JP2001141993A - Infrared optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an infrared optical system which reduces the capacity of a lens material for use to reduce the cost of the optical system. SOLUTION: The optical system is constituted which is provided with Fresnel lenses 1 and 2 having a function of a convex lens, which is made of a low- dispersion material and has one face constituted of a Fresnel surface and has the other face constituted of a plane, and satisfies conditional formulas (ϕ-ϕ1)/ϕ2>0.1 and 1.5>ϕ(ϕ2+ϕ1-ϕ)/(ϕ1ϕ2)>0.5 where ϕis the power of the entire infrared optical system and ϕ1 is the power of the Fresnel lens arranged on the object side and ϕ2 is that on the image side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an infrared optical system used for an infrared camera for photographing an object which emits infrared rays according to temperature.

[0002]

2. Description of the Related Art An infrared camera is a device for imaging infrared rays emitted by a subject. The amount of infrared radiation emitted from the subject depends on the temperature. Assuming that the temperature of the subject is about 300 K, there is a luminance peak at a wavelength of about 10 μm. In addition, since there is little absorption by moisture in the atmosphere, some infrared cameras use infrared rays having a wavelength of about 8 to 12 μm.

In order to image the infrared radiation distribution of a subject, a function of faithfully mapping infrared rays in the above-mentioned wavelength band, that is, a function of forming an image, is required for an infrared optical system. Aberration is a factor of image formation deterioration. The infrared optical system is required to have a small aberration over a wide wavelength band. Further, it is required that aberration is small in the range of the visual field covering the subject. Aberrations include chromatic aberration depending on the wavelength change of the refractive index,
There are spherical aberration, coma aberration, astigmatism, etc. depending on the lens shape.

FIG. 8 shows, for example, Japanese Patent Application Laid-Open No. 62-109014.
FIG. 1 is a cross-sectional view of a conventional infrared optical system disclosed in Japanese Patent Application Laid-Open Publication No. HEI 10-115,036.
In FIG. 8, reference numeral 11 denotes a convex meniscus lens having a convex surface facing the object side, 12 denotes a concave meniscus lens having a concave surface facing the object side, and 13 denotes a convex meniscus lens having a convex surface facing the object side. The material of each lens is germanium, and the lens shape of each lens is all spherical. Reference numeral 14 denotes an aperture. Reference numeral 15 denotes an optical axis (a rotationally symmetric axis of the lens), and reference numeral 16 denotes an image plane.

In the above structure, germanium used as a lens material has an advantage that it has a low dispersion in a wavelength range of about 8 to 12 μm, that is, a change in the refractive index is small. By employing germanium as the lens material, the occurrence of chromatic aberration depending on the wavelength change of the refractive index is suppressed. In addition, a three-element configuration is used, and the six spherical lens surfaces are arranged in the order of convex, concave, concave, convex, convex, concave from the object surface, thereby reducing aberrations over the entire field of view and reducing infrared rays of the subject. An infrared optical system for imaging the radiation distribution on the image plane 16 is obtained.

[0006]

In the above-mentioned conventional infrared optical system, germanium used as a lens material is a rare lens material and is expensive. Further, the meniscus lens needs to be cut out of a cylindrical lens material determined by the effective lens diameter and the length from the vertex of the convex surface to the peripheral portion of the concave surface. For this reason, since the conventional infrared optical system is composed of the meniscus lens as described above, there is a problem that the capacity is increased, more lens materials are required, and the cost of the entire optical system is increased. Was.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an infrared optical system capable of reducing the capacity of a lens material to be used and reducing the cost of the optical system. And

[0008]

The infrared optical system according to the present invention comprises two Fresnel lenses having a Fresnel surface on one side and a convex surface on the other side, and having the function of a convex lens, and (φ−φ1) / φ2> 0.1 (1) 1.5> φ (φ2 + φ1-φ) / (φ1φ2)> 0.5 (2) where φ is the power of the whole infrared optical system φ1 is the Fresnel lens arranged on the object side Is the power of the Fresnel lens arranged on the image side, and satisfies the above conditional expressions (1) and (2).

The two Fresnel lenses are made of a material with low dispersion.

[0010] Further, the material of the Fresnel lens is germanium or silicon.

[0011] Further, at least one of the four surfaces of the two Fresnel lenses is provided with a fine structure having a diffractive lens function.

Further, the Fresnel lens is manufactured by cutting.

Further, the Fresnel lens is manufactured by molding.

