RU2831936C1 - Infrared telephoto lens with two fields of view - Google Patents

Abstract

FIELD: optics.

SUBSTANCE: invention relates to telephoto lenses for thermal imaging devices based on cooled matrix receivers. Infrared telephoto lens with two fields of view consists of four components arranged along an optical axis. First component is fixed, consists of three lenses, the first convex-concave positive lens from germanium, facing the convex surface to the plane of objects, the second negative plano-concave lens from fluorite, facing the concave surface to the plane of the objects, third positive convex-concave zinc selenide lens whose convex surface faces the plane of objects. Second and third components are movable and have two fixed positions. Second component is made in the form of a negative convex-concave lens made of germanium with its convex surface facing the plane of objects. Third component is made in the form of a positive plano-convex lens made of germanium with its convex surface facing the image plane; the second surface of the lens is aspherical. Fourth fixed component consists of a positive convex-concave lens made of germanium with its convex surface facing the plane of objects; the first surface of the lens is aspherical.

EFFECT: reducing the number of aspherical surfaces while maintaining the number of optical elements, increasing the magnitude of changing the field of view while maintaining image quality.

1 cl, 10 dwg, 6 tbl

Description

The invention relates to the field of optical instrument making, namely to optical systems operating in the infrared (IR) region of the spectrum, and can be used in thermal imaging devices built on the basis of cooled matrix thermal radiation receivers.

An optical system of an infrared lens with two fields of view and an external aperture diaphragm is known (patent RU No. 2400784 C1, IPC G02B 13/14, G02B 15/16, G02B 9/64, priority date 21.08.2009), consisting of ten lenses. The infrared lens, made without aspherical and diffraction optical elements, has high image quality (relative aperture 1:2). But a large number of lenses reduces the transmission coefficient of the system, which is its disadvantage.

An infrared lens with a smoothly changing focal length is known (patent RU No. 2299454 C1, IPC G02B 13/16, G02B 13/14, priority date 31.01.2005), comprising a fixed first component, movable second and third components, and fixed fourth and fifth components sequentially located along the optical axis. The lens has a large focal length change multiplicity M = 5 ^× (the focal length changes from 60 mm to 300 mm). The focal length is changed by simultaneously moving the second and third components along the optical axis, with the second component moving 182 mm and the third component moving 67.6 mm from the initial position. Large values of the movable component movements lead to a significant axial dimension of the lens, which is its disadvantage.

The closest analogue in technical essence to the claimed system is an infrared telephoto lens with two fields of view (patent RU No. 2630195 C1, IPC G02B 13/02, G02B 13/14, G02B 15/14, priority date 01.0.2016). The optical system operates in the mid-infrared spectrum from 3.7 to 4.85 μm. The infrared telephoto lens contains four components along the beam path. The first fixed component contains two meniscuses facing the plane of objects with their convex surfaces, the first of which is positive and the second negative. The second surface of the first meniscus and the first surface of the second meniscus are aspherical. The second component is a movable biconcave lens, the second surface of which is aspherical. The third fixed positive component contains two meniscuses facing the plane of objects with their convex surfaces, of which the first is positive, the second is negative, the second surface of the first meniscus and the first surface of the second meniscus are aspherical. The fourth component is a fixed single positive convex-concave lens facing the plane of objects with its convexity, the second surface of which is asphero-diffractive. An intermediate image is formed in the space between the third and fourth components. The focal length of the narrow-field infrared system objective is ƒ'=-183.3 mm, wide-field ƒ'=-61 mm, the linear field of view in the image space is 9.6 mm × 7.68 mm. The function of the aperture diaphragm during operation of the objective in combination with a cooled matrix IR radiation receiver is performed by the cooled diaphragm of the receiver. The objective allows changing the field size by three times and has high image quality. But this lens is technically difficult to manufacture - out of six lenses, five have aspherical surfaces, and one has asphero-diffractive surfaces, which complicates its production and increases the cost of production, which is a disadvantage of this lens.

The task that the claimed invention is aimed at solving is the creation of a technologically advanced, cost-effective design of an infrared telephoto lens with high technical characteristics.

The technical result consists in reducing the number of aspherical surfaces while maintaining the number of optical elements, increasing the multiplicity of the field of view change while maintaining the image quality, as well as removing the entrance pupil, which will allow the use of a telephoto lens in special equipment with a periscopic beam path, which will reduce the dimensions of the protective windows of products in which the proposed scheme is used.

