaging performance of both, making it easier to achieve both.

[0078] In this embodiment, the imaging system 20 includes an imaging device 1 and a control unit 15 that analyzes various images Im1 and Im2 captured by the imaging device 1. According to this system 20, the optical system 2 of the imaging device 1 makes it easy to analyze both the visible image Im1 and the far-infrared image Im2.

[0079] (Embodiment 2) 12 to 16, a second embodiment will be described below. In the first embodiment, an example was described in which the lens materials were different between the two lens elements 31 and 32 in the optical system 2. In the second embodiment, an optical system will be described in which the lens elements are made of the same lens material.

[0080] Hereinafter, the optical system according to this embodiment will be described, omitting descriptions of the same configurations and operations as those of the optical system 2 and the imaging device 1 according to the first embodiment as appropriate.

[0081] 12 shows the configuration of optical system 2A according to embodiment 2. In embodiment 1, zinc sulfide was exemplified as the lens material of second lens element 32. In optical system 2A according to this embodiment, while having the same configuration as optical system 2 of embodiment 1, the lens material of second lens element 32A is made of chalcohalide glass, the same as first lens element 31. According to optical system 2A of this embodiment, lens group 3A can be made of glass that is highly productive and processable, making it easy to use in a variety of applications.

[0082] Fig. 13 shows a graph of the simulation results of the imaging performance of optical system 2A of this embodiment. In the numerical simulation of Fig. 13, the optical design of optical system 2A of Embodiment 2 was optimized by setting the factor Lf to various values, as in Fig. 6 of Embodiment 1. Examples 1, 2, and 3 of Embodiment 2 correspond to plot points p21, p22, and p23, respectively, in the graph of Fig. 13.

[0083] 14 shows Numerical Example 1 of the optical system 2A of Embodiment 2, in the same manner as in Embodiment 1. Specifically, surface data D21a, aspherical surface data D21b, and various data D21c in FIG. 14 each show information about the optical system 2A of Example 1 of this embodiment, in the same manner as the data D11a to D11c in FIG. 9. The optical system 2A of this example has a factor Lf that satisfies conditional expression (1), as indicated by plot point p21 in FIG.

[0084] 15 shows Numerical Example 2 of the optical system 2A of Embodiment 2, similar to the above examples. Surface data D22a, aspherical surface data D22b, and various data D22c in FIG. 15 each indicate information about the optical system 2A of Example 2 of this embodiment. As indicated by plot point p22 in FIG. 13, the optical system 2A of this example has a factor Lf that is below the lower limit of conditional formula (1).

[0085] 16 shows Numerical Example 3 of the optical system 2A of Embodiment 2, similar to the above examples. Data D23a, D23b, and D23c in FIG. 16 each show information about the optical system 2A of Example 3 of this embodiment. The optical system 2A of this Example has a factor Lf that exceeds that of Example 1, as shown by plot point p23 in FIG.

[0086] The average MTF between visible light L11 and far-infrared light L12 in the optical systems 2A of Examples 1 to 3 of this embodiment is particularly high within the range of the factor Lf defined by conditional expression (1), as shown in Fig. 13. As with Embodiment 1, the optical system 2A of this embodiment can also improve the imaging performance of both visible light L11 and far-infrared light L12 in a compact configuration.

[0087] (Embodiment 3) 17 to 21, a third embodiment will be described below. In the first and second embodiments, examples have been described in which the lens group 3 includes two lenses and has a focal length Fir of 6.8 mm, but the present disclosure is not limited to this. In the third embodiment, an example will be described in which the lens group 3 includes three lenses and has a focal length Fir of 15 mm.

[0088] Hereinafter, the optical system according to this embodiment will be described, omitting the same explanation as in the first and second embodiments as appropriate.

[0089] 17 shows the configuration of an optical system 2B according to embodiment 3. In addition to the same configuration as the optical system 2 of embodiment 1, the optical system 2B according to this embodiment further includes a third lens element 33 arranged behind the second lens element 32 in the lens group 3. The third lens element 33 is made of a lens material that transmits visible light L11 and far-infrared light L12, such as chalcohalide CHA. Numerical simulations of various examples were also performed on the optical system 2B of this embodiment, as in embodiments 1 and 2.

