WO2017090495A1

WO2017090495A1 - Infrared optical system, image pickup optical device, and digital apparatus

Info

Publication number: WO2017090495A1
Application number: PCT/JP2016/083967
Authority: WO
Inventors: 誠神
Original assignee: コニカミノルタ株式会社
Priority date: 2015-11-27
Filing date: 2016-11-16
Publication date: 2017-06-01
Also published as: JPWO2017090495A1

Abstract

An infrared optical system of the present invention is configured from a negative first group and a positive second group, which are sequentially disposed from the object side to the image side. A diaphragm is disposed between the second group and an image surface, the first group consists of a first lens configured from a negative single lens having at least one aspheric surface, the second group consists of a second lens configured from a positive meniscus lens having a concave surface on the object side, and a third lens configured from a positive meniscus lens having a convex surface on the object side, and conditional formulae of 1<f23/FL<1.2 and 0.9<t2/FL<1.7 (f23: focal point distance of the second group, FL: focal point distance of the whole system, t2: on-optical axis distance from the first lens image-side surface to the second lens object-side surface) are satisfied.

Description

Infrared optical system, imaging optical device, and digital equipment

The present invention relates to an infrared optical system, an imaging optical device, and a digital device. For example, an infrared optical system for gas detection used as a photographing optical system in the infrared region of the wavelength band of 3 to 5 μm, and an imaging optical device that captures an infrared image obtained by the infrared optical system with an imaging sensor such as a cooling sensor And a digital device with an image input function equipped with an infrared optical system.

It is known that the absorption wavelength band peculiar to hydrocarbon substances such as methane, ethane, propane, and butane is gathered around the wavelength range of 3 to 5 μm. In addition, since these hydrocarbon-based substances are gases at room temperature and there is a danger of ignition, handling is required with care. Therefore, in recent years, it has been performed to detect a gas leak or the like that cannot be captured by visible light by photographing the contrast difference due to the absorption with an infrared camera. In addition, the wavelength range of 3 to 5 μm is a region where the amount of light from sunlight is small and the amount of light emitted from an object is also small, so that even a small amount of light can be taken with high sensitivity by cooling the sensor. ing. Therefore, an infrared optical system that can be mounted on a cooling sensor and has good S / N ratio and optical performance is desired.

Examples of infrared optical systems include those described in

Patent Documents

1 and 2. Patent Document 1 discloses a first lens having a negative meniscus shape convex on the object side, a second lens having a positive meniscus shape concave on the object side, and a third lens having a positive meniscus shape convex on the object side. An optical system for infrared rays composed of three is proposed. The diaphragm is disposed between the second and third lenses, and achieves good optical performance in a so-called far infrared band of 8 to 14 μm.

Patent Document 2 proposes an infrared optical system including four or five lenses, and a cold shield is disposed between the third lens and the image plane in order to cope with a cooling sensor. Also, in order to achieve a wide field angle of 2ω≈120 degrees, the first lens has strong power to bend off-axis rays strongly, and spherical aberration, astigmatism, distortion, etc. generated there. Is corrected by a second lens as an aberration correction plate.

JP 2013-228539 A US 5,446,581

In the infrared optical system described in Patent Document 1, since the diaphragm is arranged between the second and third lenses, the diaphragm cannot be mounted in the cold shield. Therefore, there is a problem that radiation from the lens barrel (particularly, a portion arranged from the stop to the image plane portion) enters the sensor, and the S / N ratio deteriorates. In the infrared optical system described in Patent Document 2, it is necessary to sufficiently secure the first and second lens intervals and the second and third lens intervals in order to correct each aberration in a balanced manner. There is a problem of incurring an increase in length.

The present invention has been made in view of such a situation, and an object of the present invention is to be mounted on a cooling sensor and achieve a high S / N ratio and high optical performance with a relatively small number of lenses of three. It is an object of the present invention to provide an infrared optical system, an imaging optical device including the infrared optical system, and a digital apparatus.

In order to achieve the above object, an infrared optical system according to the present invention includes, in order from an object side to an image side, a first group having negative power and a second group having positive power. Because
A diaphragm is disposed between the second group and the image plane;
The first group includes a first lens composed of a negative single lens having an aspheric surface on at least one surface,
The second group includes a second lens composed of a positive meniscus lens concave on the object side, and a third lens composed of a positive meniscus lens convex on the object side,
The following conditional expressions (1) and (2) are satisfied.
1 <f23 / FL <1.2 (1)
0.9 <t2 / FL <1.7 (2)
However,
f23: focal length of the second group,
FL: focal length of the entire system,
t2: the distance on the optical axis from the image side surface of the first lens to the object side surface of the second lens,
It is.

An imaging optical device of the present invention includes the infrared optical system and an imaging sensor that converts an infrared optical image formed on an imaging surface into an electrical signal, and an object on the imaging surface of the imaging sensor. The infrared optical system is provided so that an infrared optical image is formed.

The digital apparatus according to the present invention is characterized in that at least one of a still image shooting and a moving image shooting of a subject is added by providing the imaging optical device.

The infrared camera system according to the present invention includes the infrared optical system.

