JP2737272B2 - Variable power optical system for infrared - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a so-called thermal imaging system for obtaining an infrared image by infrared light in a wavelength band of 3 to 5 μm and 8 to 13 μm. This relates to a variable power optical system.

[Conventional technology]

Conventionally, a variable power optical system for infrared has been used as an objective lens in a thermal imaging system or the like generally used in industry, medical care, and the like, and many proposals have been made.

For example, a four-unit variable magnification optical system is disclosed in U.S. Pat. No. 4,676,581.
No. 6,086,045.

[Problems to be solved by the invention]

However, in U.S. Pat. No. 4,676,581, a plurality of lenses move on the optical axis for zooming, which may complicate the zooming mechanism.

The optical material applied to the variable power optical system for infrared rays is, for example, germanium, selenium sulfide, or the like having a density of about 5.3 g / cm ³ and a normal optical glass (2 to ³ g / cm ³ ). It is much larger than the density, the load applied to the variable power drive system is increased, and the response to the variable power is degraded. In addition, the cost is extremely high compared to ordinary optical glass.

In view of the above, it is an object of the present invention to solve all of the above problems and to provide a high-performance infrared variable power optical system having a simple configuration and excellent imaging performance at low cost.

[Means to solve the problem]

In order to achieve the above object, the present invention provides a first lens group having a positive refractive power, a second lens group having a negative refractive power, and a positive refractive power, as shown in FIG. A third lens group, the second lens group is moved in the optical axis direction during zooming, and the focal length of the first lens group is f ₁ ,
When the focal length of the lens group and f _2, is obtained so as to satisfy _{-8 ≦ f 1 / f 2 ≦} -4.5.

Then, based on the basic configuration described above, providing at least a negative lens on the most object side in the second lens group, respectively r _a radius of curvature of the radius of curvature and the image side of the object side in the negative lens, when the r _b , It is desirable to configure so as to satisfy the following.

(Operation)

As described above, optical materials suitable for infrared optical systems are extremely heavy and expensive as compared with ordinary optical glass materials. It is necessary to reduce the size of the second lens group while keeping the amount of movement of the magnification lens small.

Therefore, in a situation where the absolute value of the focal length f ₂ (<0) of the second lens group is smaller than the absolute value of the focal length f ₁ (> 0) of the first lens group, For a given moving amount, a large zoom ratio can be obtained. In other words, a small moving amount is achieved to obtain a desired zoom ratio. Therefore, it is convenient to realize a simple zoom mechanism.

To achieve the above configuration, in the present invention, the optimum ratio between the focal length f ₂ of the focal length f ₁ and the second lens group of the first lens group, i.e., the power (refractive power) was found allocations These are the following conditions (1).

−8 ≦ f ₁ / f ₂ ≦ −4.5 (1) Beyond the range of this condition, not only the lens diameter of the second lens unit as the zooming unit is increased, but also the zooming mechanism is enlarged. In addition, it becomes difficult to secure good imaging performance in each magnification state. In order to achieve better aberration correction and more compact size, it is desirable to set the value of the condition (1) to −5 and to satisfy the range.

Now, the infrared light detector is cooled at a low temperature to detect the temperature of the outside world, and the infrared radiation emitted from the cooled infrared light detector is applied to each lens surface of the optical system of the system. Because the surface is reflected by the remaining weak reflection,
The infrared light detector re-detects its own image due to the reflected light of the infrared radiation emitted by itself. Since the infrared light detector is cooled at a low temperature, the temperature of the infrared light detector is extremely lower than that of the surroundings or the background of the field of view being imaged. , Causing narcissus. In other words, the narcissus is the total sum of the decrease in the image signal level caused by re-detecting the image of itself by the surface reflection of each lens surface of the optical system. Therefore, based on the detected image signal,
When image processing is performed, a ghost image usually appears as if it were a cold object near the center of the image where reflected light from each lens surface is likely to occur.