[0090] 18 shows a graph of the simulation results of the imaging performance of the optical system 2B of this embodiment. In Examples 1, 2, and 3 of Embodiment 3, the factor Lf was set to 0.182 mm, 0.111 mm, and 0.260 mm, respectively, as shown by plot points p31, p32, and p33 in the graph of FIG.

[0091] 19 shows Numerical Example 1 of the optical system 2B of Embodiment 3, similar to the above-described embodiments. Surface data D31a, aspherical surface data D31b, and various data D31c in FIG. 19 each indicate information about the optical system 2B of Example 1 of this embodiment. The optical system 2B of this example has an air-equivalent length Lvis of 15.159 mm for visible light L11 and an air-equivalent length Lir of 17.889 mm for far-infrared light L12 at the back focus, satisfying conditional formula (1).

[0092] 20 and 21 show Numerical Examples 2 and 3 of the optical system 2B of Embodiment 3, respectively, in the same manner as the above-described examples. Data D32a-D32c and D33a-D33c in FIGS. 20 and 21 show information about the optical system 2B of Examples 2 and 3 of this embodiment, respectively. The optical system 2B of Example 2 has the same air-equivalent length Lir of the far-infrared light L12 as in Example 1 and an air-equivalent length Lvis of the visible light L11 of 16.219 mm, which is below the lower limit of conditional formula (1). The optical system 2B of Example 3 has the air-equivalent length Lvis of the visible light L11 of 16.829 mm and an air-equivalent length Lir of the far-infrared light L12 of 20.724 mm, which is above the upper limit of conditional formula (1).

[0093] The average MTF in the optical system 2B of each of Examples 1 to 3 of this embodiment is also particularly high within the range of conditional expression (1) when the factor Lf is within the range of conditional expression (1), as shown in Fig. 18. The optical system 2B of this embodiment can also improve the imaging performance for both visible light L11 and far-infrared light L12, as in the above-mentioned embodiments.

[0094] (Embodiment 4) 22 to 26, the fourth embodiment will be described below. In the fourth embodiment, an example in which the number of lenses is even greater than that of the third embodiment will be described.

[0095] Hereinafter, the optical system according to this embodiment will be described, omitting the same explanation as in the first to third embodiments as appropriate.

[0096] 22 shows the configuration of an optical system 2C according to embodiment 4. In addition to the same configuration as optical system 2B according to embodiment 3, optical system 2C according to this embodiment further includes a fourth lens element 34 arranged behind first lens element 31 in lens group 3. Fourth lens element 34 is made of a lens material that transmits visible light L11 and far-infrared light L12. Numerical simulations of various examples were also performed on optical system 2C according to this embodiment, as in embodiments 1 to 3.

[0097] 23 shows a graph of the simulation results of the imaging performance of the optical system 2C of this embodiment. In Examples 1, 2, and 3 of Embodiment 4, the factor Lf was set to 0.196 mm, 0.096 mm, and 0.255 mm, respectively, as shown by plot points p41, p42, and p43 in the graph of FIG.

[0098] 24 to 26 respectively show Numerical Examples 1 to 3 of the optical system 2C of Embodiment 4, similar to the above embodiments. Data D41a to D41c, D42a to D42c, and D43a to D43c in Figures 24, 25, and 26 indicate information about the optical system 2C of Examples 1, 2, and 3 of this embodiment, respectively.

[0099] In this embodiment, an optical system 2C of Example 1 has an air-equivalent length Lvis of 12.750 mm for visible light L11 and an air-equivalent length Lir of 15.695 mm for far-infrared light L12 at the back focus, satisfying conditional expression (1). An optical system 2C of Example 2 has the same air-equivalent length Lir of far-infrared light L12 as in Example 1 and an air-equivalent length Lvis of 14.260 mm for visible light L11, which is below the lower limit of conditional expression (1). An optical system 2C of Example 3 has the same air-equivalent length Lir of far-infrared light L12 as in Example 1 and an air-equivalent length Lvis of 11.863 mm for visible light L11, which is above the upper limit of conditional expression (1).

[0100] As shown in FIG. 23, the average MTF in the optical systems 2C of Examples 1 to 3 of this embodiment is also particularly high within the range of conditional formula (1) for the factor Lf. The optical system 2C of this embodiment can also improve the imaging performance for both visible light L11 and far-infrared light L12, as in the above-described embodiments. Furthermore, in FIG. 23, the graph shape is flatter within the range of conditional formula (1) compared to FIG. 6 and the like. This embodiment makes it easier to improve imaging performance by increasing the number of lenses.