According to the present invention, it is possible to realize an infrared optical system and an imaging optical device that can be mounted on a cooling sensor and achieve a high S / N ratio and high optical performance with a relatively small number of lenses of three. it can. The infrared optical system or the imaging optical device according to the present invention is used in a digital device such as a gas detection device or a camera system (for example, a monitoring camera or an aircraft camera), thereby enabling high-performance infrared image input to the digital device. Functions can be added at low cost.

The lens block diagram of 1st Embodiment (Example 1). FIG. 6 is an aberration diagram of Example 1. The lens block diagram of 2nd Embodiment (Example 2). FIG. 6 is an aberration diagram of Example 2. The lens block diagram of 3rd Embodiment (Example 3). FIG. 6 is an aberration diagram of Example 3. The lens block diagram of 4th Embodiment (Example 4). FIG. 6 is an aberration diagram of Example 4. The lens block diagram of 5th Embodiment (Example 5). FIG. 6 is an aberration diagram of Example 5. The lens block diagram of 6th Embodiment (Example 6). FIG. 10 is an aberration diagram of Example 6. The lens block diagram of 7th Embodiment (Example 7). FIG. 10 is an aberration diagram of Example 7. The lens block diagram of 8th Embodiment (Example 8). FIG. 10 is an aberration diagram of Example 8. Sectional drawing which shows the schematic structural example of a cold shield. Sectional drawing for demonstrating the infrared cut by a cold door aperture. The schematic diagram which shows the schematic structural example of the digital apparatus carrying the optical system for infrared rays.

Hereinafter, an infrared optical system, an imaging optical device, a digital device, and the like according to embodiments of the present invention will be described. An infrared optical system according to an embodiment of the present invention is an infrared imaging optical system including a first group having negative power and a second group having positive power in order from the object side to the image side. (Power: an amount defined by the reciprocal of the focal length), a stop is disposed between the second group and the image plane, and the first group is composed of a negative single lens having an aspheric surface on at least one surface. The second lens unit includes a second lens made up of a positive meniscus lens that is concave on the object side, and a third lens made up of a positive meniscus lens that is convex on the object side. It is characterized by satisfying the expressions (1) and (2).
1 <f23 / FL <1.2 (1)
0.9 <t2 / FL <1.7 (2)
However,
f23: focal length of the second group,
FL: focal length of the entire system,
t2: the distance on the optical axis from the image side surface of the first lens to the object side surface of the second lens,
It is.

As described above, when the first lens and the second lens have a negative and positive power arrangement, so-called retrofocus is achieved, and a back focus longer than the focal length can be secured. As a result, a diaphragm (cold aperture) can be disposed between the third lens and the image plane.

Examples of the infrared optical system include an infrared photographing optical system using a wavelength band of 3 to 5 μm. As is easily understood from Planck's law, infrared rays in the 3-5 μm wavelength band are significantly less than visible light in the light emitted from the sun, and in the 8-14 μm wavelength band in the light emitted from the surrounding environment such as the ground. Significantly less than the so-called far infrared. Therefore, it is difficult to shoot using a so-called uncooled sensor used for shooting in the visible light or far-infrared region, and it is necessary to detect a small amount of light using the cooling sensor. Specific examples of the cooling sensor include so-called quantum sensors using indium antimonide (InSb), platinum silicon (PtSi), cadmium mercury telluride (HgCdTe), or the like as element materials.

¡When an infrared image sensor receives infrared light (radiant heat), it has its own heat, which causes noise. The cooling sensor suppresses this phenomenon by cooling the sensor itself to an extremely low temperature of about −200 ° C. However, since the cooling sensor detects even a small amount of emitted light, the light emitted from the diaphragm or the lens holding member cannot be ignored. When they enter the sensor, it becomes a large noise source, and the S / N ratio of image quality is greatly deteriorated. In order to prevent this, it is required to arrange a cold shield between the lens system and the image sensor.

FIG. 17 shows a schematic configuration example of the cold shield. The cold shield CS is composed of a vacuum vessel, and a diaphragm ST is formed by a part of the cold shield CS. The aperture stop ST, the band pass filter BPF, and the cooling sensor SR are set and cooled in a cold shield CS sealed by a window WI that transmits infrared rays. By disposing the stop ST between the third lens L3 and the image plane IM and cooling the stop ST to form a so-called cold aperture, light incident on the sensor other than the regular light can be cut. it can. That is, it is possible to suppress the generation of radiated light from the aperture stop ST and block the radiated light from the lens holding member. FIG. 18 shows a state where infrared light from the lens barrel BL, which is a lens holding member, is cut by the stop ST.

Structuring the first lens with a single lens has an effect of suppressing the occurrence of inter-surface reflection. Light rays are incident on the first group at various angles from the outside. If a plurality of lenses are arranged here, inter-surface reflection is likely to occur. It may enter the imaging sensor and cause a decrease in the S / N ratio. If the first lens is a single lens, this can be prevented.

