RU2386156C1 - Optical system with remote apertures for infrared spectrum - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: optical system has an objective lens which constructs real intermediate image, a projection objective lens which inverts the image, and an aperture diaphragm lying between the projection objective lens and the image plane. The objective lens has a positive meniscus whose convex surface faces the object space and a biconcave lens. The projection objective lens has three menisci lying close to each other. The first and third menisci are positive and their convex surfaces face each other. The second meniscus is negative and its concave surface faces the image plane. All refracting surfaces are spherical. Relationships given in the formula of invention hold in the system.

EFFECT: increased technological effectiveness, increased distance of the input aperture from the first surface, and the output aperture from the last surface of the system, increased back focal distance, reduction of possible calculated focal distances to 15 mm while maintaining the aperture ratio, field angle and high image quality within the entire field.

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Description

The invention relates to the field of optical instrumentation and can be used in optical systems of thermal imaging devices.

For the operation of a thermal imaging device using scanning elements, an optical system is needed that performs the function of a focusing lens with remote input and output pupils, which provides optical pairing with a cold diaphragm of a photodetector device (FPU), combined with the output pupil of the optical system, and a scanner located in the input pupil of the system, calculated for the infrared (IR) region of the spectrum corresponding to the spectral characteristic of the FPU.

Known optical lens systems for the infrared region intended for thermal imaging devices, for example a fast lens [RU 2183340 C1, 2002], consisting of four single lenses, having a relative aperture of 1: 1.65, field of view 18 degrees and a spectral range of operation from 8 to 12.5 microns. The disadvantage of the lens is the lack of pupils, which does not allow its rational use in thermal imaging devices with modern infrared FPU.

Also known is an infrared lens with a remote entrance pupil [RU 2281536 C1, 2006], consisting of three single lenses, designed to work in the far or medium IR range in thermal imaging devices. The lens has a focal length of 74.55 mm, a relative aperture of 1: 2.4, an angular field in the space of objects of 17 °, a back focal segment of 15.27 mm, an extension of the entrance pupil of 16 mm. The disadvantage of the lens is a small aperture, a small amount of projection of the entrance pupil and the absence of a valid exit pupil, which when paired with modern IR FPUs leads to an increase in the noise level of the thermal imaging device.

A known optical system for the infrared region of the spectrum [RU 2006128691 A, 2008], containing seven lenses and having remote input and output pupils. The disadvantage of this system is the large number of components that reduce the transmittance and effective aperture of the system.

It is known that for the rational design of thermal imaging devices, optical systems must have a remote exit pupil, mating with the IR FPU diaphragm [Tarasov VV, Yakushenko Yu.G. "Looking" type infrared systems. - M .: Logos, 2004 .-- 444 p. See p. 94-95], and to accommodate scanners, optical systems must also have an exit pupil.

The closest in technical essence, adopted for the prototype, is an optical system with remote pupils for the infrared region of the spectrum [US Patent No. 6274868 B1, 2001. Optical design - figure 3, design parameters - tables 1 and 1a], containing sequentially located along the way ray lens, building a valid intermediate image containing the first positive meniscus in the direction of the rays, convex to the space of objects, and the second negative lens, a projection lens containing three single claim, the first and third of which along the beam is positive and convex sides facing each other along the second beam is a negative meniscus and a concavity facing towards the image plane, and the aperture stop disposed between the projection lens and the image plane. In the lens, the second negative lens is made in the form of a meniscus facing the concave side to the image plane. In the projection lens, the distance between the first positive meniscus and the second negative is 0.9 of the absolute value of the focal length of the optical system. In each of the lenses of the optical system, one of the refracting surfaces is aspherical. Moreover, in the optical system of the prototype the following relationships between the optical forces are observed:

φ ₁ : φ ₂ : φ = 0.85: 1: 1; φ ₃ = -0.66φ ₄ ; φ ₅ : φ ₆ : φ ₇ = 0.44: (- 0.55): 1; φ ₇ = 3.45φ,

where φ ₁ , φ ₂ , φ are the optical forces of the lens, projection lens, and optical system, respectively, with remote pupils for the infrared region of the spectrum;

φ ₃ , φ ₄ - optical forces, respectively, of the first along the rays of the positive meniscus and the second biconcave lens in the lens;

φ ₅ , φ ₆ , φ ₇ are the optical powers of the first, second, and third menisci along the rays in the projection lens, respectively.