Due to such a Narcissus mechanism, the infrared light detector is regarded as a kind of light source, and a light beam emitted from the infrared light detector is reflected by each lens surface of the optical system, and a very small ratio (so-called light source) is obtained. , Cold return), it is an effective method to reduce narcissus by examining whether to return to the infrared light detector and reducing the ratio.

Here, the ratio of this return light (cold return) is described in Applied Optics, Vol. 21, No. 18, pp. 3393-3397, 198.
2, James W. Howard, “Narcissus: Reflection on Retrore
flections in Thermal Imaging Systems. "

From the above, in the design of the variable power optical system for infrared in the present invention, by making the lens shape such that the occurrence of narcissus in the lens located closest to the object side of the second lens group can be minimized at the same time as aberration correction, When the infrared light detector is regarded as a light source, the luminous flux generated therefrom can be diverged, and the narcissus can be greatly reduced.

This to achieve a practical implementation to simultaneously good Naru aberration correcting the object side curvature radius and the image side of the curvature radius of each r _a negative lens L ₂₁ located closest to the object side in the second lens group, where r _b It is desirable to configure so as to satisfy the following.

Explaining this condition from the viewpoint of aberration correction, deviation from this condition is not preferable because aberration fluctuations become extremely large. That is, when the upper limit of the condition (2) is exceeded, the astigmatism moves in the positive direction at the wide end W on the low magnification side, and the high-order astigmatism moves at the telephoto end T on the high magnification side. Occur. In each magnification state, coma greatly occurs.

On the other hand, when the value goes below the lower limit of the condition (2), the spherical aberration generated in the lens closest to the object side in the second lens unit becomes excessively corrected, and the amount of the aberration becomes undesirably large.

Next, the effectiveness of this condition will be described in detail from the viewpoint of reducing narcissus.

To that replaced the meniscus lens having a convex surface directed toward the object side having a shape deviating from the negative lens L ₂₁ with condition (2) located closest to the object side in the second lens group in the structure in line with the examples described below, Negative lens satisfying condition (2)
Comparing to that of the embodiment having the L _21, the ratio of the return light in the low magnification state (wide-angle end W) (cold return)
Can be reduced to 1/10 in the first embodiment and 1/20 in the second embodiment.

At this time, for the purpose of comparison, the negative meniscus lens having a shape deviating from the condition (2) with the convex surface facing the object side has a center of curvature of the image side surface in this lens in a low magnification state (wide-angle end W). Are provided so as to substantially coincide with the image forming position of the first lens group in the above.

The effect of the narcissus reduction in this way can be easily understood from FIGS. 2A and 2B, which schematically show the principle of narcissus generation, and will be described with reference to FIG.

As shown in both figures, when light rays (g ₁ , g ₂ ) parallel to the optical axis enter the variable power optical system for infrared light according to the structure of the embodiment described later, the positive light constituting the first lens group G ₁ is formed. Meniscus lens L
Undergo a Osamu儉operation by _11, Osamu儉positive lens L ₃₁ which then negative lens L ₂₁ and subjected to diverging effect by the biconcave negative lens the second lens group G ₂ consisting of L _22, constituting the third lens group G ₃ An image is formed by the action.

In Fig. 2A, the negative lens is out of condition (2).
Has a structure with L _21, in Figure 2B has a configuration with a negative lens L ₂₁ having a shape satisfying the condition (2).

Now, for ease of explanation, assuming that an infrared light detector D is arranged at an image forming position of the infrared variable power optical system,
In both FIGS. 2A and 2B, the radiation rays (j ₁ , j ₂ ) emitted from the detector D cooled at a low temperature are applied to the positive lens L ₃₁ of the third lens group G _{3 and} the light rays of the second lens group G ₂ . after passing through the biconcave negative lens, and enters the image-side surface of the negative lens L ₂₁ located closest to the object side in the second lens group G _2.

Here, first, as shown in FIG. 2A, considering the construction having a negative lens L ₂₁ having a shape out of the condition (2),
Return light on the image side surface of the negative lens L ₂₁ of the second lens group G ₂ is reflected towards a detector D as shown by rays h ₁ and beam h _2, it will be re-detected by the detector D .

