RU2594955C1 - Telescopic lens for infrared spectrum - Google Patents

Abstract

FIELD: heat engineering.

SUBSTANCE: invention can be used in thermal imagers sensitive within the spectral range from 8 to 12 µm. Telescopic lens has four menisci, from which the first, the second and the fourth menisci on the beam path are positive, and the third one is negative. All meniscus by concave surfaces face image plane. First meniscus is made from germanium, and the rest ones - from zinc selenide. Third meniscus is made movable along the optical axis. Following ratios are implemented: φ₁:φ₂:φ₃:φ₄ = (1.15÷1.27):(0.9÷1.15):-(2.8÷3.8):(1.5÷2.7), where φ₁, φ₂, φ₃, φ₄ are relative optical forces of the first, the second, the third and the fourth menisci; D2/f′ = 0.15÷0.35; D6/f′ = 0.25÷0.45, where D2 is an air gap between the first and the second menisci, f^' is equivalent focal distance of the lens; D6 is an air gap between the third and the fourth menisci.

EFFECT: technical result is reduction of length and weight of the telescopic lens while maintaining high image contrast and providing constant focal distance within the range of temperatures from -40 °C to +50 °C.

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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 thermal imagers built on the basis of matrix photodetector devices sensitive in the spectral range from 8 to 12 microns.

A known infrared lens according to RF patent No. 2365952 of February 13, 2008, contains four components, the first of which is a positive meniscus convex to the object, the second is a negative lens, the third is a positive lens, and the fourth is a positive meniscus, facing concavity to the image. The menisci are made of germanium, the negative and positive lenses are made of oxygen-free glass. The focal lengths of the components satisfy the following conditions: f ′ ₁ / f ′ = 1.1 ÷ 1.3; f ′ ₂ / f ′ = - 0.69 ÷ -0.74; f ′ ₃ / f ′ = 0.84 ÷ 0.98; f ′ ₄ / f ′ = 0.93 ÷ 1.6, where f ′ ₁ , f ′ ₂ , f ′ ₃ , f ′ ₄ are the focal lengths of the first, second, third and fourth components, respectively, f ′ is the equivalent focal length the lens.

Focusing the lens at a finite distance and compensating for the displacement of the image plane in the range of operating temperatures from -40 ° C to + 50 ° C is achieved by performing a negative lens moving along the optical axis.

The specified lens has a high relative aperture of 1: 1 and good image quality at a temperature of 20 ° C. However, in the temperature range from -40 ° C to + 50 ° C, the image contrast is significantly reduced. In addition, the lens at a focal length of 100 mm has a large length along the optical axis equal to 145 mm, i.e. L / f ′ = 1.45, where L is the lens length, f ′ is the equivalent focal length of the lens. In addition, focusing the lens at a finite distance and compensating for the displacement of the image plane in the operating temperature range leads to a change in focal length, which, in turn, will lead to a change in the scale of the electronic scale of the sight lens.

The closest to the claimed technical solution - the prototype - is a lens for the infrared region of the spectrum (RF patent No. 1155514, IPC G02B 13/14, January 11, 2012), which contains four menisci, of which the second is negative, the rest are positive, the first and the fourth menisci are made of germanium and face the image plane with their concave surfaces, the second and third menisci are made of zinc selenide and face the image plane with their convex surfaces, the third and fourth menisci are installed with the possibility of one belt movable along the optical axis between the relative optical powers menisci following relations hold: φ _1: φ _2: φ _3: φ ₄ = (0,75 ÷ 0,85) :-( 2,0 ÷ 2,5): - (1.40 ÷ 1.66) :( 1.7 ÷ 1.9), where φ ₁ , φ ₂ , φ ₃ , φ ₄ are the relative optical powers of the first, second, third, and fourth menisci, respectively.

For comparison, the specified lens is counted at a focal length of 100 mm. With a relative aperture of 1: 1.3, the lens has good image contrast over the entire temperature range from -40 ° C to + 50 ° C.

The lens has the following disadvantages.

1. The lens has a large relative length equal to L / f ′ = 1.32, which increases its mass. The length of the lens along the optical axis at a focal length of 100 mm is 132 mm.

2. With thermal compensation in the range of operating temperatures from -40 ° С to + 50 ° С and focusing to a finite distance by the joint movement of the third and fourth menisci, a significant change in the focal length occurs. Since the movable component has a positive optical power, the change in the focal length of the lens is 1.19 mm (1.7%).