The relative aperture of the optical system - the prototype is from 1: 1.65 to 1: 3.16, the system can be converted to a specific focal length lying in the range from 1.89 to 2.4 inches, i.e. from 48 to 60 mm, the spectral range of operation is from 8 to 11.5 microns. The angular field of the system in the space of objects is 18 °. Removal of the entrance pupil from the first surface of the system is 0.505 of the absolute value of the focal length | f '| system, the removal of the exit pupil from the last surface of the system is 0.1 | f '|, the back focal segment (the distance from the last refracting surface to the rear focal plane) is 0.52 | f' |.

The disadvantages of the prototype are low-tech design due to the use of a large number of aspherical surfaces, a small amount of removal of the entrance pupil from the first surface of the system, a small amount of removal of the exit pupil from the last surface of the system, a small size of the back focal segment, the presence of a limitation (48 mm) of the smallest calculated value focal length.

The solution of the optical system with remote pupils for the infrared region of the spectrum contained in the prototype does not allow its use in conjunction with IR FPUs of 2 and 3 generations of SOFRADIR and with existing and promising FPUs of 2 and 3 generations of domestic production due to the inability to coordinate the cold diaphragm with the output pupil of the system, and also limits the possibility of using the optical system in small-sized thermal imagers.

To match the optical system with the pupils for the infrared region of the spectrum from the cold diaphragm of the above FPUs, it is necessary that the distance from the exit pupil of the optical system to the image plane be equal to the distance from the FPU cold diaphragm to the plane of the FPU sensitive elements. The value of the latter for ruled FPUs is approximately 11.6 mm, for matrix FPUs - about 20 mm. It is also necessary that the removal of the exit pupil from the last surface of the system takes into account the design features of the FPU cryostat and allows for adjustment of the scale, focusing, and thermal compensation of the optical system. Removal of the entrance pupil from the first surface of the system should provide the possibility of placing a scanner or micro-scanning device.

The invention solves the problem of improving the manufacturability of the structure by using only spherical refractive surfaces, increasing the distance of the entrance pupil from the first surface, increasing the distance of the exit pupil from the last surface of the system, increasing the back focal length, lowering the lower boundary of the possible estimated focal lengths to 15 mm while maintaining the relative aperture , angular field and high quality images within the entire field.

To solve this problem, it is necessary to provide such a combination of the optical powers of the lens, the projection lens, the lenses included in their composition, the shapes of the individual lenses, as well as the distances between the lenses in the projection lens, so that the distance between the entrance and exit pupils from the first and last surfaces increases, the posterior focal segment increased, and the optimal ratio between the parameters of the lens, projection lens, and the lenses included in them was found, which would allow it to provide l high values of the frequency-contrast characteristic (FM) and the elimination of distortion for all points of the field when using only spherical refractive surfaces.

The problem is solved in that in the known optical system with remote pupils for the infrared region of the spectrum, containing a lens sequentially located along the rays, constructing a real intermediate image containing the first positive meniscus along the rays, convex to the space of objects, and the second negative lens, a projection lens containing three single menisci, the first and third of which are positive along the rays and face each other with convex sides yeah, the second meniscus along the rays of the lens is negative and facing concavity to the image plane, and the aperture diaphragm located between the projection lens and the image plane, the second negative lens is biconcave in the lens, the menisci in the projection lens are close to each other, while the refracting surfaces optical systems are spherical and the following relationships are true:

where φ ₁ , φ ₂ , are the optical powers of the lens and the projection lens, respectively;

| φ | - the absolute value of the optical power of the optical system with remote pupils for the infrared region of the spectrum;

φ ₃ , φ ₄ - the optical power of the first along the rays of the positive meniscus and the second biconcave lens in the lens, respectively;

φ ₅ , φ ₆ , φ ₇ are the optical powers of the first, second, and third menisci along the rays in the projection lens, respectively.