Using the second lens as a positive meniscus lens concave on the object side and the third lens as a positive meniscus lens convex on the object side has an effect of correcting spherical aberration and coma with a good balance. The second lens and the third lens have a bright F value and are arranged relatively close to the stop. Therefore, there is not much difference between the passing positions of the on-axis light beam and the off-axis light beam. When the surface shapes of the second and third lenses are set as described above, it is possible to make a difference in the passing state of the surface of the axial light beam and the off-axis light beam, so that it is easy to correct the axial aberration and the off-axis aberration at the same time. . Therefore, even when the stop is disposed between the third lens and the image plane, it is possible to achieve both correction of spherical aberration and coma aberration.

¡In order to ensure sufficient back focus so that a cold shield can be placed, it is necessary to increase the degree of retrofocus. Specifically, it is necessary to increase the distance between the first and second lenses while increasing the negative power of the first group and the positive power of the second group. Astigmatism and spherical aberration generated by increasing the power of the first lens can be suppressed by the aspherical surface disposed on the first lens. That is, by arranging an aspherical surface on the first lens, it is possible to suppress the amount of astigmatism and spherical aberration generated while giving strong power to the first lens. As a result, the total length can be shortened by shortening the distance between the second and third lenses.

There are two types of preferable aspherical surfaces arranged on the first lens. For example, when an aspherical surface is disposed on one side of the first lens (Examples 2, 3, and 6 to be described later), the curvature becomes gentler (that is, the radius of curvature becomes larger) from the center of the first lens to the periphery. An aspheric surface having such a shape is preferable. When aspherical surfaces are disposed on both surfaces of the first lens (Examples 1, 4, 5, 7, and 8 to be described later), the curvature of each surface increases toward the periphery (that is, the radius of curvature decreases). An aspheric surface having such a shape is preferable.

Conditional expression (1) defines the condition range regarding the power of the second group. If the upper limit of conditional expression (1) is not reached, an appropriate power can be given to the second lens group, and the effect of configuring retrofocus can be obtained without increasing the overall length. If the lower limit of conditional expression (1) is surpassed, the power of the second group is suppressed, so that an effect of suppressing the occurrence of aberrations occurring in the second group and correcting favorably only by the first group is obtained. Further, when the power of the second group is within the range of the conditional expression (1), the distance between the first group and the second group can be appropriately maintained while ensuring a sufficient back focus. If the distance between the first group and the second group is too narrow, inter-surface reflection occurs, and the S / N ratio deteriorates in a highly sensitive cooling sensor.

Conditional expression (2) defines a condition range regarding the first and second lens intervals. When less than the upper limit of conditional expression (2), the effect of suppressing the increase in the total length is obtained. If the lower limit of conditional expression (2) is exceeded, the lens interval between the first group and the second group is moderately separated, so that an effect of suppressing the occurrence of inter-surface reflection can be obtained. Further, if the distance between the first and second lenses is within the range of the conditional expression (2), it is possible to secure the back focus without increasing the power of each group excessively or increasing the total length. Become.

According to the above characteristic configuration, an infrared optical system that can be mounted on a cooling sensor and achieves a high S / N ratio and high optical performance with a relatively small number of lenses, such as three, and an imaging optical device including the same Can be realized. The infrared optical system or imaging optical device is used in a digital device such as a gas detection device or a camera system (for example, a surveillance camera or an aircraft camera), thereby providing a high-performance infrared image input function for the digital device at a low price. It is possible to contribute to the reduction in cost, performance and functionality of digital equipment. An infrared optical system compatible with a cooling sensor can be realized with three lenses while achieving a good balance between S / N ratio and optical performance as described above. Desirable condition settings and the like for achieving light weight and downsizing will be described below.

Preferably, the first lens is an aspheric lens made of germanium (Ge) or silicon (Si), and the second and third lenses are both spherical lenses made of silicon. When a material having a high refractive index such as silicon or germanium is used, an incident angle and a reflection angle with respect to the lens can be reduced, and an effect of suppressing the amount of aberration generated on each lens surface can be obtained. If the amount of aberration generated on each lens surface is large, it is necessary to arrange a large number of lenses in order to correct the aberration. As a result, the inter-surface reflection increases and the S / N ratio deteriorates. Speaking of a glass material having a high transmittance and refractive index in the wavelength band of 3 to 5 μm, it is germanium or silicon. When aspherical processing is performed on germanium or silicon, it is necessary to perform grinding processing, which increases processing time and costs. Since a spherical lens can be polished, mass production is possible while reducing costs.

It is desirable that the first lens is a negative meniscus lens. A negative meniscus lens has the functions of both a positive lens and a negative lens. By using this, the generation of spherical aberration can be suppressed. And there exists an effect which prevents that correction | amendment of another aberration and correction | amendment of spherical aberration will be in a trade-off state.

It is desirable to satisfy the following conditional expression (3).
-4.9 <f1 / f23 <-1.3 (3)
However,
f1: focal length of the first group,
f23: focal length of the second group,
It is.

Conditional expression (3) defines a preferable condition range regarding the power ratio between the negative first group and the positive second group. If the upper limit of conditional expression (3) is exceeded, the power of the first group can be appropriately given to the second group, and the effect of reducing the distance between the first group and the second group can be obtained. If the lower limit of conditional expression (3) is exceeded, the occurrence of aberrations in the second group can be suppressed, and aberrations can be corrected favorably in the first group. Further, when the negative / positive power ratio is within the range of the conditional expression (3), it is possible to suppress the entire length while reducing the occurrence of distortion.