On the other hand, it emitted light from the detector which has passed through the image side surface of the negative lens L ₂₂ of the second lens group _{G 2 (j 1, j 2} ) , the second lens group
Upon reaching the object side surface of the negative lens L ₂₁ of G _2, the emitted light (j _1, j ₂₎ and the radiation beam with respect to the tangent plane at the point where the lens surface intersects (j _1, j ₂₎ is vertically Because of the incidence, the return light at this surface is specularly reflected and follows the opposite path from the emitted rays (j ₁ , j ₂ ), as shown by rays i ₁ and i ₂ ,
It will be detected again by the detector D.

From the above, it can be understood that the condition (2) is a negative lens image quality greatly occurred Narcissus by return light due to both sides of the L ₂₁ having a deviating shape deteriorates significantly.

On the other hand, a negative lens L having a shape satisfying the condition (2)
Considering the structure of the present invention having a _21, as shown in Figure 2B, the emitted light from the detector D on the image side surface of the negative lens L ₂₁ closest to the object side in the second lens group G ₂ (j _1, j ₂ ) return light when the incident diverges to the peripheral as indicated by rays h ₁ and beam h _2.

On the other hand, it emitted light from the detector D which has passed through the image side surface of the negative lens L ₂₂ of the second lens group G ₂ (j _1, j ₂₎ is on the object side surface of the negative lens L ₂₁ of the second lens group G ₂ Upon reaching, the return light in the object side surface diverges to the peripheral as indicated by rays i ₁ and actinic i _2.

From the above, return light by both surfaces of the negative lens L ₂₁ having a shape that satisfies the condition (2) is to be diverging to the periphery, it can be understood that it is possible to realize an excellent picture quality that can reduce the occurrence of Narcissus.

Also in the third lens group G _3, while a shape can reduce the effect of Narcissus realizes a good Naru aberration correction.

Specifically, the radius of curvature of the third lens group closest to the object side
r _c , when the radius of curvature of the third lens unit closest to the image side is r _d , It is desirable to configure so as to satisfy the following.

Beyond the range of this condition, various aberrations, especially spherical aberrations, become extremely large, and the aberration balance is extremely disrupted.
Various aberrations are magnified by the lens group having a function of magnifying the image formed by the first and second lens groups, and the image is significantly deteriorated. Furthermore, the radiation emitted by the low-temperature cooled detector is reflected back to the detector by the surface of the lens exceeding the condition (3), and this return light is re-detected, and narcissus is generated. .

In the case where the third lens group is composed of only the positive lens, when the focal length of the third lens group is f ₃ , r _d / f ₃ > 1.2 and r _d / f, particularly from the viewpoint of reducing narcissus. ₃ <0.8 It is more preferable to satisfy the following condition (4).

That is, the condition (4) that the radius of curvature r _d of the image side surface of the third lens substantially coincides with the focal length f ₃ of the third lens group.
Taking the structure beyond the range of, as shown in Figure 3A, the emitted light rays emanating from the detector D which is cryogenically cooled (j _1, j ₂₎ is regularly reflected by the final surface of the third lens group G ₃ The return light (k ₁ , k ₂ ) is directly detected by the detector D.

Therefore, in the present invention, if the radius of curvature r _d of the final surface of the third lens group is made smaller than the focal length f ₃ of this group so as to satisfy the condition (4), the detection is performed as shown in FIG. 3B. The radiation rays (j ₁ , j ₂ ) emitted from the vessel D are in the third lens group
The reflected light (k ₁ , k ₂ ) is reflected at the final surface of G ₃ and diverges around the detector D.

If the radius of curvature r _d of the final surface of the third lens group is made larger than the focal length f ₃ of this group to satisfy the condition (4), the light is emitted from the detector D as shown in FIG. 3C. radiation beam (j _1, j ₂₎ is reflected by the final surface of the third lens group G _3,
This return light (k ₁ , k ₂ ) diverges around the detector D.

Therefore, if the condition (4) is satisfied as compared with the case where the narcissus is maximized in the state shown in FIG. 3A, the narcissus can be reduced to about 1/400 or less. In an embodiment to be described later, the narcissus can be suppressed to about 1/1000.