This is achieved by the fact that the optical scheme of the infrared telephoto lens with two fields of view, like the analogue, consists of four components located along the beam. The first fixed component contains the first positive lens, facing the plane of objects with its convex surface, and, unlike the analogue, made of germanium, the second negative lens, which, unlike the analogue, is made flat-concave, facing the plane of objects with its concave surface and made of fluorite. The first component differs from the analogue in that after the second lens, a third positive convex-concave lens made of zinc selenide is introduced, facing the plane of objects with its convex surface. The second movable component is a negative lens made of germanium, made, unlike the analogue, convex-concave, facing the plane of objects with its convex surface. The third component consists of a positive lens, the second surface of which is aspherical and, unlike the analogue, the lens is made plano-convex from germanium, with its convex surface facing the plane of images. The lens is installed with the possibility of movement along the optical axis. The fourth fixed component is a single positive convex-concave lens made of germanium, with its convex surface facing the plane of objects, unlike the analogue, the first surface of this lens is aspherical.

An intermediate image is formed between the second and third components.

In this case, the following relationships are fulfilled:

where: ƒ ₁ , ƒ ₂ , ƒ ₃ are the focal lengths of the first, second, third lenses of the first component, respectively;

ƒ _UP - focal length of a narrow field of view lens;

d ₂ - air gap between the first and second lenses of the first component;

d ₄ - air gap between the second and third lenses of the first component;

n ₂ is the refractive index of the second lens of the first component for λ=4 nm.

The dual field infrared telephoto lens operates with a photodetector that has an input window and a cooled diaphragm, which is the aperture diaphragm of the optical system.

The proposed invention is illustrated by the following graphic materials:

Fig. 1 - optical diagram of an infrared telephoto lens with two fields of view (wide field of view);

Fig. 2 - optical diagram of an infrared telephoto lens with two fields of view (narrow field of view);

Fig. 3 - modulation transfer function or frequency-contrast characteristic (MCF) of an infrared telephoto lens with two fields of view in a wide field of view;

Fig. 4 - modulation transfer function or frequency-contrast characteristic (MCF) of an infrared telephoto lens with two fields of view in a narrow field of view;

Fig. 5 - diagram of the scattering spot of an infrared telephoto lens with two fields of view in a wide field of view;

Fig. 6 - diagram of the scattering spot of an infrared telephoto lens with two fields of view in a narrow field of view;

Fig. 7 - energy concentration function (ECF) of an infrared telephoto lens with two fields of view in a wide field of view;

Fig. 8 - energy concentration function (ECF) of an infrared telephoto lens with two fields of view in a narrow field of view;

Fig. 9 - relative illumination in the image plane of an infrared lens with two fields of view in a wide field of view;

Fig. 10 - Relative illumination in the image plane of an infrared lens with two fields of view in a narrow field of view.

An infrared telephoto lens with two fields of view (Fig. 1 and Fig. 2) consists of four components located along the beam path. The first component I is fixed and consists of three lenses: the first convex-concave positive lens made of germanium, pos. 1, with its convex surface facing the plane of objects, the second negative plano-concave lens made of fluorite, pos. 2, with its concave surface facing the plane of objects, the third positive convex-concave lens made of zinc selenide, pos. 3, with its convex surface facing the plane of objects. The second component II, containing a negative convex-concave lens made of germanium, pos. 4, with its convex surface facing the plane of objects, is mounted so as to be able to move along the optical axis. The third component III consists of a positive plano-convex lens made of germanium, pos. 5, with its convex surface facing the plane of images, the second surface of the lens is aspherical and is mounted with the possibility of movement along the optical axis. The fourth component IV is fixed, consists of a positive convex-concave lens made of germanium pos. 6, with its convex surface facing the plane of objects, the first surface of the lens is aspherical.

In this case, the following relationships are fulfilled:

where: ƒ ₁ , ƒ ₂ , ƒ ₃ , are the focal lengths of the first, second, third lenses of the first component, respectively;

ƒ _UP - focal length of a narrow field of view lens;

d ₂ - air gap between the first and second lenses of the first component;

d ₄ - air gap between the second and third lenses of the first component;

n ₂ is the refractive index of the second lens of the first component for λ=4 nm.

The infrared telephoto lens with two fields of view works with a photodetector (cooled matrix infrared radiation receiver) pos. 7, which has an input window pos. 8 and a cooled diaphragm pos. 9, which is the aperture diaphragm of the optical system of the infrared telephoto lens.

In accordance with the proposed solution, an optical design of an infrared telephoto lens with two fields of view was calculated, the technical characteristics of which are given in Table 1.

The design parameters of the proposed optical design of an infrared telephoto lens with two fields of view are given in Table 2.

Table 3 shows the variable air gap values for the two fields of view of a dual-field infrared telephoto lens.

The optical scheme of the infrared telephoto lens with two fields of view operates as follows: infrared radiation emanating from infinitely distant points of the object passes through lenses pos. 1, pos. 2, pos. 3 of the first component I, lens pos. 4 of the second component II, forms a real image in the plane of the intermediate image, then the radiation passes through lens pos. 5 of the third component III and lens pos. 6 of the fourth component IV within the angular field determined by the dimensions of the cooled matrix receiver of infrared radiation and the focal length of the infrared telephoto lens (Fig. 1, Fig. 2), then through the input window pos. 8 and the diaphragm pos. 9 of the photodetector pos. 7 is transferred to the plane of the lens images, providing for each point of the object focusing into a small spot, which in size is comparable with the scattering spot caused by diffraction. The plane of the sensitive elements of the photodetector is aligned with the plane of images of the infrared lens.

