RU2400784C1 - Infrared objective lens with two fields of vision and remote aperture diaphragm - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: objective has fixed positive first and last components between which there is a movable negative meniscus whose convex surface faces the image plane and a positive component. The first component contains a positive and a negative meniscus whose concave side faces the image plane. The last component is in form of two positive menisci whose convex surfaces face each other, and a negative meniscus whose concave side faces the image plane. The movable component includes a divergent and a convergent lens and a negative meniscus whose convex side faces the image plane lying tightly in front of the divergent lens of this component. The aperture diaphragm is placed between the objective lens and the image plane. The entrance pupil is superposed with the first surface of the objective lens.

EFFECT: exclusion of aspherical and diffraction optical elements, reduced diameter of the objective lens and distance from the aperture diaphragm to the image plane with maintenance of high aperture ratio, drop in magnification and image quality.

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Description

The invention relates to the field of optical instrumentation, namely to lenses for the infrared (IR) region of the spectrum, and can be used in optical systems of thermal imagers built on the basis of cooled matrix thermal radiation detectors.

For optimal conjugation in the thermal imaging device of the lens with a cooled thermal radiation receiver having a cooled diaphragm, a lens design is needed that would form an exit pupil placed in the space between the last surface of the lens and the image plane. The mismatch of the exit pupil with the cooled diaphragm leads to an increase in spurious radiation from structural elements, as well as to the vignetting of oblique beams and a decrease in irradiation at the edge of the image and, as a result, to a decrease in the characteristics of the thermal imaging device or complex as a whole. To achieve maximum characteristics of the thermal imaging device, the cooled diaphragm must fulfill the function of the aperture diaphragm and, accordingly, the exit pupil in the optical system of the lens. In this case, the optical system of the lens becomes asymmetric with respect to the aperture diaphragm and the stronger, the closer the cooled diaphragm is located to the plane of the sensitive area of the receiver. As is known, in optical systems symmetric with respect to the aperture diaphragm, partial compensation of some aberrations occurs due to the symmetrical path of the rays in the left and right parts of the system relative to the diaphragm. The asymmetry of the optical system with respect to the aperture diaphragm complicates the optical system and requires the search for new circuit solutions of the lens that are not amenable to analytical methods of solution, but which can provide a degree of aberration correction at which an optimal match is achieved between the image quality provided by the lens and the parameters of the thermal radiation receiver .

Known optical system containing 8 lenses, providing a three-stage change of field of view, four times the difference in focal lengths, relative aperture 1: 1 [patent US 6091551, 2000], as well as a lens of 5 lenses with a relative aperture 1: 1.2 with a three-fold difference in focal lengths distances and two-stage change of field of view [patent RU 2339983, 2006. Lens with a variable focal length for working in the infrared region of the spectrum (options)]. To change the field of view in these systems, the movement of two components at unequal distances along the optical axis is used. The disadvantage of analogues is the placement of the diaphragm inside the system, and the exit pupil at infinity, which involves the use of an optical system in conjunction with uncooled bolometric radiation detectors, but does not allow the efficient use of these optical systems with cooled matrix radiation detectors.

Known optical system lens with a remote aperture diaphragm [patent US 7136235, 2006. Optical apparatus]. The lens contains 7 lenses, has a relative aperture of 1: 3.5, a focal length of 100 mm, and an angular field in the space of objects of 5 °. The disadvantage of the analogue is the inability to change the field of view, a small relative aperture, the presence of three aspherical surfaces that reduce manufacturability and increase the cost of construction, as well as a significant decrease in resolution and irradiation at the edge of the image compared to a point on the axis due to its large size (more than 50 %) narrowing the diameters of the inclined beams in comparison with the axial beam.

An optical system for imaging an object in two fields of view is also known [patent RU 2355003, 2009], in which the cooled diaphragm of the radiation receiver performs the function of the aperture diaphragm of the optical system. The optical system is a mirror lens and contains 10 optical elements. The change in the field of view and, accordingly, the focal length of the lens is carried out by input-output of lenses and mirrors from the path of the rays, the change ratio is 2.7 times. Relative aperture 1: 2.05. The disadvantage of the analogue is the presence of two aspherical surfaces that reduce manufacturability and increase the cost of the structure, as well as a decrease in the effective relative aperture in the narrow field channel due to the presence of central shielding.