As described above, it is effective to reduce the narcissus to make the lens shape capable of reducing the influence of the return light, and it is extremely important in designing an infrared optical system.

Now, in order to achieve good chromatic aberration correction in the infrared lens of the present invention, it is desirable that each lens group is made of the following glass material.

First, since the first lens group and the third lens group have positive refractive power, germanium Ge or the like is used for light having a wavelength of 8 to 13 μm, and silicon is used for light having a wavelength of 3 to 5 μm.
It is necessary to select and configure a high refractive index and low dispersion glass material such as Si. In addition, since the second lens group has a negative refractive power, zinc selenide ZnS is used for light having a wavelength of 8 to 13 μm band.
e, it is necessary to select a glass material such as zinc sulfide ZnS and calcium fluoride CaF ₂ for light having a wavelength in the 3 to 5 μm band, such as zinc sulfide ZnS. That is, since this glass material has a lower refractive index and a higher dispersion than the glass materials of the first and third lens groups,
It is possible to correct various aberrations including chromatic aberration of the entire lens system.

Specifically, the refractive index of the glass material for the 8 μm wavelength light is
n ₈ , n ₁₀ , 12μ
The refractive index of the glass material for the wavelength light of m is n ₁₂ , the refractive index of the glass material for the light wavelength of ₃ μm is n ₃ , the refractive index of the glass material for the light wavelength of ₄ μm is n ₄ , and the refractive index of the glass material for the light wavelength of 5 μm is n ₅ , the glass material constituting the first and third lens groups is: Or It is desirable to be configured to satisfy
The glass material of at least one lens constituting the lens group is Or Is preferably satisfied.

〔Example〕

 Next, this embodiment will be described.

FIG. 1 is a schematic diagram showing a framework of the double-magnification optical system of the present embodiment, and FIG. 1 (a) shows a low magnification state ( Wide-angle end W),
(B) shows a high magnification state (telephoto end T).

As shown in the figure, a parallel ray from infinity passes through the first lens group and is converged to form an image point O _1.
O ₁ becomes an object point of the second lens group, and when subjected to the diverging action of the second lens group, an image point O ₂ is created. This time, this image point
O ₂ becomes an object point of the third lens group, and when subjected to the convergence action of the third lens group, an image is formed at an image point O ₃ to form an image I.

As shown in the FIG. 1 (a), image point O ₂ of the distance from the principal point of the second lens group to the object point O ₁ a, from the principal point of the second lens group
Assuming that the distance is b, the coupling magnification β _2W in the second lens group satisfies the relationship β _2W = b / a (a), and the relationship between a and b is a> 0, b <0, | a Since | >> | b |, it can be understood that the imaging magnification β _2W is a reduction magnification (β _2W <1).

On the other hand, as shown in FIG. 1 (b), when the magnification is changed from the low magnification state of the first magnification state (wide-angle end W) to the high magnification state of the second magnification state (telephoto end T), in a state of fixing the first lens group and the third lens group, resulting in, only the second lens group as illustrated so that the position of the object point O ₁ and the image point O ₂ of the second lens group does not move It is moved to the image side along the optical axis.

That is, only the second lens group is moved in the optical axis direction by the amount of | a |-| b | so that | a | + | b | is always constant in each magnification state.

At this time, the distance from the principal point of the second lens group to the object point O ₁ is -b, the distance from the principal point of the second lens group to the image point O ₂ is -
a, and the imaging magnification β _2T in the second lens group is β _2T =
-A / -b = a / b (b) is established, and the relationship between a and b is a> 0, b <0, | a |> | b |. _It can be understood that _2T is the magnification (β _2T > 1).

As described above, from the equations (a) and (b) obtained above, the relationship between the imaging magnification of the second lens in the high magnification state and the low magnification state is β _2W = 1 / β _2T. c).