[0101] In the above description, an example has been described in which the number of lenses in the lens group 3 of the optical system 2C is four or less, but the number of lenses may be five or more. In this case, too, the imaging performance can be improved as in the above embodiments.

[0102] (Other embodiments) As described above, embodiments 1 to 4 have been described as examples of the technology disclosed in this application. However, the technology in this disclosure is not limited to these, and can be applied to embodiments in which appropriate modifications, substitutions, additions, omissions, etc. are made. Furthermore, it is also possible to combine the components described in each of the above embodiments to create new embodiments. Therefore, other embodiments will be described below as examples.

[0103] In each of the above-described embodiments, the optical systems 2, 2A to 2C are exemplified in which the light branching element 21 transmits visible light L11 and reflects far-infrared light L12, but the present disclosure is not limited to this. A modification in this respect will be described with reference to FIG. 27.

[0104] 27 illustrates the configuration of an optical system 2D according to a modification of the first embodiment. The optical system 2D according to this modification has the same configuration as the first embodiment, but includes a light branching element 23 that reflects visible light L11 and transmits far-infrared light L12 instead of the light branching element 21 of FIG. 2. For example, the light branching element 23 according to this modification is configured with a bandpass filter whose transmission band is preset to the wavelength band of the far-infrared light L12. Furthermore, in the optical system 2D according to this modification, the far-infrared transmission filter 22 is disposed, for example, on the +Z side of the light branching element 23.

[0105] In the optical system 2D of this modified example, of the incident light L10 emitted from the lens group 3 in the +Z direction, visible light L11 is reflected by the optical branching element 23 and emitted in the +X direction, while far-infrared light L12 is transmitted through the optical branching element 23 and emitted in the +Z direction. In this modified example, the air-equivalent length Lvis of the optical path of the visible light L11 and the air-equivalent length Lir of the optical path of the far-infrared light L12 may satisfy the above-mentioned conditional expression (1). As with the above, the optical system 2D of this modified example also makes it easier to realize an imaging device 1 that can capture both visible light and far-infrared light.

[0106] In the above-described embodiments, examples have been described in which the optical branching elements 21 and 23 are configured with bandpass filters. In this embodiment, the optical branching elements 21 and 23 are not limited to bandpass filters, and may be configured with various band splitters, such as highpass filters or lowpass filters.

[0107] In each of the above embodiments, an example has been described in which the far-infrared transmission filter 22 is provided in the optical system 2, 2A to 2D. In this embodiment, the far-infrared transmission filter 22 may be provided integrally with the far-infrared imaging sensor 12 or the light branching element 21. The far-infrared transmission filter 22 may be omitted from the optical system 2, 2A to 2D.

[0108] In the above-described embodiments, examples have been described in which no optical elements other than the far-infrared transmission filter 22 are arranged between the light branching elements 21, 23 and the respective imaging positions P1, P2, but the present disclosure is not limited to this. In the present embodiments, various optical elements may be arranged between the light branching elements 21, 23 and the respective imaging positions P1, P2, such as various wavelength filters, polarizing filters, polarizing plates, and mirrors. Furthermore, in the present embodiments, lens elements do not necessarily have to be excluded from the optical elements that can be arranged between the light branching elements 21, 23 and the respective imaging positions P1, P2; for example, lens elements can also be arranged depending on the smallness of their refractive power.

[0109] In the above embodiments, the optical systems 2, 2A to 2D include aspherical lens surfaces. The optical systems of the present embodiments do not need to include aspherical lens surfaces, and for example, all of the lens elements included in the lens group 3 may be spherical lenses. Furthermore, the optical systems of the present embodiments may include lens elements in the lens group 3 that have free-form surfaces that are not rotationally symmetric.

[0110] As described above, the embodiments have been described as examples of the technology in the present disclosure, and for that purpose, the accompanying drawings and detailed description have been provided.

[0111] Therefore, the components shown in the accompanying drawings and detailed description may include not only essential components for solving the problem, but also components that are not essential for solving the problem in order to illustrate the above technology. Therefore, the fact that these non-essential components are shown in the accompanying drawings or detailed description should not be interpreted as immediately indicating that these non-essential components are essential.