It is desirable to satisfy the following conditional expression (4).
1.0 <BF / FL <1.8 (4)
However,
BF: air conversion length from the image side surface of the third lens to the image surface,
FL: focal length of the entire system,
It is.

Conditional expression (4) defines a preferable condition range regarding back focus. If the upper limit of conditional expression (4) is not reached, the power of each group constituting the retrofocus can be suppressed, and as a result, the occurrence of aberrations in each group can be effectively suppressed. If the lower limit of conditional expression (4) is exceeded, a sufficient space for arranging the cold shield between the lens and the sensor can be effectively secured. If the back focus is within the range of the conditional expression (4), it is easy to secure a space for arranging the cold shield between the lens and the sensor, and conversely, aberration caused by intentionally extending the back focus. There is relatively little deterioration.

It is desirable that a band pass filter is disposed between the third lens and the image plane, and the band pass filter has a characteristic of transmitting only light having a wavelength of 3.1 to 3.5 μm. In the wavelength band of 3-5 μm, there are absorption wavelength bands for various hydrocarbon substances such as methane. In order to detect the absorption of hydrocarbon substances, an optical system in this wavelength band is required. is necessary. In the configuration having the band pass filter between the third lens and the image plane, the band pass filter can be disposed in the vicinity of the cold aperture, so that the band pass filter can be cooled in the same manner as the cold aperture. It becomes possible. By cooling the bandpass filter, the effect of reducing the emitted light from the bandpass filter to a minimum can be obtained. Therefore, a band pass filter may be arranged on the image side of the cold door aperture, and a band pass filter may be arranged on the object side of the cold door aperture.

It is further desirable that the bandpass filter has a characteristic of transmitting only light having a wavelength of 3.2 to 3.4 μm. If it is a band-pass filter having this transmission characteristic, the effect based on the above viewpoint can be further increased.

In the wavelength band of 3.1 to 3.5 μm or even 3.2 to 3.4 μm, the amount of light may be insufficient, so the lens system is required to have an F-number brighter than usual. Therefore, it is desirable that the F-number is smaller than 1.7 (Fno <1.7) for an infrared imaging optical system using a wavelength band of 3 to 5 μm.

It is desirable that none of the surfaces of the first, second, and third lenses have a surface relief-like diffractive surface or a Fresnel surface. In an optical system using so-called far infrared rays in the wavelength band of 8 to 14 μm, chromatic aberration is corrected using a diffractive surface. However, diffracted light due to higher-order terms tends to become ghost light and flare components. Further, due to the characteristics of the diffractive surface, a standing wall is inevitably generated. However, reflection and refraction at the standing wall cause ghost light and flare components similar to those described above. Since the cooling sensor has high sensitivity, the unnecessary light may cause a large noise and reduce the S / N ratio. This is not limited to a surface relief-like diffractive surface, but also applies to a Fresnel surface.

An infrared optical system according to an embodiment of the present invention is suitable for use as an imaging optical system for a digital device with an infrared image input function (for example, a gas leak detection system with an infrared camera). By combining with a cooling sensor or the like, an infrared imaging optical device that optically captures an infrared image of a subject and outputs it as an electrical signal can be configured. The imaging optical device is an optical device that constitutes a main component of a camera used for still image shooting or moving image shooting of an object. For example, an infrared optical device that forms an infrared optical image of an object in order from the object (ie, object) side. And an imaging sensor (e.g., corresponding to a cooling sensor) that converts an infrared optical image formed by the infrared optical system into an electrical signal. Then, the infrared optical system having the above-described characteristic configuration is arranged so that an infrared optical image of the subject is formed on the light receiving surface (that is, the imaging surface) of the imaging sensor. An imaging optical device having performance and a digital device including the same can be realized.

Examples of digital devices with an infrared image input function include infrared cameras, surveillance cameras, aircraft cameras, marine cameras, fire detection cameras, etc., as well as gas detection devices, gas leak detection systems, night vision devices, and thermography. , Infrastructure monitoring systems (high-voltage power lines, monitoring of abnormal heat sources in factories and plants, monitoring of deterioration of structures, etc.).

As can be seen from the above examples, not only can an infrared camera system be configured by using an imaging optical device for infrared rays, but also an infrared camera function, night vision function, It is possible to add a temperature measurement function or the like. For example, a gas detector with an infrared camera equipped with an infrared image input function that captures the contrast difference due to infrared absorption in the absorption wavelength band specific to hydrocarbon substances (methane, ethane, propane, butane, etc.) By doing so, it is possible to detect a gas leak or the like that cannot be captured by visible light.