Also known is an optical system with a variable field of view containing 11 lenses [Japanese patent 2002-14283, 2002]. The cooled diaphragm of the radiation receiver performs the function of a remote aperture diaphragm of the optical system. The change in the field of view and, accordingly, the focal length of the lens is carried out by moving two components along the optical axis, while the difference in magnification is 4. The system additionally includes a thick plane-parallel plate equivalent to a scan of the Pehan prism, which allows its use in panoramic instruments. The disadvantage of the analogue is the presence of two aspherical surfaces that reduce manufacturability and increase the cost of construction.

The closest analogue in technical essence to the claimed device is an infrared lens [patent US 6424460, 2002. Figure 3] with two fields of view and a remote aperture diaphragm located between the last component of the lens and the image plane containing optically connected first and the last positive component and the movable component located between them, including the negative and positive lenses, having two fixed positions on the optical axis to change the sp Nia; the first component is made in the form of two menisci facing the concave side to the image plane, the last component includes two positive meniscus facing the convex surfaces to each other, and a negative meniscus facing the concave side of the image plane. The function of the aperture diaphragm during the operation of the lens in combination with a cooled matrix receiver of infrared radiation is performed by the cooled diaphragm of the receiver. In the closest analogue, two surfaces are aspherical, on one of them a diffractive optical element is applied (in the form of a hologram). The focal length takes two values: 53 and 160 mm, i.e. three times the difference in focal length (field of view) is provided. The relative aperture is 1: 2.5. The distance p 'from the cooled aperture diaphragm to the image plane is 47 mm in a specific embodiment, which is 0.88 of the smallest focal length. The device is such that the projection of the aperture diaphragm into the space of objects - the entrance pupil - is imaginary and does not coincide with the first component of the lens. This leads to the fact that the diameter of the first component exceeds the diameter of the entrance pupil for the greatest focal length. In a specific embodiment, this excess is 1.6 times. It is impossible to combine the entrance pupil with the first lens in the closest analogue due to the fact that the internal movable component is negative.

So, the disadvantages of the closest analogue are:

1) the presence of two aspherical surfaces and a diffractive optical element in the form of a hologram, which reduces manufacturability and increases the cost of the device;

2) the diameter of the first lens exceeds the largest diameter of the entrance pupil due to the fact that the entrance pupil does not coincide with the first lens. This leads to an increase in the mass of the lens;

3) a large distance from the aperture diaphragm to the image plane, which is 0.88 from the smallest focal length, which limits the possibility of use with modern cooled matrix IR detectors, in which this distance can reach 10-20 mm.

The task to which the claimed device is directed is to create a technologically advanced, cost-effective lens design with high technical characteristics, which provides the possibility of interfacing with modern cooled matrix receivers of infrared radiation.

The technical result achieved in solving this problem is to eliminate aspherical and diffractive optical elements in the optical system, reducing the largest diameter of the objective lenses, decreasing the distance from the aperture diaphragm to the image plane while maintaining a high relative aperture, the difference in magnifications and image quality.

The problem is solved, and the technical result is achieved due to the fact that, in contrast to the closest analogue in the first component, the first meniscus is positive, the second negative. after the first positive component, a movable negative meniscus is introduced, convex side to the image plane, having two fixed positions on the optical axis, the distances between the vertices of the refracting surfaces of the moving negative meniscus and the moving component are different in each of the two fixed positions; the movable component is made positive, a negative meniscus is introduced into it, the convex side facing the image plane, located right in front of the negative lens of this component; all the refracting surfaces of the infrared lens are made spherical.

The combination of all the introduced features in the proposed infrared lens with two fields of view and a remote aperture diaphragm allows you to combine the entrance pupil with the first surface of the lens and reduce the diameter of the latter to the diameter of the entrance pupil, eliminate aspherical and diffractive optical surfaces in the device, and correct aberrations of axial and off-axis beam beams with asymmetric placement of the aperture diaphragm, while reducing the distance from the aperture diaphragm to the image plane, and to ensure such sizes of scattering spots within the field of view, at which diffraction-limited frequency-contrast characteristics (FSH), high values of the energy concentration function (PFC) and acceptable distortion values for each of the two fields of view are achieved while maintaining the relative aperture. As a result, a technologically advanced, cost-effective lens design with high technical characteristics was obtained, which makes it possible to interface with modern cooled matrix IR radiation detectors.

