KR20150096265A - Athermalized diffractive lens system - Google Patents

Abstract

An objective lens system for a thermal image detection device includes a first lens group and a second lens group from the object side in order. The first lens group includes a first lens having negative refractive power and a second lens having positive refractive power and has positive refractive power in general. The second lens group includes a third lens having negative refractive power, a fourth lens having positive refractive power, and a fifth lens having positive refractive power and has positive refractive power in general. At least one lens among lenses included in the first and second lens groups is a diffractive aspheric lens. The first lens has the greatest refractive index among the lenses.

Description

[0001] Athermalized diffractive lens system [0002]

The present invention relates to an objective optical system, and more specifically, to a non-totalizing objective optical system.

Temperature is one of the important environmental influences on optical systems used in military and aeronautical electro-optical equipment. The change of the thermal expansion coefficient of the lens with temperature causes the change of the focal length of the optical system, and the change of the focal distance beyond the depth of focus leads to the degradation of the thermal performance.

An active method using a motor for focus correction and a passive method for designing an optical system such that the thermal change of the lens material is canceled with each other or for compensating the focus by photodetection is used.

A problem to be solved by the present invention is to provide a non-heat-dissipative objective optical system having an optical clock and without deterioration in performance due to a temperature change.

The objective optical system according to a feature of the present invention for a technical object of the present invention is characterized in that the objective optical system for a thermal image detection apparatus includes a first lens having a negative refracting power and a second lens having a positive refracting power A first lens group having a positive refracting power as a whole; And a second lens group having a positive refracting power as a whole and including a third lens having negative refracting power, a fourth lens having positive refracting power, and a fifth lens having positive refracting power, At least one of the lens group and the lenses included in the second lens group is a diffractive aspheric lens, and the first lens has the largest refractive index among the lenses.

At least one of the first lens, the third lens, and the fifth lens may be a diffractive aspherical lens. At least one of the first lens, the third lens, and the fifth lens may be made of germanium (Ge). The second lens or the fourth lens may be formed of one of zinc sulfide (ZnS) and zinc selenide (ZnSe).

The first lens may be a convex meniscus aspherical lens convex on the object side, the second lens may be a convex spherical lens, and the third lens may be a diffracted aspheric lens including a concave diffraction pattern, The fourth lens may be a spherical lens having a biconvex shape, and the fifth lens may be an aspherical lens having a meniscus shape concave toward the object side.

Such an objective optical system can satisfy the following conditions.

-1.77? F1 / f? -1.61

+2.25? F2 / f? +2.42

-2.74? F3 / f? -2.58

+2.58? F4 / f? +2.74

+2.25? F5 / f? +2.42

+1.45? T12 / f? +1.77

+0.16? T23 / f? +0.48

+0.16? T34 / f? +0.48

+1.13? T45 / f? +1.61

F1, f2, f3, f4 and f5 are effective focal distances of the first lens, the second lens, the third lens, the fourth lens and the fifth lens, respectively, and T12 T23 is a distance between the second lens and the third lens, T34 is a distance between the third lens and the fourth lens, T45 is a distance between the fourth lens and the fifth lens, Lt; / RTI >

In addition, the diaphragm may be located between the first lens group and the second lens group. In this case, the objective optical system satisfies the condition of +0.64? D / f? +1.01 (where f is the effective focus of the entire objective optical system Distance, and D represents the aperture of the aperture).

Further, the position of the diaphragm can be set based on the position of the third lens, and the position of the diaphragm is -2? P1? 0, 0? P2? + 2 And P2 denotes a diaphragm position when the diaphragm is located on the upper side of the third lens).

A distance Dxx between the second lens and the diaphragm, and a distance dxx between the diaphragm and the third lens satisfy the following conditions: +0 Dxx / f? +0.33, +0? Dxx / f? +0.33.

On the other hand, an image correction shutter may be disposed on the upper side of the second lens group.

According to the embodiment of the present invention, it is possible to provide an objective optical system having an optical clock of a diagonal 90 DEG or more and having no performance deterioration due to a temperature change. In addition, distortion according to the optical clock can be minimized.

1 and 2 are views showing the structure of an objective optical system according to an embodiment of the present invention.
3 to 5 are graphs showing modulation transfer function characteristics of an objective optical system according to an embodiment of the present invention.
FIG. 6 is a graph showing a focal length variation according to a temperature change of an objective optical system according to an embodiment of the present invention.
7 is a graph showing a distortion aberration of an objective optical system according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily carry out the present invention. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. In order to clearly illustrate the present invention, parts not related to the description are omitted, and similar parts are denoted by like reference characters throughout the specification.

Throughout the specification, when an element is referred to as "comprising ", it means that it can include other elements as well, without excluding other elements unless specifically stated otherwise.