Here, 2 in the low magnification state (wide-angle end W) of the first magnification state.
The focal length of the variable power optical system is f _W , the focal length of the two variable power optical system in the high magnification state (telephoto end T) in the second magnification state is f _T , the focal length of the first lens group is f ₁ , and the first is low magnification state of the magnification state imaging magnification of the second lens group for focusing from the object point O ₁ at the (wide-angle end W) to image point O ₂ beta _2W, high magnification state of the second magnification state (telephoto end T) third which forms the object point O ₁ imaging magnification beta _2T of the second lens group for focusing the image point O _2, from the object point O ₂ to the image point O ₃ of the
Assuming that the imaging magnification of the lens group is β ₃ , as can be seen from FIG. 1, f _W = f ₁ β _2W β ₃ ... (D) f _T = f ₁ β _2T β ₃ = f ₁ β ₃ / β _2W (E) is established, and it can be easily understood that the focal length changes due to the movement of the second lens group during zooming.

The zoom ratio V at this time is as follows from the above equations (d) and (e).

From the above, during zooming, (c) expression so as to satisfy, by moving along only the second lens group along the optical axis of the second lens group object point O ₁ and the image point O ₂ By making the positions invariable, image plane fluctuation is suppressed, and a two-magnification optical system with a magnification ratio V of 1 / β _2W ² (= β _2T ² ) can be achieved.

Therefore, when a zoom ratio suitable for a desired purpose is determined, the imaging magnification of the second lens unit in the low magnification state (wide-angle end W) and the high magnification state (telephoto end T) is uniquely determined. In order to set each focal length between the magnification state (wide-angle end W) and the high magnification state (telephoto end T) to a desired target value, the focal length f1 of the _first lens group and the imaging magnification of the third lens group are set. β ₃
By appropriately selecting the above, it is possible to achieve a bivariable optical system having desired specifications.

Now, it can be understood from the equations (d) and (e) that the focal length in each magnification state does not depend on the focal length of the third lens group.

However, as shown in FIG. 1, when the distance from the object point O ₂ with respect to the third lens group to the principal point of the third lens group and l, the group distance between the second lens group and the third lens group In order to secure a large zoom ratio, it is necessary to satisfy the relationship of | a | + | b | <| l | (f).

Here, the focal length f _W in the low magnification state (wide-angle end W)
And high magnification state to increase the: focal length f _T in (telephoto end T) is, (d) type and (e) or techniques to increase the focal length f ₃ of the first lens group from the formula, or the third lens group A method of increasing the imaging magnification β ₃ of the above is conceivable.

In the former method of increasing the focal length of the third lens group, the convergence state of the light beam passing through this group is weakened.
Since the lens diameter of the optical system after the second lens group becomes large, there is a problem in practical use.

On the other hand, the latter method of increasing the imaging magnification β ₃ of the third lens group uses the formula (e) in order to secure the group distance between the second lens group and the third lens group for zooming. to satisfy, it is necessary to increase the focal length f ₃ of the inevitably third lens group.

However, if both the focal length f ₃ and the imaging magnification β ₃ in the third lens group are increased, various high-order aberrations, particularly high-order spherical aberrations, occur significantly in this group.

For example, when the third lens group is configured by a single spherical positive lens having a shape that minimizes the amount of spherical aberration,
The maximum is 13.9 mm in the first embodiment described later, and the maximum in the second embodiment.
Since spherical aberration as large as 8 mm occurs, the imaging performance is significantly deteriorated.

Therefore, in the present invention, the positive lens L ₃ of the third lens group, by providing a progressively aspherical positive refractive power becomes weaker toward the peripheral from the optical axis, it is a very small number of lenses However, not only the negative spherical aberration generated in the group itself but also other aberrations are corrected in a very well-balanced manner.

Here, an aspheric surface in which the positive refracting power of the third lens group gradually becomes weaker from the optical axis toward the periphery means that when this aspheric surface is provided on the convex surface of the lens, From the optical axis to the periphery when this aspherical surface is provided on the concave surface of the lens when the aspheric surface is provided on the concave surface of the lens. The shape is such that the surface refracting power gradually increases.