The change of the field of view (focal length) of the infrared telephoto lens from a wide field of view to a narrow field of view is achieved by simultaneously moving the lens pos. 4 of the second component II and the lens pos. 5 of the third component III, wherein the lens pos. 4 of the second component II is shifted in the direction of the lenses pos. 1, pos. 2 and pos. 3 of the first component I (the amount of shift is Δ=51.22-29.14=22.08 mm), and the lens pos. 5 of the third component III is shifted in the direction of the lens pos. 6 of the fourth component IV (the amount of shift is Δ=43.37-32.30=11.07 mm).

The focal length of the lens changes from ƒ'=-34.6 mm (minimum) to ƒ'=-138.88 mm (maximum). The multiplicity M of the change in focal length is:

The analogue has a 3x change in focal length.

When changing the field of view from a narrow to a wide field of view, the angular field of view of a telephoto lens also increases fourfold.

When changing the angular fields of view, the lens maintains high image quality and ensures a constant relative aperture of 1:4.

The image quality of the lens optical system is estimated using the parameters of the scattering circle and the modulation transfer function. The parameters of the modulation transfer function of the optical system of the infrared telephoto lens in comparison with the diffraction-limited system are given in Table 4 for a wide field of view and in Table 5 for a narrow field of view, and are also shown in Fig. 3 and Fig. 4.

Fig. 5 and Fig. 6 show the diagrams of the scattering spot. The geometric radius of the scattering circle of a point does not exceed 19.9 μm in the center of the field of view and 43.9 μm at the edge of the field of view for a wide field of view and 23.8 μm in the center of the field of view and 28.9 μm at the edge of the field of view for a narrow field of view.

The energy concentration values for the two fields of view are given in Table 6 and in the graphs of Fig. 7 and Fig. 8.

The optical system of the infrared telephoto lens has a very high image quality; in a wide field of view, the energy concentration in a circle with a radius of 15 µm is 71% at the diffraction limit of 79% for the on-axis beam, 63% for the off-axis beam (2ω=20°), for a narrow field of view, the energy concentration value is 72% for the on-axis beam, 61% for the off-axis beam (2ω=5°).

The claimed optical system has no vignetting, therefore the drop in relative illumination in the image plane (on the plane of the sensitive elements of the photodetector matrix) from the center to the edge is from 1 to 0.89 for a wide and from 1 to 0.86 for a narrow field of view, which is confirmed by the graphs in Fig. 9 and Fig. 10.

The infrared telephoto lens with two fields of view allows the field size to be changed four times, has high image quality and all but two of its surfaces are spherical.

Thus, a technical result has been achieved - an infrared telephoto lens with two fields of view, an extended aperture diaphragm, which coincides with the cooled diaphragm of the receiver, has been created, which provides the possibility of coupling the telephoto lens with modern cooled matrix receivers of IR radiation, with a reduced number of aspherical surfaces with an increase in the multiplicity of the field of view change, high image quality and the same number of lenses in the telephoto lens, namely six, as well as an extension of the entrance pupil, which will allow the use of the telephoto lens in special equipment with a periscopic beam path, which will reduce the dimensions of the protective windows of products in which the proposed scheme is used.

Claims (8)

An infrared telephoto lens with two fields of view, comprising four components along the beam path, the first of which is fixed, consisting of a first positive lens facing the plane of objects with its convex surface, and a second negative lens, the second movable component made in the form of a negative lens made of germanium, the third component made in the form of a positive lens, the second surface of which is aspherical, the fourth fixed component, including a single positive convex-concave lens made of germanium facing the plane of objects with its convex surface, characterized in that in the first component the first positive lens is made of germanium, and the second negative lens is made plano-concave made of fluorite, facing the plane of objects with its concave surface, also in the first component behind the second negative lens a third lens made of zinc selenide is added in the form of a positive meniscus facing the plane of objects with its convex surface, the negative lens of the second component is made convex-concave, facing the convex surface to the plane of objects, in the third component the positive lens is made plano-convex from germanium, facing the convex surface to the plane of the image and is installed with the possibility of movement along the optical axis, in addition, the first surface of the positive convex-concave lens of the fourth component is made aspherical, and the following relationships are fulfilled: n ₂ ≤1.45; where: ƒ ₁ , ƒ ₂ , ƒ ₃ , are the focal lengths of the first, second, third lenses of the first component, respectively; ƒ _UP - focal length of a narrow field of view lens; d ₂ - air gap between the first and second lenses of the first component; d ₄ - air gap between the second and third lenses of the first component; n ₂ is the refractive index of the second lens of the first component for λ=4 nm.