Further, the Fresnel lens is manufactured by an etching technique.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 FIG. 1 is a cross-sectional view showing a basic configuration of an infrared optical system according to Embodiment 1 of the present invention. The infrared optical system has two Fresnel lenses each having a Fresnel surface on one side and a flat surface on the other side. And In FIG. 1, reference numeral 1 denotes a Fresnel lens made of germanium, which has a Fresnel surface 1a having a refraction function of a convex lens on the object side and a flat surface 1b on the image side, and has a refraction function of a convex lens as an optical component. Reference numeral 2 also designates a Fresnel lens made of germanium, which has a Fresnel surface 2a having a convex lens refraction function on the object side and a flat surface 2b on the image side, and has a refraction function of a convex lens as an optical component.

As described above, germanium is 8 to 12 μm
Since the dispersion is low and the change in the refractive index is very small in the wavelength range of the order, the chromatic aberration generated in the Fresnel lenses 1 and 2 is negligibly small. Reference numeral 15 denotes an optical axis, which is a rotationally symmetric axis of the Fresnel surfaces 1a and 2a. Reference numeral 16 denotes an image plane on which an infrared image of a subject formed by the Fresnel lenses 1 and 2 is formed. By using a Fresnel lens structure having the other surface as a flat surface, the lens can be made thin, so that the capacity of the lens material used can be extremely reduced.

When only one Fresnel lens is used,
The aberration can be reduced in a very narrow range near the intersection of the optical axis 15 and the image plane 16, but asymmetric aberrations (coma, astigmatism, etc.) increase when the distance from the optical axis 15 increases. Therefore, an infrared image of the subject cannot be obtained. By arranging the Fresnel lenses 1 and 2 having the function of refracting convex lenses at appropriate intervals, all aberrations can be reduced and a clear infrared image can be obtained.

The Fresnel lenses 1 and 2 are set so as to satisfy the conditions relating to the back focus and the lens interval shown in the following equations (1) and (2). (Φ−φ1) / φ2> 0.1 (1) 1.5> φ (φ2 + φ1−φ) / (φ1φ2)> 0.5 (2) where φ is the power of the whole infrared optical system φ1 is the object side Φ2 is the power of the Fresnel lens placed on the image side

That is, the conditional expression (1) is a condition relating to the back focus. In order to visualize an infrared image formed by the infrared optical system on a display means such as a monitor, it is necessary to convert the infrared image into an electric signal by an infrared detector. In order to arrange the infrared detector, it is necessary to secure a sufficient back focus. By satisfying conditional expression (1), it is possible to secure a sufficient back focus while suppressing coma. If the lower limit of conditional expression (1) is exceeded, it becomes impossible to secure the back focus and coma aberration increases.

On the other hand, conditional expression (2) is a condition relating to the lens interval. By satisfying conditional expression (2), the lens interval can be set in the range of 0.5 to 1.5 times the focal length of the entire system, and astigmatism and field curvature can be suppressed. . When the value exceeds the range of the conditional expression (2), the astigmatism and the curvature of field become insufficiently corrected, and cannot be put to practical use.

FIG. 2 is an explanatory view showing the shape of the Fresnel surface. Assuming that the displacement from the optical axis (rotationally symmetrical axis) 15 is x, the radius of curvature of the Fresnel surface is r, and the cone coefficient is b, f (x) shown in Expression (3) is divided by a constant step. , The shape of the Fresnel surface.

[0022]

(Equation 1)

Both Fresnel lenses 1 and 2 have a Fresnel surface on the object side and a flat surface on the image side. Also,
Since the lens material of both Fresnel lenses 1 and 2 is germanium, the generated chromatic aberration is negligibly small. Further, the stop of the optical system is arranged on the Fresnel surface of the Fresnel lens 1 on the object side.

Hereinafter, the present invention will be described with reference to specific examples. Embodiment 1 FIG. The following shows specific numerical data of the infrared optical system of the present invention. The specifications are as follows: the wavelength band is 8 to 12 μm, the F number is 1.6, the focal length is 100 mm, the half angle of view is 8.0 °,
The step is 0.1 mm. Table 1 shows specific numerical data of the first embodiment. In Table 1, S is the surface number, R is the radius of curvature (mm), b is the cone coefficient, d is the surface interval (mm), n
Is the refractive index at a wavelength of 10 μm.