In addition, the surface refracting power referred to here is an arbitrary one of a certain refracting surface.
The difference between the angle of incidence and the angle of exit of any ray incident on a point,
That is, the declination is defined as the surface power of a minute surface near the refraction point, and when the parallel rays incident near the refraction point converge after refraction, the surface power of the surface is defined as a positive surface power. ,
When diverging after refraction, the surface power of the surface is defined as negative surface power.

As mentioned above, by providing an aspherical surface in the positive lens L ₃ of the third lens group, spherical aberration, compared with when constituting the positive lens L ₃ only spherical, approximately in Example 1 below
It is reduced to 1/40, and in the embodiment to about 1/70.

The aspheric surface to be provided in the third lens group has a maximum value of the distance (so-called sag amount) along the optical axis direction from the tangent plane to the aspheric surface at the vertex of the lens at δ _MAX , and the paraxial refraction of the aspheric surface. when the curvature radius of the spherical force equal power r ^*, a value obtained by dividing the focal length of the lens in the unit length (1mm) having an aspheric surface in the third lens group and f _ASP, It is desirable to configure so as to satisfy the following.

Beyond the range of this condition, if priority is given to aberration correction, the lens shape must be changed in consideration of the reduction of narcissus, and as a result, narcissus will occur greatly and performance will be greatly deteriorated, while narcissus reduction Is performed, the aberration balance among various aberrations is greatly destroyed, and it is difficult to achieve good imaging performance.

In order to obtain the effect of the aspherical surface more effectively and better correct various aberrations generated in the lens itself,
More preferably, the lower limit of this condition is 0.01 and the upper limit is 0.5.

The infrared variable power optical system of this embodiment is 3 μm to 5 μm.
This is applied as an objective lens system of a so-called infrared optical system for a so-called thermal imaging system for obtaining an infrared image by infrared light in a wavelength band of 8 µm to 13 µm.

The optical system of this system includes a telescope system including a two-magnification optical system as an objective lens and an eyepiece, and an optical system provided behind an exit pupil formed by the telescope system and including an optical scanning system. And an infrared detector provided at a position where an image is formed by the optical system.

FIGS. 4 and 6 show the first embodiment of the present invention, respectively.
FIG. 9 is a diagram illustrating a lens configuration in a state where an eyepiece system E is disposed behind an infrared variable power optical system O according to a second embodiment. (A) of each drawing shows a low magnification state (wide-angle end W), and (b) shows a high magnification state (telephoto end T).

As shown in the configuration diagram of both lenses, a bivariable optical system O for infrared
Incident on the first lens group G ₁ undergoes a converging action when passing through the first lens group G ₁ , undergoes a diverging action when passing through the second lens having a variable power function, and passes through a third lens group G ₃ having an imaging function. Under the convergence effect again, an image is formed at the rear focal position of this optical system, and an intermediate image I is formed.

Then, the light beam forming the intermediate image by the two-magnification optical system passes through the eyepiece system E and forms the exit pupil P.

Explaining a specific lens configuration in the first embodiment,
As shown in Figure 4, in order from the object side, a first lens group G ₁ is a positive meniscus lens L ₁₁ made of only the convex surface facing the object side, the second lens group G ₂ is a stronger curvature by two object-side Consists of a negative lens L ₂₁ facing the surface and a biconcave negative lens L ₂₂ ,
The third lens group G ₃ is composed of a positive meniscus lens L ₃₁ toward the convex surface facing the object side.

As shown in FIG. 6 in the second embodiment has the same lens configuration as in Example 1, none of the examples, a positive meniscus lens constituting the first lens group G ₁ L
₁₁ and the positive meniscus lens that constitutes the third lens group
Aspherical surface is provided on the object side surface of the L _31.

Further, in each of the embodiments, as shown, an eyepiece lens system E is provided behind the variable power optical system O having a function as an objective lens system. includes, in order from the object side, is composed of a positive meniscus lens Le ₂ having a convex surface directed toward the positive meniscus lens Le ₁ and the object side with a convex surface facing the image side.