Hereinafter, a non-thermalized objective optical system according to an embodiment of the present invention will be described.

1 and 2 are views showing the structure of an objective optical system according to an embodiment of the present invention.

The objective optical system 100 according to the embodiment of the present invention functions as an optical part of the thermal image detecting apparatus 200 and is located at the front of the thermal image detecting unit 220. Here, the front end represents the side to which the image (or light) is input. A protective glass window 210 is disposed between the objective optical system 100 and the thermal image detecting unit 220. The image of the object is imaged by the objective optical system 100 on the thermal image detector 220 of the thermal imaging apparatus 200. [

As shown in Fig. 2, the objective optical system 100 comprises, in order from the object side, a first lens group I and a second lens group II. A diaphragm A is positioned between the first lens group I and the second lens group II and the first lens group I has a positive refractive power as a whole and the second lens group II is entirely It has a positive refractive power.

The first lens group I includes, in order from the object side, a first lens 10 having a negative meniscus shape aspherical lens convex on the object side, a first lens 10 having a positive refractive power, And a second lens 20.

The second lens group II includes a third lens 30 having a negative refractive power, a fourth lens 40 having a positive refractive power and a convex spherical lens, And a fifth lens 50 having a positive refractive power as a whole, which is a meniscus-shaped aspherical lens concave toward the object side.

In the objective optical system 100 having such a structure, the first lens 10 is an aspherical lens, and both surfaces of the lens may be aspherical. The first lens 10 may be made of germanium (Ge).

The higher the refractive index of the first lens 10 is, the easier it is to realize a wide angle and the shape can be easily processed. Therefore, it is preferable that the first lens 10 is formed of a material having a refractive index of 3 or more. The refractive index of the first lens 10 at a wavelength of 10 um is 4.003.

At this time, the effective focal length F1 of the first lens 10 satisfies the following expression.

On the other hand, the second lens 20 has a positive refractive power in a biconvex shape. The second lens 20 is a spherical lens, and may be made of zinc selenide (ZnSe) or zinc sulfide (ZnS).

At this time, the effective focal length F2 of the second lens 20, and the distance T12 between the first lens 10 and the second lens 20 satisfy the following equation.

The third lens 30 has negative refractive power in both concave shapes. The third lens 30 is a diffracted aspheric lens including a diffraction pattern, and germanium (Ge) can be used as the material of the first lens 10. As the level of processing the pattern increases, the diffraction efficiency increases, so that the number and spacing of the diffraction patterns can be varied.

In the embodiment of the present invention, the diffractive aspherical surface is applied on the upper surface of the third lens 30, but the present invention is not limited thereto. The diffractive aspherical surface can be applied to at least one of the first lens 10, the third lens 30, and the fifth lens 50.

At this time, the effective focal length F3 of the third lens 30 satisfies the following expression.

The distance T23 between the second lens 20 and the third lens 30 satisfies the following equation.

A diaphragm A is formed between the second lens 20 and the third lens 30 and the position of the diaphragm A satisfies the following formula based on the position of the third lens 30.

Here, P1 represents the diaphragm position when the diaphragm is located on the side of the object side of the third lens 30, and P2 represents the diaphragm position when the diaphragm is located on the upper side of the third lens 30. When the diaphragm position does not satisfy the above expression (5), the size of the lens constituting the optical system becomes large, making miniaturization difficult.

The distance Dxx between the second lens 20 and the diaphragm A and the distance dxx between the diaphragm A and the third lens 30 satisfy the following equation.

Further, the aperture of the diaphragm A satisfies the following expression.

Here, f represents the effective focal length of the entire objective optical system, and D represents the aperture of the diaphragm.

On the other hand, the fourth lens 40 has a positive refractive power in a biconvex shape. The fourth lens 40 is a spherical lens, and may be made of zinc selenide (ZnSe) or zinc sulfide (ZnS).

At this time, the effective focal length F4 of the fourth lens 40, and the distance T34 between the third lens 30 and the fourth lens 40 satisfy the following equation.

The fifth lens 50 has a meniscus shape concave toward the object side and has a positive refractive power. The fifth lens 50 is an aspherical lens, and both surfaces of the lens may be aspherical. The material of the fifth lens 50 may be germanium (Ge) as in the first lens or the third lens.

At this time, the effective focal length F5 of the fifth lens 50, and the distance T45 between the fourth lens 40 and the fifth lens 50 satisfy the following equation.

An image correcting shutter may be disposed between the fifth lens 50 and the thermal imaging apparatus 200 of the objective optical system 100 having such a structure.