The specified solution, in our opinion, has novelty and inventive step. The authors are not aware of the optical systems of infrared lenses with two fields of view and a remote aperture diaphragm in which these signs would be realized.

The proposed invention is illustrated by the following graphic materials:

Figa - optical diagram of an infrared lens with two fields of view and a remote aperture diaphragm (narrow field of view);

Fig. 1b is an optical diagram of an infrared lens with two fields of view and a remote aperture diaphragm (wide field of view);

Figure 2a shows the frequency response of an infrared lens for a narrow field of view;

Fig.2b - frequency response of the infrared lens for a wide field of view;

Figa - FKE in the infrared lens for a narrow field of view;

Figb - FKE in the infrared lens for a wide field of view;

Figa - astigmatism and distortion in an infrared lens for a narrow field of view;

Figb - astigmatism and distortion in an infrared lens for a wide field of view.

On figa shows the proposed optical scheme of the infrared lens with two fields of view and a remote aperture diaphragm in one of two fixed positions of the moving components, which implements a narrow field of view. The lens contains, along the rays of the beam, optically coupled a fixed positive component 1, a moving negative meniscus 2, convex side to the image plane, a moving positive component 3, a fixed positive component 4, in the space between which and the image plane there is an aperture diaphragm. Component 1 is made in the form of a positive meniscus 5 and a negative meniscus 6, facing the concave side to the image plane. The movable positive component 3 includes the closely spaced negative meniscus 7, convex side to the image plane, the negative lens 8 and the positive lens 9. The stationary positive component 4 consists of closely spaced negative meniscus 10, two positive menisci 11 and 12 facing convex surfaces to each other friend, and the negative meniscus 13, facing the concave side to the image plane. All refractive surfaces of the lens are made spherical. The movable meniscus 2 and component 3 have two fixed positions on the optical axis to change the field of view, while the distances between the vertices of the refracting surfaces of the movable meniscus 2 and the movable component 3 are different in each of the two fixed positions. Additionally, a protective glass is shown in pos. 14 of the cryostat, into which the IR radiation receiver is placed, and the cooled diaphragm of which serves as the aperture diaphragm of the lens. On figb shows the relative position of the components of the lens in the second fixed position of the moving elements, which implements a wide field of view.

The optical system operates as follows. In the position of the components shown in Fig. 1a, the lenses of components 1 and 2 focus the infrared radiation coming from each point of the distant objects within the angular field determined by the dimensions of the cooled matrix receiver of infrared radiation and the focal length of the lens, and create a real image of objects in the plane of the intermediate image, which is then transferred by the lenses of components 3 and 4 through the protective glass 14 to the plane of the image of the lens, providing for each point of the object focusing in the spot of a small area size, comparable in magnitude to the diffusion spot. The cooled aperture diaphragm provides a high relative aperture of the lens, the absence of vignetting for tilted beams of rays and minimizes the input of infrared radiation to the matrix receiver of infrared radiation. The plane of the sensitive elements of the matrix receiver of infrared radiation is combined with the plane of the image of the lens. To change the field of view, the components 2 and 3 are moved along the optical axis to the position shown in Fig. 1b, while the distance between the vertices of the first surfaces of the components 2 and 3 changes. In this position, an intermediate real image of objects is formed by the lenses of components 1, 2, and 3, and then transferred by the lenses of component 4 to the image plane of the lens. In this case, the equivalent focal length of the infrared lens is reduced by three times, and accordingly, the angular field in the space of objects increases three times. The position of the image plane remains unchanged both with a narrow and a wide field of view.

For an example of a specific implementation, Table 1 shows the optical powers of the components and lenses of the proposed lens, the distance between the lenses, lens materials and their diameters. The values of the parameters in table 1 are given during normalization of the equivalent focal length of the lens f '= - 1.