The proposed optical system with remote pupils for the infrared region of the spectrum makes it possible to increase manufacturability, provides an increase in the distance of the entrance pupil from the first surface, an increase in the distance of the exit pupil from the last surface of the system, an increase in the back focal length, a decrease in the lower boundary of the possible estimated focal lengths to 15 mm while maintaining the relative holes, angular field and high quality images within the entire field.

These advantages in comparison with the prototype in the optical system with remote pupils for the infrared region of the spectrum are achieved by a new set of distinctive features:

- the implementation of the second negative lens in the lens in the form of a biconcave;

- the location of the menisci in the projection lens close to each other;

- the implementation of all the refracting surfaces of the optical system spherical;

- observance of relations (1) and (2).

Performing in the optical system with remote pupils for the infrared region of the spectrum of all refracting surfaces spherical allows you to increase manufacturability, because labor-intensive aspherical surfaces are excluded.

Compliance with the indicated relations (1) provides such a combination of the optical forces of the lens and the projection lens, in which the values increase: removal of the entrance pupil from the first surface to 2 | f '|, removal of the exit pupil from the last surface to 0.8 | f' |, back focal segment up to 1.3 | f '|. As a result, it is possible to reduce the lower boundary of the focal length to 15 mm, while maintaining the possibility of interfacing with modern infrared FPUs of 2 and 3 generations of SOFRADIR and with existing and promising FPUs of 2 and 3 generations of domestic production, which makes it possible to use the optical system in small-sized thermal imaging devices.

The implementation of the second negative lens in the lens in the form of a biconcave, the location of the menisci in the projection lens close to each other simultaneously with the observance of the dependencies indicated in relation (2) between the optical forces allows the distribution of optical forces, lens shapes and their relative position so that, while maintaining the relative aperture of the system is not lower than 1: 1.65 and the angular field is not less than 18 °; th diffraction scattering circle that determines high values of the frequency-contrast characteristics (MTF), and eliminating the distortion for all field points using only spherical refractive surfaces.

In addition, it can be noted that a large amount of removal of the entrance pupil allows for a kink in the optical axis in front of the lens, which is necessary when constructing a compact instrument. The distance between the lens 1 and the projection lens 2 is such that it allows you to install, for example, prism systems for using the optical system in panoramic instruments or additional mirrors for breaking the optical axis.

The specified solution, in our opinion, has novelty and inventive step. The invention is based on the relationship between the optical forces, the shape, the relative position of the lenses in the optical system with remote pupils for the infrared region, which was first established by the applicants.

The authors are not aware of optical systems with remote pupils for the infrared region of the spectrum in which these features would be implemented.

The proposed invention is illustrated by the following graphic materials:

Figure 1. The optical scheme of the optical system with remote pupils for the infrared region of the spectrum.

Figure 2. Frequency response of the optical system with pupils for the infrared region of the spectrum.

Figure 3. The function of energy concentration (PCE) of an optical system with remote pupils for the infrared region of the spectrum.

Figure 4. Distortion graph of the optical system with pupils for the infrared region of the spectrum.

An optical system with remote pupils for the infrared region of the spectrum (Fig. 1) comprises a lens 1 that constructs a real intermediate image, a projection lens 2 that rotates the image with a linear increase (in absolute value) of less than 1 ^x , and an aperture diaphragm located between the projection lens and the image plane and being the exit pupil of the optical system. The lens 1 contains the first positive meniscus 3 along the rays, convex to the space of objects, and the second biconcave lens 4. The projection lens 2 contains three meniscus 5, closely spaced, meniscus 5 and the third meniscus 7 are positive and convex sides to each other, and the second meniscus 6 along the rays of the meniscus is negative and faces with a concavity to the image plane. All refracting surfaces in the system are spherical. Relations (1) are satisfied in the system.