Defocus caused by focusing on an object from infinity to a finite distance and a change in ambient temperature are caused by the lenses constituting the bi-magnification optical system O of the present invention or the eyepiece lens system E.
This is achieved by moving any one of the lenses constituting the lens unit along the optical axis direction. In particular, when the first lens group is provided with a focusing function, the amount of focus for focusing on an object at the same distance is constant in each magnification state, and when the second lens group is provided with a focusing function, the magnification for zooming is reduced. These two methods are effective because they can be shared with the moving mechanism.

Tables 1 and 2 below show the values of specifications of each embodiment. In each table, the leftmost digit represents the order from the object side, and r
Is a radius of curvature of the lens surface, d is a lens thickness and a lens surface interval, and a refractive index n is a value with respect to a B line (λ = 10 μm).
GE indicates germanium and ZnSe indicates zinc selenide.
Here, the refractive indexes of the germanium GE with respect to the C line (λ = 12 μm) and the A line (λ = 8 μm) are 4.0023, respectively.
0, 4.00530, and C line (λ) in zinc selenide ZnSe.
= 12 μm) and the refractive index for the A-line (λ = 8 μm) are 2.39400 and 2.41800, respectively.

Further, M telescope magnification, f _W is a low magnification state (wide-angle end W)
Is the focal length of the variable power optical system O, f _T is the focal length of the variable power optical system O in a high magnification state (wide-angle end T), F _NO
The F-number of 2 variable magnification optical system O, d _P is the distance from the vertex of the last lens surface of the eyepiece system E to the exit pupil P, D is the effective diameter of the lens system, V is a zoom ratio.

Note that the first to eighth surfaces indicate lens data of the variable power optical system O, and the ninth to twelfth surfaces indicate lens data of the eyepiece lens diameter E. The aspheric shape is expressed as follows. .

Where X (y) is the distance from the tangent plane at the lens vertex to the aspheric surface along the optical axis direction, y is the height of the aspheric surface from the optical axis, k is the conic constant, _An is the aspheric coefficient, C Is the curvature, and r is the radius of curvature at the vertex of the lens. Also, r
^* Is the radius of curvature of a spherical surface having a refractive power equal to the paraxial refractive power of an aspheric surface.

Table 3 below shows a numerical value table corresponding to the condition of the present embodiment.

As described above, each embodiment according to the present embodiment has been described. FIGS. 5A and 5B show various aberration diagrams in the low magnification state (wide-angle end W) in Embodiment 1 and the high magnification state ( 7A and 7B show various aberrations at the telephoto end T).
The figures respectively show various aberration diagrams in the low magnification state (wide-angle end W) in Example 2 and various aberrations in the high magnification state (telephoto end T) in Example 2.

In each aberration diagram, SA indicates spherical aberration, AST. Indicates astigmatism, COMA. Indicates coma aberration, DIS. Indicates distortion aberration (distortion), and A in each aberration diagram indicates an A line (λ = 8μ).
m), B is B line (λ = 10 μm), C is C line (λ = 12 μm)
m). Further, a broken line in the astigmatism diagram indicates a meridional image plane, and a solid line indicates a sagittal image plane.

It is apparent from comparison of the aberration diagrams that the lens has excellent imaging performance in each magnification state.

As described above, zooming can be achieved with a simple configuration in which the second lens group is simply moved in the optical axis direction, and high-performance that can achieve excellent aberration correction while greatly reducing the effect of narcissus is achieved. A variable power optical system for infrared can be achieved.

〔The invention's effect〕

As described above, according to the present invention, only the second lens group in the lens system having the three groups of positive, negative, and positive is zoomed along the optical axis direction, and the infrared zooming is performed with a simple mechanism. Double optics can be achieved.

As in the present embodiment, a bright F-number of about 1.3 to 1.6 and a high transmittance can be obtained despite the small number of lens elements of only about four, and both good aberration correction and reduction of narcissus can be achieved. And a high-performance infrared variable power optical system capable of realizing the above.

In addition, since it can be realized with a small number of lens components of only about four, not only the cost can be reduced and the weight of the lens can be reduced, but also the responsiveness to zooming and the operability are very advantageous, which is extremely effective. .