As an example of a digital device with an infrared image input function, FIG. 19 shows a schematic configuration example of the digital device DU in a schematic cross section. The imaging optical device LU mounted on the digital device DU shown in FIG. 19 includes an infrared optical system LN (AX: optical axis) that forms an infrared optical image (image plane) IM of an object in order from the object (namely, subject) side. ) And an imaging sensor (cooling sensor) SR that converts the optical image IM formed on the light receiving surface (imaging surface) SS by the infrared optical system LN into an electrical signal. The infrared optical system LN is a three-lens single-focus imaging optical system including three single lenses, a cold aperture, and a bandpass filter, on the light receiving surface SS that is a photoelectric conversion unit of the image sensor SR. The optical image IM composed of infrared rays is formed.

A plane parallel plate other than the bandpass filter built in the infrared optical system LN may be arranged as necessary. As such a parallel flat plate, for example, a window WI (FIGS. 17 and 18) may be disposed in the vicinity of the cold door aperture in order to seal the cold shield CS (FIGS. 17 and 18) constituting the cooling unit CU. In consideration of scratch resistance, chemical resistance, and the like, a cover member or window material that transmits infrared light having a wavelength of 3 to 5 μm may be disposed outside the first lens located closest to the object side. In addition, when the digital device DU with an image input function is configured by the imaging optical device LU, the imaging optical device LU is usually disposed inside the body. It is possible to adopt. For example, the unitized imaging optical device LU can be configured to be detachable or rotatable with respect to the main body of the digital device DU.

The digital device DU includes a signal processing unit 1, a control unit 2, a memory 3, an operation unit 4, a display unit 5 and the like in addition to the imaging optical device LU. The signal generated by the image sensor SR is subjected to predetermined digital image processing, image compression processing, and the like as required by the signal processing unit 1 and recorded as a digital video signal in the memory 3 (semiconductor memory, optical disk, etc.) In some cases, it is transmitted to other devices via a cable or converted into an infrared signal or the like (for example, a communication function of a mobile phone). The control unit 2 is composed of a microcomputer, and controls functions such as a shooting function (still image shooting function, moving image shooting function, etc.), an image reproduction function, etc .; control of a lens moving mechanism for focusing, etc. . For example, the control unit 2 controls the imaging optical device LU so as to perform at least one of still image shooting and moving image shooting of a subject. The display unit 5 includes a display such as a liquid crystal monitor, and performs image display using an image signal converted by the imaging sensor SR or image information recorded in the memory 3. The operation unit 4 is a part including operation members such as an operation button (for example, a release button) and an operation dial (for example, a shooting mode dial), and transmits information input by the operator to the control unit 2.

FIGS. 1, 3,..., And 15 show first to eighth embodiments of the infrared optical system LN in an infinitely focused state in optical cross sections. In any of the first to eighth embodiments, the infrared optical system LN includes, in order from the object side to the image side, the first group Gr1 having negative power, the second group Gr2 having positive power, (Cold door aperture) ST is arranged (all power values are paraxial). Between the second group Gr2 and the image plane IM, a window WI and a band are provided in addition to the aperture ST. A pass filter BPF is arranged. The band pass filter BPF has a configuration in which a coating (multilayer film) is applied to a silicon flat plate, and the window WI has a configuration in which an antireflection film (multilayer film) is applied to a silicon flat plate. Yes. The first group Gr1 is composed of a first lens L1 made of a negative single lens having an aspheric surface on at least one surface, and the second group Gr2 is a second lens made of a positive meniscus lens concave on the object side. The lens L2 and a third lens L3 made of a positive meniscus lens convex on the object side are configured, and the second lens L2 and the third lens L3 are all spherical lenses.

In any of the first to eighth embodiments, the second lens L2 is made of silicon. In the first, second, fourth, sixth, seventh, and eighth embodiments, the first lens L2 is made of silicon. The lens L1 is made of germanium. In the third and fifth embodiments, the first lens L1 is made of silicon. In the first, fourth, fifth, seventh, and eighth embodiments, aspheric surfaces are arranged on both surfaces of the first lens L1, and in the second, third, and sixth embodiments, An aspherical surface is disposed only on the object side surface of one lens L1. In the first, fourth to eighth embodiments, the first lens L1 is a negative meniscus lens convex toward the object side. In the second and third embodiments, the first lens L1 is concave toward the object side. Negative meniscus lens. In the first, fourth to eighth embodiments, the bandpass filter BPF is disposed on the object side of the stop ST. In the second and third embodiments, the bandpass filter BPF is an image of the stop ST. It is arranged on the surface side.

In any of the first to eighth embodiments, a window WI for sealing the cooling unit CU (FIG. 19) is disposed in the vicinity of the cold door aperture ST, but is omitted if the cooling effect is sufficient. May be. A cover member or window material that passes the corresponding infrared rays may be disposed outside the first lens L1 positioned closest to the object side in consideration of scratch resistance, chemical resistance, and the like.

As can be understood from the above description, the following characteristic configurations (# 1) to (# 11) and the like are included in each of the above-described embodiments and examples described later.