In Fig.1, in addition between the last meniscus 7 of the optical system and the aperture diaphragm there is a plane-parallel plate 8, which acts as a protective glass of the FPU cryostat.

Infrared radiation coming from each point of the distant object and passing through the entrance pupil of the optical system, passing sequentially the meniscus 3 and the biconcave lens 4, forming the lens 1, is focused by them in the plane of the intermediate image. Next, a projection lens, consisting of menisci 5, 6 and 7, projects an intermediate image into the image plane of the optical system. The aperture diaphragm, placed along the rays after the last meniscus 7, acts as the exit pupil of the optical system, in fact, determining the aperture ratio of the optical system with remote pupils for the infrared region of the spectrum. The projection lens in the reverse beam projectes the real aperture diaphragm into the space of objects of the projection lens, and the lens, respectively, into the plane of the entrance pupil of the optical system. Thus, a valid entrance pupil is formed at the required distance from the first surface of the optical system.

As a specific example of execution, Table 1 shows an example implementation of an optical system with remote pupils for the infrared region of the spectrum.

Table 1 Parameters of an example implementation of an optical system with remote pupils for the infrared region of the spectrum Name or item number in accordance with figure 1 The number of lenses in accordance with figure 1 Relative optical power Distance along the optical axis Lens material Entrance pupil 1.91 one 3 1,300 0.13 Ge four -0.951 3.61 Ge 2 5 0.525 0.01 Ge 6 -0.284 0.08 Znse 7 1,000 0.66 Ge Aperture diaphragm (exit pupil) 0.47 Image plane 0.66

As follows from table 1, in a specific example of an optical system with remote pupils for the infrared region of the spectrum, the following distribution of optical forces is performed:

φ ₁ : φ ₂ : | φ | = 0.62: 1.16: 1; φ ₃ = -0.73 φ ₄ ; φ ₅ : φ ₆ : φ ₇ = 0.52: (- 3.52): 1; φ ₇ = | φ |, where | φ | - the absolute value of the optical power of the optical system with remote pupils for the infrared region of the spectrum;

φ _i - the optical power of the components and lenses of the system (i is the position number in accordance with figure 1),

those. the above relations (1) and (2) are observed.

The position of the image plane in table 1 is indicated as the distance between the aperture diaphragm and the image plane, i.e. the back focal segment of the system is 1.13 | f '|, which is 2.1 times higher than the rear focal segment in the prototype. As follows from table 1, the removal of the entrance pupil from the first surface is 1.91 | f '|, i.e. 3.8 times the corresponding value in the prototype. As follows from table 1, the removal of the exit pupil from the last surface is 0.66 | f '|, i.e. 6.6 times the corresponding value in the prototype.

The exact values of the optical forces, radii of the refractive surfaces and thicknesses along the optical axis for specific values of the refractive index and the specific value of the focal length of the optical system with remote pupils for the infrared region of the spectrum and the specific spectral range determined by the spectral sensitivity of the FPU used are set by standard least squares optimization , which is part of all modern programs for optical calculations.

An example of the implementation of an optical system with remote pupils for the infrared region of the spectrum was analyzed in the Zemax program for the following values: focal length of the optical system 24.5 mm, relative aperture 1: 1.65, linear image size 8.06 mm (corresponds to an angular field in space objects 18.6 °), the spectral range of wavelengths from 7.7 to 10.3 microns, i.e. for values of aperture and angular field similar to the prototype.

The frequency response of the optical system of this implementation example is shown in FIG. 2, and the FEC in FIG. 3 for various image points. The graphs show the coordinates y 'of the image point in the image plane (0, 2, and 4.03 mm) for the meridional (m) and sagittal sections (s), as well as the aberration-free frequency response and photomultiplier (the designation is “differ”).