FIG. 3 is a graph showing the imaging performance, and shows the angle of view dependence of the spot diameter obtained by ray tracing. Expressing the spread of diffraction by the circular aperture by the diameter of the first dark zone of Airy is approximately (2.5 × F number × wavelength). In this case, according to the specifications of this embodiment, approximately 40 μm
m. It can be seen that the spot diameter of the infrared optical system according to the present invention is about diffraction, the aberration is suppressed to a small value, and an image is formed.

[0026]

[Table 1]

Embodiment 2 FIG. The specifications are F-number 1.3,
The half angle of view is 5.0 °, and the others are the same as in the first embodiment. Table 2 shows specific numerical data of the second embodiment. FIG.
The imaging performance shows the dependence of the spot diameter on the angle of view.

[0028]

[Table 2]

Embodiment 3 FIG. The specifications are F number 2.0,
Others are the same as the first embodiment. Table 3 shows specific numerical data of the third embodiment. FIG. 5 shows the angle of view dependence of the spot diameter as the imaging performance.

[0030]

[Table 3]

Embodiment 4 FIG. The specifications are the same as those of the first embodiment, except that the half angle of view is 1.4. Table 4 shows specific numerical data of the fourth embodiment. FIG. 6 shows the field angle dependence of the spot diameter as the imaging performance.

[0032]

[Table 4]

Next, Table 5 shows values for the respective conditional expressions in Examples 1 to 4. As is clear from Table 5, the numerical values of Examples 1 to 4 satisfy the conditional expressions (1) and (2).

[0034]

[Table 5]

In the above, a Fresnel lens having a Fresnel surface on the object side and a plane on the image side has been described.
Conversely, even if the Fresnel surface is arranged on the image side and the plane is arranged on the object side, if conditional expressions (1) and (2) are satisfied,
Similar effects can be obtained. Further, although the aperture of the optical system is arranged on the Fresnel surface on the object side of the Fresnel lens 1, the position of the aperture is not limited to this, and may be arranged at any of the optical systems.

Further, the shape of the Fresnel surface is determined based on f (x) which is a quadratic aspheric surface expressed by the equation (3). The effect is obtained.
For example, a general aspherical expression (4) expressed by the following equation may be used. Where A, B, C, D
Is a coefficient representing the shape of the aspherical surface.

[0037]

(Equation 2)

Although the shape of each zone on the Fresnel surface is determined based on the same function f (x), the function may be changed for each zone. When the ring zone number is represented by i, the fresnel lenses 1 and 2 can be formed by determining f _i (x) representing the shape of the Fresnel surface for each ring zone. Also,
In the above embodiment, the step of the Fresnel surface is set to 0.1 mm. However, the value is not limited to this value, and may be a value other than 0.1 mm as long as the value is sufficiently larger than the wavelength.

In the above description, germanium is used as the lens material, but it goes without saying that other materials are effective as long as the material has a small wavelength dependence of the refractive index. In the wavelength range of about 8 to 12 μm,
Silicon is an example of a material in which the change in the refractive index due to the wavelength change is small. Since silicon is a lens material having relatively large absorption, it cannot be used as a thick lens in the above wavelength band. However, if it is a thin lens like the Fresnel lens used in the infrared optical system of the present invention,
Absorption by the lens material can be suppressed.

Further, since the Fresnel surface of the Fresnel lens 1 or 2 is rotationally symmetric, it can be manufactured by cutting with a precision lathe. Further, it can also be manufactured by molding using a mold manufactured by cutting with a precision lathe. Furthermore, germanium and silicon exemplified as lens materials are semiconductors,
It can also be manufactured using a semiconductor manufacturing technique such as an etching technique.

Embodiment 2 FIG. 7 is a sectional view showing an infrared optical system according to Embodiment 2 of the present invention. In FIG. 7, reference numeral 1 denotes a Fresnel lens, which has a Fresnel surface 1a having a refractive function of a convex lens on the object side, and a surface 1 having a fine structure (annular structure) having a diffractive lens function having a convex surface on the image side.
and has the function of a convex lens as an optical component. 2
Denotes a Fresnel lens, which has a Fresnel surface 2a having a refractive function of a convex lens on the object side and a flat surface 2b on the image side, and has a convex lens function as an optical component.

When the dispersion of the lens material cannot be neglected, it is necessary to reduce chromatic aberration in order to obtain a clear object image. A general method of reducing chromatic aberration is to use lenses of different materials to have a convex refraction effect on a lens of a low dispersion material and a concave refraction effect on a lens of a high dispersion material. However, in this method, the aberration cannot be reduced by arranging the two convex lenses at an appropriate interval.