(# 1): An infrared imaging optical system comprising, in order from the object side to the image side, a first group having negative power and a second group having positive power,
A diaphragm is disposed between the second group and the image plane;
The first group includes a first lens composed of a negative single lens having an aspheric surface on at least one surface,
The second group includes a second lens composed of a positive meniscus lens concave on the object side, and a third lens composed of a positive meniscus lens convex on the object side,
An infrared optical system satisfying the following conditional expressions (1) and (2);
1 <f23 / FL <1.2 (1)
0.9 <t2 / FL <1.7 (2)
However,
f23: focal length of the second group,
FL: focal length of the entire system,
t2: the distance on the optical axis from the image side surface of the first lens to the object side surface of the second lens,
It is.

(# 2): The infrared ray according to (# 1), wherein the first lens is an aspherical lens made of germanium or silicon, and the second and third lenses are both spherical lenses made of silicon. Optical system.

(# 3): The infrared optical system according to (# 1) or (# 2), wherein the first lens is a negative meniscus lens.

(# 4): The infrared optical system according to any one of (# 1) to (# 3), which satisfies the following conditional expression (3):
-4.9 <f1 / f23 <-1.3 (3)
However,
f1: focal length of the first group,
f23: focal length of the second group,
It is.

(# 5): The infrared optical system according to any one of (# 1) to (# 4), wherein the following conditional expression (4) is satisfied;
1.0 <BF / FL <1.8 (4)
However,
BF: air conversion length from the image side surface of the third lens to the image surface,
FL: focal length of the entire system,
It is.

(# 6): A band-pass filter is disposed between the third lens and the image plane, and the band-pass filter has a characteristic of transmitting only light having a wavelength of 3.1 to 3.5 μm. The infrared optical system according to any one of (# 1) to (# 5).

(# 7): Any one of (# 1) to (# 6) is characterized in that none of the first, second and third lenses has a surface relief-like diffractive surface or Fresnel surface. The infrared optical system according to claim 1.

(# 8): The infrared optical system according to any one of (# 1) to (# 7), wherein the aperture is a cold door aperture formed of a part of a cold shield.

(# 9): the infrared optical system according to any one of (# 1) to (# 8), an imaging sensor that converts an infrared optical image formed on the imaging surface into an electrical signal, And an infrared optical system is provided so that an infrared optical image of a subject is formed on an imaging surface of the imaging sensor.

(# 10): A digital apparatus provided with the imaging optical device according to (# 9), to which at least one function of still image shooting and moving image shooting of a subject is added.

(# 11): An infrared camera system comprising the infrared optical system according to any one of (# 1) to (# 8).

Hereinafter, the configuration of the infrared optical system embodying the present invention will be described more specifically with reference to the construction data of the examples. Examples 1 to 8 (EX1 to 8) listed here are numerical examples corresponding to the first to eighth embodiments, respectively, and are lens configuration diagrams showing the first to eighth embodiments. (FIG. 1, FIG. 3,..., FIG. 15) show the lens cross-sectional shape, lens arrangement, and the like of the corresponding Examples 1 to 8, respectively.

In the construction data of each example, as surface data, in order from the left column, the surface number, the radius of curvature R (mm) in the paraxial axis, the distance d (mm) between the axial upper surfaces, and the refractive index N3.3 for a wavelength of 3.3 μm. And the dispersion value ν for a wavelength of 3 to 5 μm. The dispersion value ν represents the nature of dispersion, and is defined by ν = (N4-1) / (N3-N5) (where N3: refractive index for wavelength 3 μm, N4: refractive index for wavelength 4 μm, N5: wavelength 5 μm) Is the refractive index for. Table 1 shows refractive indexes N5, N4, N3.3, N3 and dispersion value ν of silicon (Si) and germanium (Ge).

A surface with * in the surface number is an aspheric surface, and the surface shape is defined by the following expression (AS) using a local orthogonal coordinate system (x, y, z) with the surface vertex as the origin. . As aspheric data, an aspheric coefficient or the like is shown. It should be noted that the coefficient of the term not described in the aspherical data of each embodiment is 0, and ^En = × 10 ⁻ⁿ for all data.
z = (C · h ² ) / [1 + √ {1− (1 + K) · C ² · h ² }] + Σ (Ai · h ⁱ ) (AS)
However,
h: height in the direction perpendicular to the z axis (optical axis AX) (h ² = x ² + y ² ),
z: the amount of sag in the direction of the optical axis AX at the position of the height h (based on the surface vertex),
C: curvature at the surface vertex (reciprocal of paraxial radius of curvature R),
K: conic constant,
Ai: i-th order aspheric coefficient (Σ represents the sum of 4th order to ∞ order for i),
It is.

Table 2 shows various data as focal length FL (mm), F number (Fno), total angle of view 2ω (°), total lens length TT (mm), back focus BF (mm), and image height Y ′. (Mm), focal length f1 (mm) of the first lens L1, focal length f2 (mm) of the second lens L2, focal length f3 (mm) of the third lens L3, focal length f23 of the second group Gr2 (mm) ), And a distance t2 (mm) between the first lens L1 and the second lens L2, and Table 3 shows values corresponding to the conditional expressions of the respective examples (all are values at a wavelength of 3.3 μm). The back focus BF in Table 2 expresses the distance from the lens final surface to the paraxial image plane by the air conversion length (the flat portions such as the window WI and the bandpass filter BPF are converted to the air conversion length. )), The total lens length TT is obtained by adding the back focus BF to the distance from the lens front surface (the object side surface of the first lens L1) to the lens final surface.