From the above graphs it follows that the contrast transfer coefficients for the spatial frequency of 40 mm ^-1 are 0.26 for the point on the axis, 0.24 and 0.26 for the image point with the coordinate y '= 2 mm, respectively, for the meridional and sagittal sections , for the image point with the coordinate y '= 4.03 mm - 0.15 and 0.20 respectively for the meridional and sagittal sections, i.e. are close to the value of the contrast transfer coefficient for the indicated frequency in the non-aberration system equal to 0.27. FKE in a spot with a radius of 0.0125 mm for points with coordinates y 'equal to 0; 2 and 4.03 mm, respectively 0.73; 0.75 and 0.65, i.e. close to the corresponding value of the PCE in the non-aberration system equal to 0.76.

The calculated integral image quality characteristics shown in FIGS. 2 and 3 indicate a high degree of correction of residual aberrations achieved by observing the claimed ratios of optical forces, lens shapes and their refractive surfaces, their relative positions in the optical system with remote pupils for the infrared region of the spectrum .

Figure 4 shows a graph of distortion for an example implementation: the y-axis indicates the image y 'in the image plane, the abscissa shows the relative distortion in percent. As follows from the graph, the distortion for the field edge does not exceed 2.3%, which is an acceptable value for thermal imaging lenses.

An analysis of the frequency response of the prototype described in [US Patent No. 6274868, 2001. FIG. 4] shows that the values of modulation transmission coefficients for different points of the field are in the range from 0.27 to 0.14. Based on a comparison of the implementation example with the prototype, we can conclude that the proposed optical system provides high image quality while maintaining aperture ratio and relative aperture.

Thus, the claimed optical system with remote pupils for the infrared region of the spectrum, having a combination of these distinctive features, in comparison with the prototype allows to increase the manufacturability of the design by using only spherical refractive surfaces, to increase the distance of the entrance pupil from the first surface, to increase the distance of the exit pupil from the last the surface of the system, increase the back focal segment, provide a decrease in the lower boundary of the possible calculated focal lengths distances up to 15 mm while maintaining the relative aperture, the angular field and high image quality over the entire field.

Using the proposed optical system with remote pupils for the infrared region of the spectrum allows us to increase manufacturability and create small-sized thermal imaging devices that work in conjunction with infrared FPUs of 2 and 3 generations of SOFRADIR and with existing and promising FPUs of 2 and 3 generations of domestic production.

Literature

1. RU 2183340 C1, 2002

2. RU 2281536 C1, 2006.

3. RU 2006128691 A, 2008.

4. Tarasov VV, Yakushenko Yu.G. "Looking" type infrared systems. - M .: Logos, 2004 .-- 444 p.

5. US patent No. 6274868 B1, 2001 (prototype).

Claims (1)

An optical system with remote pupils for the infrared region of the spectrum, comprising a lens sequentially located along the rays, constructing a real intermediate image, containing the first positive meniscus convex to the space of objects, and the second negative lens, a projection lens containing three single menisci, the first and third of which along the rays of the rays are positive and face with convex sides to each other, the second along the rays of the meniscus is negative the lens is concave to the image plane, and the aperture diaphragm located between the projection lens and the image plane, characterized in that the second negative lens is biconcave in the lens, the menisci in the projection lens are close to each other, while the refractive surfaces of the optical system are spherical , and the following relations are satisfied:
φ ₁ : φ ₂ : | φ | = (0.55 ÷ 0.70) :( 0.90 ÷ 1.25): 1;
φ ₃ = - (0.7 ÷ 0.8) φ ₄ ; φ ₅ : φ ₆ : φ ₇ = (0.46 ÷ 0.58) :-( 2.9 ÷ 4.3): 1; φ ₇ = (0.8 ÷ 1.2) | φ |,
where φ ₁ , φ ₂ , are the optical powers of the lens and the projection lens, respectively;
| φ | - the absolute value of the optical power of the optical system with remote pupils for the infrared region of the spectrum;
φ ₃ , φ ₄ - the optical power of the first along the rays of the positive meniscus and the second biconcave lens in the lens, respectively;
φ ₅ , φ ₆ , φ ₇ are the optical powers of the first, second, and third menisci along the rays in the projection lens, respectively.

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