In the thermal image detecting apparatus 200, the brightness and the blur of the image are largely changed according to the signal gain value (sensitivity) of each pixel of the sensitive surface (image detector) as compared with the visible light detector, and the detected image is uneven. Such image unevenness can be corrected by measuring the signal gain ratio of each detection element and electronically adjusting the offset value. Such correction must be performed each time the luminescence detecting apparatus is turned on. In the case where the temperature of the lasing apparatus is suddenly changed even during the use of the lasing detecting apparatus, the gain of the detecting element becomes nonlinear and the image becomes uneven. Therefore, in the case of correcting the unevenness of the image, the image correction shutter can be disposed in the optical path of the thermal imaging apparatus 200, and the offset value can be calculated when the shutter momentarily blocks the optical path. That is, if the shutter momentarily blocks the optical path, the photodetector measures the offset value, and if the shutter is opened again, the external image can be detected by adjusting the image unevenness.

When the image correcting shutter that performs such a function is disposed between the fifth lens 50 and the thermal image detecting device 200, the fifth lens 50 is placed between the fifth protective film 50 and the thermal image detecting device 200, It is necessary to design the image correction shutter to have a long focal length in consideration of the space to be inserted.

As described above, the objective optical system according to the embodiment of the present invention is an objective optical system according to the embodiment of the present invention, in which the amount of change in the focal length, which is a result of a change in refractive index with temperature, The change can be minimized. Specifically, the first lens 10 is made of a germanium material having a high refractive index in consideration of the radius of curvature and the NA value due to the wide angle and the second lens 20 and the fourth lens 40 are made of zinc selenide (ZnSe ) Lens to minimize the change in focal length due to aberration control and non - heating.

In the case of the refractive lens, the change of the refractive index depending on the temperature greatly affects the variation of the focal length, so that the performance of the image detected by the image detecting device 200 Degradation. In the embodiment of the present invention, a passive non-thermalization method is used in which the variation of the focal length depending on the temperature is made to be in the depth of focus through the diffractive lens and the aspheric lens.

The focal length variation of the diffractive lens can be expressed as a function of the thermal expansion coefficient of the lens, which is suitable for the passive non-thermalization method because it has no influence on the change of the refractive index. Accordingly, in the embodiment of the present invention, as described above, the third lens 30 of the second lens group II is composed of the diffractive aspheric lens. The third lens 30 is formed of a germanium material in place of a general optical glass material, and the third lens 30 is used. The wavelength is long and the aspherical surface is easily processed.

Next, an embodiment of the present invention, which is implemented to satisfy the above equations as described above, will be described.

Here, f represents the focal length, and ri (i = 1 to 14) represents the radius of curvature of the lens surface. The unit of the length value is mm.

The embodiment according to the present invention is shown in Table 1 below.

division Radius of curvature Thickness or spacing (Ti) Effective diameter type material r1 29.687 5 11.3 Aspherical surface Ge r2 13.414 9.45 7.3 Aspherical surface r3 92.924 5.8 6.3 Spherical ZnSe r4 -25.154 0.1 5.8 Spherical r5 infinity 1.9 5.2 iris r6 -134.88 4 5.7 Spherical Ge r7 70 2 6.2 Diffraction aspherical surface r8 185.546 6 8.1 Spherical ZnSe r9 -25.546 8.8 9.3 Spherical r10 -200 7 10.6 Aspherical surface Ge r11 -36.991 8 11.4 Aspherical surface r12 infinity One 7.3 Sensor window (object side) Ge r13 infinity 0.92 7.2 Sensor window (upper side) r14 infinity 6.8 Sensor face

The total effective focal length of the objective optical system 100 according to Table 1 is 6.2 mm, and the principal wavelength is 10 m, which is a standard for all the analysis and numerical calculations, and aberration correction is possible up to the range of 8 to 12 m. 2, r12 and r13 are the object side surface and the upper surface side of the protective window glass 210 of the thermal image detecting apparatus 200, respectively. r 14 is the focal plane of the objective optical system as the sensitive surface of the image detector 220.

In the image detecting apparatus 200 according to the embodiment of the present invention associated with the objective optical system 100, the image detector 220 is an uncooled detector, and its pixel size is approximately 17 um. The image detector 220 has a pixel array structure of 640 x 480 pixels.

The objective optical system according to the embodiment of the present invention according to the above Table 1 is designed to have a Nyquist frequency of 29 cy / mm.

Considering the pixel size (17 mu m x 17 mu m) of the image detecting apparatus 200, the modulation transfer function (MTF) indicating the total optical system performance was determined to be 25% or more at a resolution of 29 cycles / mm, The MTF is 56%.

3 to 5 are graphs showing modulation transfer function characteristics of an objective optical system according to an embodiment of the present invention. In the graph, the vertical axis represents the normalized modulation transfer function value, and the horizontal axis represents the resolution (cy / mm).