2, FIG. 4,..., FIG. 16 are aberration diagrams corresponding to Examples 1 to 8 (EX1 to 8), respectively, (A) is a spherical aberration diagram, (B) is an astigmatism diagram, and (C ) Is a distortion diagram. The spherical aberration diagram shows the amount of spherical aberration at the design wavelength (evaluation wavelength) of 3.3 μm indicated by the solid line, the amount of spherical aberration at the wavelength of 3.0 μm indicated by the alternate long and short dash line, and the amount of spherical aberration at the wavelength of 3.5 μm indicated by the broken line. This is represented by the amount of deviation (mm) in the optical axis AX direction from the axial image plane, and the vertical axis represents the F number. In the astigmatism diagram, the broken line T is the tangential image plane at the design wavelength of 3.3 μm, the solid line S is the sagittal image plane at the design wavelength of 3.3 μm, and the amount of deviation in the optical axis AX direction from the paraxial image plane (mm) The vertical axis represents the half angle of view ω (°). In the distortion diagram, the horizontal axis represents the distortion (%) at the design wavelength of 3.3 μm, and the vertical axis represents the half angle of view ω (°).

Example 1
Unit: mm
Surface data Surface number R d N3.3 ν
1 (OB) ∞ ∞
2 * 50.203 4.000 4.036 103
3 * 37.945 37.244
4 -133.554 9.300 3.433 234
5 -68.805 0.200
6 95.739 3.216 3.433 234
7 336.082 14.000
8 ∞ 1.000 3.433 234 (WI)
9 ∞ 1.500
10 ∞ 0.500 3.433 234 (BPF)
11 ∞ 0.000
12 (ST) ∞ 19.040
13 (IM) ∞

Aspheric data Surface number K A4 A6 A8 A10
2 5.21 2.360E-05 -2.293E-08 6.578E-11 -4.222E-13
3 4.32 3.378E-05 -3.294E-08 5.651E-10 -3.176E-12

Example 2
Unit: mm
Surface data Surface number R d N3.3 ν
1 (OB) ∞ ∞
2 * -35.370 3.000 4.036 103
3 -61.939 19.364
4 -59.293 5.000 3.433 234
5 -44.123 0.300
6 58.917 4.824 3.433 234
7 164.599 13.500
8 ∞ 1.500 3.433 234 (WI)
9 ∞ 1.500
10 (ST) ∞ 0.000
11 ∞ 1.000 3.433 234 (BPF)
12 ∞ 19.500
13 (IM) ∞

Aspheric data Surface number K A4 A6 A8 A10
2 3.27 3.352E-06 1.196E-08 2.092E-11 2.825E-13

Example 3
Unit: mm
Surface data Surface number R d N3.3 ν
1 (OB) ∞ ∞
2 * -40.922 3.000 3.433 234
3 -92.866 19.081
4 -70.354 7.000 3.433 234
5 -50.161 0.300
6 64.238 3.749 3.433 234
7 216.477 13.500
8 ∞ 1.500 3.433 234 (WI)
9 ∞ 1.500
10 (ST) ∞ 0.000
11 ∞ 1.000 3.433 234 (BPF)
12 ∞ 19.600
13 (IM) ∞

Aspheric data Surface number K A4 A6 A8 A10
2 4.24 2.369E-07 1.894E-09 5.028E-11 5.781E-14

Example 4
Unit: mm
Surface data Surface number R d N3.3 ν
1 (OB) ∞ ∞
2 * 51.871 4.241 4.036 103
3 * 38.722 34.748
4 -84.015 7.015 3.433 234
5 -48.319 0.300
6 48.870 6.197 3.433 234
7 60.274 10.000
8 ∞ 1.000 3.433 234 (WI)
9 ∞ 1.500
10 ∞ 0.500 3.433 234 (BPF)
11 ∞ 0.000
12 (ST) ∞ 19.500
13 (IM) ∞

Aspheric data Surface number K A4 A6 A8 A10
2 5.92 2.213E-05 -1.847E-08 6.157E-11 -3.134E-13
3 3.58 3.480E-05 -4.632E-09 4.815E-10 -1.931E-12

Example 5
Unit: mm
Surface data Surface number R d N3.3 ν
1 (OB) ∞ ∞
2 * 58.159 5.537 3.433 234
3 * 40.909 36.153
4 -69.540 2.907 3.433 234
5 -42.521 0.300
6 39.721 2.838 3.433 234
7 50.065 10.000
8 ∞ 1.000 3.433 234 (WI)
9 ∞ 1.500
10 ∞ 0.500 3.433 234 (BPF)
11 ∞ 0.000
12 (ST) ∞ 19.265
13 (IM) ∞

Aspheric data Surface number K A4 A6 A8 A10
2 7.07 1.756E-05 -1.858E-08 6.848E-11 -1.808E-13
3 2.22 3.320E-05 7.007E-09 3.434E-10 -4.564E-13