[0112] Furthermore, since the above-described embodiments are intended to illustrate the technology of the present disclosure, various modifications, substitutions, additions, omissions, etc. may be made within the scope of the claims or their equivalents. [Industrial Applicability]

[0113] The present disclosure is applicable to a variety of applications that combine visible and far-infrared imaging.

Claims

1. An optical system that forms an image at a first imaging position using first light having a wavelength in the visible range, and forms an image at a second imaging position using second light having a wavelength in the far-infrared range that is longer than the wavelength of the first light, a lens group having an optical axis extending from a front side where the first and second light beams are incident to a rear side where the first and second light beams are emitted, a focal length of the first light beam, and a focal length of the second light beam; a light branching element disposed behind the lens group, which branches the first and second light from the lens group into each other by reflection or transmission, and guides the first light to the first imaging position and the second light to the second imaging position, the lens group is composed of lens elements that transmit the first and second light so as to make the first image-forming position correspond to a focal length of the first light and make the second image-forming position correspond to a focal length of the second light, separately from the first image-forming position; no lens element is provided behind the light branching element, and the lens elements of the lens group are provided in front of the light branching element; A first optical path of the light reflected by the optical branching element, of the first and second light beams, from emitting from a rear end position of the lens group until being reflected by the optical branching element, and a second optical path of the light beam after being reflected by the optical branching element do not pass through the same optical element, and a first air-equivalent length of the first light beam passing from the rear end position to the first image-forming position is shorter than a second air-equivalent length of the second light beam passing from the rear end position to the second image-forming position. optical system.

2. The following formula (1) is satisfied based on the first air-equivalent length, the second air-equivalent length, and the focal length of the second light of the lens group: 0.13<(Lir-Lvis) / Fir<0.23...(1) where: Lvis: the first air equivalent length, Lir: the second air equivalent length, Fir: focal length of the second light of the lens group is The optical system of claim 1 .

3. An optical system that forms an image at a first imaging position using first light having a wavelength in the visible range, and forms an image at a second imaging position using second light having a wavelength in the far-infrared range that is longer than the wavelength of the first light, a lens group having an optical axis extending from a front side where the first and second light beams are incident to a rear side where the first and second light beams are emitted, a focal length of the first light beam, and a focal length of the second light beam; a light branching element disposed behind the lens group, branching the first and second light from the lens group into each other, and guiding the first light to the first imaging position and the second light to the second imaging position, the lens group is composed of lens elements that transmit the first and second light so as to make the first image-forming position correspond to a focal length of the first light and make the second image-forming position correspond to a focal length of the second light, separately from the first image-forming position; no lens element is provided behind the light branching element, and the lens elements of the lens group are provided in front of the light branching element; The following formula (1a) is satisfied based on a first air-equivalent length that the first light passes through from the rear end position of the lens group to the first image-forming position, a second air-equivalent length that the second light passes through from the rear end position to the second image-forming position, and a focal length of the lens group for the second light: 0.08<(Lir-Lvis) / Fir<0.28...(1a) where: Lvis: the first air equivalent length, Lir: the second air equivalent length, Fir: focal length of the second light of the lens group That is, the optical system.

4. The lens group includes two lens elements and a stop disposed between the two lens elements. The optical system according to any one of claims 1 to 3.

5. At least one lens element in the lens group is made of chalcohalide glass. The optical system according to any one of claims 1 to 4.

6. The optical branching element has an optical property of transmitting the first light and reflecting the second light. The optical system according to any one of claims 1 to 5.

7. The optical branching element transmits the first light from the lens group and reflects the second light from the lens group, thereby branching the first and second light from each other. The optical system according to any one of claims 1 to 5.

8. When the wavelength of the first light is 587 nm and the wavelength of the second light is 10 μm, the formula (1) is satisfied. The optical system according to claim 2 .

9. The wavelength in the visible range is 400 nm to 750 nm, and the wavelength in the far-infrared range is 3 μm to 20 μm. The optical system according to any one of claims 1 to 8.

10. An optical system according to any one of claims 1 to 9; a first imaging unit that is disposed at the first imaging position and captures an image using the first light; a second imaging unit that is disposed at the second imaging position and captures an image using the second light; An imaging device comprising:

11. The imaging device according to claim 10; a control unit that analyzes an image captured by the imaging device; An imaging system comprising:

Images