Table 1 - Parameters of an infrared lens with two fields of view and a remote aperture diaphragm Component number in accordance with figa Optical power The lens number in accordance with figa Optical power Axis distance Material Diameter one 1,296 5 1,399 0,038 Ge 0.53 6 -0.128 0.490 / 0.104 Znse 0.52 2 -1,264 2 -1,264 0.270 / 0.067 Ge 0.20 3 2,366 7 -2,083 0.010 Ge 0.24 8 -0.193 0,029 Gaas 0.29 9 3,873 0.161 / 0.750 Ge 0.30 four 8,137 10 -2,712 0.010 Gaas 0.16 eleven 6,010 0.010 Ge 0.18 12 4,780 0.024 Ge 0.16 13 -2.693 0,046 Ge 0.12 fourteen 0 0.014 Ge 0,065 Hell. 0,094

The “/” sign indicates the distances between the components for narrow and wide fields of view, respectively. With a wide field of view, the focal length of the lens is -0.33.

As follows from table 1, figa and 1b, the signs of the optical forces of the components and their lenses, as well as the shape of the lenses correspond to the claimed. The relative aperture of the lens is 1: 2. The entrance pupil is aligned with the first surface of the lens, and the largest diameter of the objective lens is matched with the diameter of the entrance pupil. As a result, the mass of the lens decreases. The refractive surfaces of all lenses are spherical, i.e. the manufacturability of the device increases and its cost decreases. The distance from the aperture diaphragm (A.D.) to the image plane is 0.094 from the largest focal length and, accordingly, 0.28 from the smallest, i.e. less than in the closest analogue.

The specific radii of the spherical refracting surfaces are determined by the required focal lengths in each of the fixed positions of the movable lenses and the refractive indices of the materials corresponding to the working spectral range of the radiation detector used, by using the optimization of any modern optical design program.

An example of the implementation of an infrared lens with two fields of view and a remote aperture diaphragm was carried out for a variant with a triple change of the field of view, with focal lengths of 210 and 70 mm, angular fields of 3.34 ° and 10.5 °, respectively, determined by the diagonal of the receiver (12.4 mm). For each of the focal lengths, the relative aperture is 1: 2. The spectral range (from 7.7 to 10.3 μm) corresponds to the spectral sensitivity of a cooled infrared radiation receiver from Sofradir. In the example implementation p '= 19.8 mm, which is 0.28 of the smallest focal length.

The plots of the frequency response and the photomultiplier are presented in FIG. 2a and FIG. 3a, respectively, for the positions of the components in the lens providing a narrow field, and FIG. 2b and FIG. 3b show a wide field of view. Graphs of astigmatism and distortion are presented in Fig.4a for the positions of the components in the lens, providing a narrow field, and Fig.4b - a wide field of view. From the presented graphs it follows that the inventive lens provides high image quality for each of the fields of view, close to diffraction, and distortion of not more than 2.2% for the edge of the field of view. The proposed lens retains a three-fold difference in the field of view, the relative aperture is 1: 2, and a two-field observation mode is implemented.

Thus, the implementation of the technical advantages of the proposed infrared lens with two fields of view and a remote aperture diaphragm, having a combination of these distinguishing features, allows you to create a technological, cost-effective design of the lens with high technical characteristics, to use it in conjunction with modern cooled matrix receivers of infrared radiation, including including in small-sized thermal imaging devices.

Information sources

1. Patent US 6091551, 2000.

2. Patent RU 2339983, 2006.

3. Patent US 7136235, 2006.

4. Patent RU 2355003, 2009.

5. Japanese Patent 2002-14283, 2002.

6. Patent US 6424460, 2002.

Claims (1)

An infrared lens with two fields of view and a remote aperture diaphragm located between the last component of the lens and the image plane, containing optically connected first and last positive components located along the rays and a moving component located between them, including a negative and a positive lens, having two fixed positions on optical axis for changing visual fields; wherein the first component is made in the form of two meniscuses facing the concave side to the image plane, the last component includes two positive meniscuses facing the convex surfaces to each other, and a negative meniscus facing the concave side to the image plane, characterized in that the first component the meniscus is positive, the second is negative, after the first positive component, a movable negative meniscus is introduced, with the convex side facing the image plane having two fixed positions on the optical axis, the distance between the peaks of the negative meniscus movable and the movable component refracting surfaces are different in each of the two fixed positions; the movable component is made positive, a negative meniscus is inserted into it, facing the convex side to the image plane, located right in front of the negative lens of this component, a negative meniscus is placed in the last component, located right in front of the positive menisci of this component, all the refracting surfaces of the infrared lens are made spherical.

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