On the other hand, since the diffractive lens function has a negative dispersion characteristic, the chromatic aberration can be reduced both in the convex refraction function by the convex diffractive lens function. Therefore, both the Fresnel lenses 1 and 2 can have a convex lens function, and can satisfy the conditional expressions (1) and (2) for obtaining a clear image.

The diffraction lens having a fine structure can be formed not only on a flat surface but also on a Fresnel surface having a refracting action. In Embodiment 2 shown in FIG. 7, the fine structure is provided on the image-side plane of the Fresnel lens 1 on the object side, but may be provided on the Fresnel surface on the object side. Of course, it may be provided on the plane of the Fresnel lens 2 or on the Fresnel surface.

In the above embodiment, the wavelength band is set to 8 to 12 μm, which is a thermal infrared region, but the same effect can be obtained even in another infrared region.

[0046]

As described above, according to the present invention, there are provided two Fresnel lenses each having a Fresnel surface on one side and a flat surface on the other side and having the function of a convex lens, and conditional expressions (1) and (2) ), The capacity of the lens material used can be reduced, and the cost of the optical system can be reduced.

Further, since the two Fresnel lenses are made of a material with low dispersion, the chromatic aberration that occurs is so small that it can be neglected, and a clear image of the subject can be obtained.

Further, since the Fresnel lens is made of germanium or silicon, the generated chromatic aberration is negligibly small, and a clear image of the subject can be obtained. Further, a change in the refractive index due to a change in the wavelength can be made small.

Further, since a fine structure having a diffractive lens function is provided on at least one of the four surfaces of the two Fresnel lenses, the cost of the optical system can be reduced by using a material having low dispersion. it can.

The Fresnel lens can be easily manufactured by cutting.

Further, the Fresnel lens can be easily manufactured by molding.

Further, the Fresnel lens can be easily manufactured by an etching technique.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a basic configuration of an infrared optical system according to Embodiment 1 of the present invention.

FIG. 2 is an explanatory diagram showing a shape of a Fresnel surface of the lens of FIG.

FIG. 3 is an explanatory diagram showing a field angle dependence of a spot diameter, which is an image forming performance of Example 1, as a specific example of numerical data of an infrared optical system.

FIG. 4 is an explanatory diagram showing the angle-of-view dependence of the spot diameter, which is the imaging performance of Example 2 as a specific example of numerical data of the infrared optical system.

FIG. 5 is an explanatory diagram showing the angle-of-view dependency of the spot diameter, which is the imaging performance of Example 3, as a specific example of numerical data of the infrared optical system.

FIG. 6 is an explanatory diagram showing the angle of view dependence of the spot diameter, which is the imaging performance of Example 3, as a specific example of numerical data of the infrared optical system.

FIG. 7 is a sectional view showing an infrared optical system according to Embodiment 2 of the present invention.

FIG. 8 is a cross-sectional view of a conventional infrared optical system disclosed in JP-A-62-109014.

[Explanation of symbols]

1, 2 Fresnel lens, 1a, 2a Fresnel surface of Fresnel lens 1, 2, 1b, 2b Fresnel lens 1,
2 plane, 1c surface provided with a fine structure having the function of a convex diffraction lens of a Fresnel lens, 16 image surfaces.

Claims (7)

[Claims] 1. A fresnel lens having a function of a convex lens having one surface formed as a Fresnel surface and the other surface formed as a flat surface, wherein (φ−φ1) / φ2> 0.1 (1) 1.5> φ (Φ2 + φ1-φ) / (φ1φ2)> 0.5 (2) where φ is the power of the whole infrared optical system φ1 is the power of the Fresnel lens disposed on the object side φ2 is the power of the Fresnel lens disposed on the image side Power An infrared optical system that satisfies the conditional expressions (1) and (2). 2. The infrared optical system according to claim 1, wherein said two Fresnel lenses are made of a low-dispersion material. 3. The infrared optical system according to claim 1, wherein said Fresnel lens is made of germanium or silicon. 4. The infrared optical system according to claim 1, wherein at least one of the four surfaces of said two Fresnel lenses has a fine structure having a diffractive lens function. . 5. The infrared optical system according to claim 1, wherein said Fresnel lens is manufactured by cutting. 6. The infrared optical system according to claim 1, wherein said Fresnel lens is manufactured by molding. 7. An infrared optical system according to claim 1, wherein said Fresnel lens is manufactured by an etching technique.