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a skeleton of a variable power optical system according to this embodiment.
FIG. 2A is a diagram showing how narcissus is generated in the second lens group, FIG. 2B is a diagram showing how narcissus is reduced in the second lens group, and FIG. 3A is a diagram showing narcissus in the third lens group. FIG. 3B and FIG. 3C are diagrams showing how narcissus is reduced by the third lens group, and FIG. 4 is a diagram showing a lens configuration and an optical path of Embodiment 1 of the present invention. FIG. 5A is a diagram showing various aberrations in Example 1 in a low magnification state (wide-angle end W), FIG. 5B is a diagram showing various aberrations in Example 1 in a high magnification state (telephoto end T), and FIG. FIG. 7A is a diagram showing a lens configuration and an optical path of Example 2, FIG. 7A is a diagram of various aberrations in Example 2 at a low magnification state (wide-angle end W), and FIG. 7B is a graph of Example 2 at a high magnification state (Telephoto end T). It is a some aberration figure. [Explanation of Signs of Main Parts] G ₁ ... First lens group G ₂ ... Second lens group G ₃ .

Claims (8)

(57) [Claims] A first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having only a positive lens and having a positive refractive power; during zooming, the second lens group moved in the optical axis direction, further, f ₁ the focal length of the first lens group, the focal length of the second lens group f _2, the third lens group The focal length
f ₃ , when the radius of curvature of the most object side lens surface of the third lens group is r _c , and the radius of curvature of the most image side lens surface of the third lens group is r _d , A variable power optical system for infrared, characterized by satisfying the following. Wherein said second lens group includes at least a negative lens closest to the object side, the curvature of the object side in the negative lens radius and the image-side radius of curvature, respectively r _a, when the r _b, 2. The variable power optical system for infrared rays according to claim 1, wherein the following is satisfied. 3. A glass material constituting said first and third lens groups has a refractive index of n ₈ , 10 μm for a light having a wavelength of ₈ μm.
When the refractive index of the glass material for light having a wavelength of n is n ₁₀ and the refractive index of the glass material for light having a wavelength of 12 μm is n ₁₂ , The variable power optical system for infrared rays according to claim 1 or 2, wherein the following is satisfied. 4. A glass material constituting the first and third lens groups has a refractive index of n ₃ , 4 μm for light having a wavelength of ₃ μm.
When the refractive index of the glass material for light having a wavelength of n ₄ is n ₄ , and the refractive index of the glass material for light having a wavelength of 5 μm is n ₅ , The variable power optical system for infrared rays according to claim 1 or 2, wherein the following is satisfied. 5. At least one of the second lens groups
The glass materials of the two lenses have a refractive index of n ₈ for the light of ₈ μm wavelength and a refractive index of the glass for the light of 10 μm.
n ₁₀ , when the refractive index of the glass material with respect to 12 μm wavelength light is n ₁₂ , The variable power optical system for infrared rays according to any one of claims 1 to 4, characterized by satisfying the following. 6. At least one lens constituting the second lens group
The glass materials of the two lenses have a refractive index of n ₃ for the light having a wavelength of ₃ μm, a refractive index of n ₄ for a light having a wavelength of 4 μm,
When the refractive index of a glass material with respect to wavelength of 5μm and n _5, The variable power optical system for infrared rays according to any one of claims 1 to 4, characterized by satisfying the following. 7. The first lens group according to claim 1, wherein said third lens group includes a positive lens having an aspheric surface whose positive refractive power gradually becomes weaker from the optical axis toward the periphery.
Item 7. The variable power optical system for infrared light according to any one of items 7 to 6. 8. The aspheric surface in the third lens group has a maximum value of a distance along the optical axis direction from the tangent plane at the vertex of the lens to the aspheric surface, δ _MAX , and paraxial refraction of the aspheric surface. When a radius of curvature of a spherical surface having a refractive power equal to a force is r ^* , and a value obtained by dividing a focal length of the positive lens having the aspheric surface in the third lens group by a unit length (1 mm) is f _ASP , The variable power optical system for infrared rays according to claim 7, wherein the following is satisfied.