3, it can be seen that the MTF of the objective optical system designed based on the room temperature (+ 20 ° C) satisfies 30% or more, which is higher than the design target at all viewing angles. It can be predicted that optical performance is maintained without degradation of MTF performance due to temperature change.

FIG. 4 shows the characteristics of the modulation transfer function (MTF) of the objective optical system at low temperature (-40 degrees), and FIG. 5 shows the characteristics of the modulation transfer function (MTF) of the objective optical system at high temperature .

4 and 5, it can be seen that the objective optical system according to the embodiment of the present invention satisfies an MTF of 30% or more at all viewing angles even at a low temperature (-40 DEG C) and a high temperature (+80 DEG C). Therefore, it can be predicted that the optical performance is maintained without degrading the MTF performance due to the temperature change.

FIG. 6 is a graph showing a focal length variation according to a temperature change of an objective optical system according to an embodiment of the present invention.

For example, when the design target of the depth of focus is selected within ± 15 μm in consideration of the far-infrared wavelength region (8 to 12 μm), the focal length, the circle of confusion and the manufacturing tolerance, As described above, it can be predicted that the variation amount of the focal length according to the temperature change of the objective optical system according to the embodiment of the present invention is within the design target.

7 is a graph showing a distortion aberration of an objective optical system according to an embodiment of the present invention.

In FIG. 7, when the angle of view is 90 degrees or more, it is found that the distortion aberration is within a range of ± 5% (Example -3.5%). Therefore, it can be seen that the objective optical system according to the embodiment of the present invention has excellent resolution and non-aberration correction of aberration at a wide angle of 90 degrees or more.

According to the embodiment of the present invention, it is possible to provide an objective optical system having an optical clock with a diagonal 90 DEG or more and without performance degradation due to a temperature change. In addition, distortion according to the optical clock can be minimized.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, It is clear.

Claims (10)

An objective optical system for a lattice detection apparatus,
In order from the object side,
A first lens group including a first lens having a negative refractive power and a second lens having a positive refractive power and having a positive refractive power as a whole; And
A third lens having a negative refractive power, a fourth lens having a positive refractive power, and a fifth lens having a positive refractive power,
/ RTI >
At least one of the lenses included in the first lens group and the second lens group is a diffractive aspherical lens, and the first lens has the largest refractive index among the lenses. The method of claim 1, wherein
Wherein at least one of the first lens, the third lens, and the fifth lens is a diffractive aspherical lens. The method according to claim 2, wherein
Wherein at least one of the first lens, the third lens, and the fifth lens is made of germanium (Ge). The method according to claim 2, wherein
And the second lens or the fourth lens is made of one of zinc sulfide (ZnS) and zinc selenide (ZnSe). The method according to claim 2, wherein
Wherein the first lens is an aspherical lens having a convex meniscus shape on the object side, the second lens is a spherical lens having a convex shape,
Wherein the third lens is a diffraction aspherical lens including a concave diffraction pattern, the fourth lens is a biconvex spherical lens, and the fifth lens is a meniscus aspherical lens concave toward the object side. The method according to any one of claims 1 to 5, wherein
The objective optical system satisfying the following condition.
-1.77? F1 / f? -1.61
+2.25? F2 / f? +2.42
-2.74? F3 / f? -2.58
+2.58? F4 / f? +2.74
+2.25? F5 / f? +2.42
+1.45? T12 / f? +1.77
+0.16? T23 / f? +0.48
+0.16? T34 / f? +0.48
+1.13? T45 / f? +1.61
F1, f2, f3, f4 and f5 are effective focal distances of the first lens, the second lens, the third lens, the fourth lens and the fifth lens, respectively, and T12 T23 is a distance between the second lens and the third lens, T34 is a distance between the third lens and the fourth lens, T45 is a distance between the fourth lens and the fifth lens, Lt; / RTI > The method according to claim 6,
Wherein an aperture stop is located between the first lens group and the second lens group, and further satisfies the following condition.
+0.64? D / f? +1.01
Here, f represents the effective focal length of the entire objective optical system, and D represents the aperture of the diaphragm. The method of claim 7, wherein
The position of the diaphragm is set with reference to the position of the third lens, and the position of the diaphragm satisfies the following condition.
-2? P1? 0
0? P2? +2
Here, P1 represents a diaphragm position when the diaphragm is located on the side of the object side of the third lens, and P2 represents the diaphragm position when the diaphragm is located on the upper side of the third lens. The method of claim 6, wherein
A distance Dxx between the second lens and the diaphragm, and a distance dxx between the diaphragm and the third lens satisfies the following condition.
+0? Dxx / f? +0.33
+0? Dxx / f? +0.33 The method of claim 1, wherein
And an image correction shutter is disposed on the upper side of the second lens group.

Images