Example 6
Unit: mm
Surface data Surface number R d N3.3 ν
1 (OB) ∞ ∞
2 * 188.402 3.000 4.036 103
3 71.688 24.959
4 -86.152 4.660 3.433 234
5 -43.985 0.300
6 35.913 8.524 3.433 234
7 34.731 10.000
8 ∞ 1.000 3.433 234 (WI)
9 ∞ 1.500
10 ∞ 0.500 3.433 234 (BPF)
11 ∞ 0.000
12 (ST) ∞ 19.071
13 (IM) ∞

Aspheric data Surface number K A4 A6 A8 A10
2 -9.68 -5.945E-06 -1.220E-08 9.107E-12 -1.022E-13

Example 7
Unit: mm
Surface data Surface number R d N3.3 ν
1 (OB) ∞ ∞
2 * 70.573 5.803 4.036 103
3 * 47.633 30.267
4 -48.733 3.480 3.433 234
5 -33.906 0.300
6 46.064 2.787 3.433 234
7 67.615 8.000
8 ∞ 1.000 3.433 234 (WI)
9 ∞ 1.500
10 ∞ 0.500 3.433 234 (BPF)
11 ∞ 0.000
12 (ST) ∞ 19.000
13 (IM) ∞

Aspheric data Surface number K A4 A6 A8 A10
2 14.56 3.347E-05 -4.863E-08 2.356E-10 -5.247E-13
3 -0.27 6.288E-05 3.430E-08 8.272E-10 1.787E-12

Example 8
Unit: mm
Surface data Surface number R d N3.3 ν
1 (OB) ∞ ∞
2 * 140.751 3.000 4.036 103
3 * 102.721 28.393
4 -91.480 2.860 3.433 234
5 -46.503 0.300
6 31.616 5.288 3.433 234
7 30.324 8.000
8 ∞ 1.000 3.433 234 (WI)
9 ∞ 1.500
10 ∞ 0.500 3.433 234 (BPF)
11 ∞ 0.000
12 (ST) ∞ 19.159
13 (IM) ∞

Aspheric data Surface number K A4 A6 A8 A10
2 -5.88 1.703E-05 -2.022E-08 1.065E-10 -3.339E-13
3 -19.60 2.436E-05 -2.384E-08 2.305E-10 -7.006E-13

DU Digital equipment LU Imaging optical device CU Cooling unit LN Infrared optical system Gr1 First group Gr2 Second group L1 First lens L2 Second lens L3 Third lens ST Aperture (cold aperture)
CS Cold shield WI Window BPF Bandpass filter SR Imaging sensor (cooling sensor)
SS Photosensitive surface (imaging surface)
IM image plane (optical image)
AX Optical axis 1 Signal processing unit 2 Control unit 3 Memory 4 Operation unit 5 Display unit

Claims

An infrared imaging optical system comprising, in order from the object side to the image side, a first group having negative power and a second group having positive power,
A diaphragm is disposed between the second group and the image plane;
The first group includes a first lens composed of a negative single lens having an aspheric surface on at least one surface,
The second group includes a second lens composed of a positive meniscus lens concave on the object side, and a third lens composed of a positive meniscus lens convex on the object side,
An infrared optical system satisfying the following conditional expressions (1) and (2);
1 <f23 / FL <1.2 (1)
0.9 <t2 / FL <1.7 (2)
However,
f23: focal length of the second group,
FL: focal length of the entire system,
t2: the distance on the optical axis from the image side surface of the first lens to the object side surface of the second lens,
It is.
The infrared optical system according to claim 1, wherein the first lens is an aspherical lens made of germanium or silicon, and the second and third lenses are both spherical lenses made of silicon.
The infrared optical system according to claim 1 or 2, wherein the first lens is a negative meniscus lens.
The infrared optical system according to any one of claims 1 to 3, which satisfies the following conditional expression (3):
-4.9 <f1 / f23 <-1.3 (3)
However,
f1: focal length of the first group,
f23: focal length of the second group,
It is.
The infrared optical system according to any one of claims 1 to 4, which satisfies the following conditional expression (4):
1.0 <BF / FL <1.8 (4)
However,
BF: air conversion length from the image side surface of the third lens to the image surface,
FL: focal length of the entire system,
It is.
The bandpass filter is disposed between the third lens and the image plane, and the bandpass filter has a characteristic of transmitting only light having a wavelength of 3.1 to 3.5 μm. The optical system for infrared rays described in 1.
The infrared optical system according to any one of claims 1 to 6, wherein none of the first, second, and third lenses has a surface relief-like diffractive surface or a Fresnel surface.
The infrared optical system according to any one of claims 1 to 7, wherein the aperture is a cold door aperture comprising a part of a cold shield.
An infrared optical system according to any one of claims 1 to 8, and an imaging sensor for converting an infrared optical image formed on the imaging surface into an electrical signal, the imaging surface of the imaging sensor An imaging optical apparatus provided with the infrared optical system so that an infrared optical image of a subject is formed thereon.
A digital device to which at least one function of still image shooting and moving image shooting of a subject is added by including the imaging optical device according to claim 9.
An infrared camera system comprising the infrared optical system according to any one of claims 1 to 8.