The objective of the invention is to create a telephoto lens with the following technical results: reducing the length of the telephoto lens and its mass while maintaining a high image contrast in the temperature range from -40 ° C to + 50 ° C, ensuring constant focal length.

The specified technical results are achieved as follows. The telephoto lens for the infrared region of the spectrum, like the prototype, contains four menisci, of which the first and fourth along the meniscus beam are positive facing concave surfaces to the image plane, the first meniscus made of germanium, and the second and third of zinc selenide. In contrast to the prototype in the inventive telephoto lens the following is performed. The second meniscus is positive and has a concave surface facing the image plane, the third meniscus is negative and has a concave surface facing the image plane, and the fourth meniscus is made of zinc selenide. The third meniscus is mounted to move along the optical axis. The following relationships are true:

φ ₁ : φ ₂ : φ ₃ : φ ₄ = (1.15 ÷ 1.27) :( 0.9 ÷ 1.15) :-( 2.8 ÷ 3.8) :( 1.5 ÷ 2, 7)

where φ ₁ , φ ₂ , φ ₃ , φ ₄ are the relative optical powers of the first, second, third and fourth menisci, respectively;

D2 / f ′ = 0.15 ÷ 0.35;

D6 / f ′ = 0.25 ÷ 0.45;

where D2 is the air gap between the first and second menisci;

D6 - air gap between the third and fourth menisci;

f ′ is the equivalent focal length of a telephoto lens.

An example of a specific implementation of a telephoto lens is shown in the drawings.

In FIG. 1 is an optical diagram of a telephoto lens.

In FIG. Figure 2 shows the point scattering function.

In FIG. Figure 3 shows the contrast of the image and the function of energy concentration during the operation of the telephoto lens over the entire temperature range.

The infrared telephoto lens (Fig. 1) contains four menisci. Meniscus 1 - positive, made of Germany. Meniscus 2 - positive, made of zinc selenide (ZnSe). Meniscus 3 - negative, made of zinc selenide. Meniscus 4 - positive, made of zinc selenide. All menisci face a concave surface to the image plane 5. Meniscus 3 is arranged to move along the optical axis to focus the lens at a finite distance and compensate for the displacement of the image plane in the operating temperature range from -40 ° C to + 50 ° C, while the focal length changes within 0.02 mm, i.e. virtually constantly.

Table 1 shows the characteristics of a telephoto lens, considered as an example of the implementation of the inventive telephoto lens. The entrance pupil of the telephoto lens is located on its first surface.

Table 2 shows the optical characteristics of a telephoto lens.

Telephoto lens calculations were performed under the assumption that the lens body is made of aluminum, and the refractive indices for germanium and zinc selenide are taken from the ZEMAX database (″ INFRARED ″ catalog). When using domestic standards (for germanium RTM3-1640-83, and for zinc selenide TU AH 23-83), to obtain the achieved image quality, it is necessary to move the negative meniscus 3 along the axis within ± 0.01 mm.

The lens design elements shown in Table 1 provide the following relationships between the relative optical powers φ ₁ , φ ₂ , φ ₃ , φ _{4 of} menisci 1-4: φ ₁ : φ ₂ : φ ₃ : φ ₄ = 1.189: 1.02: -3.43: 2.26,

where φ ₁ = f ′ / f ′ ₁ , φ ₂ = f ′ / f ′ ₂ , φ ₃ = f ′ / f ′ ₃ , φ ₄ = f ′ / f ′ ₄ ;

f ′ is the equivalent focal length of the entire telephoto lens;

f ′ ₁ , f ′ ₂ , f ′ ₃ , f ′ ₄ are the focal lengths of menisci 1, 2, 3, and 4, respectively. Moreover, the air gaps between menisci D2 and D6 are equal to:

D2 = 0.28 f ′; D6 = 0.318 f ′.

The telephoto lens works as follows. Beams of rays from an object (an axial beam of rays is shown) sequentially pass through menisci 1, 2, 3, 4 and build an image in the image plane 5. To obtain a high quality image in the temperature range, the meniscus 3 moves along the axis along arrows A and B, while the values of segments D4 and D6 change. The same movement is used to focus the telephoto lens to a finite distance.

The main idea of constructive implementation of the telephoto lens is that the fourth meniscus is made of zinc selenide, while the optical power of the negative meniscus 3 has increased, which translated the proposed lens into the category of telephoto lenses, because its length does not exceed its focal length.

Changes in the optical forces of the menisci φ ₁ , φ ₂ , φ ₃ , φ ₄ in the ranges, respectively (1.15 ÷ 1.27) :( 0.9 ÷ 1.15) :-( 2.8 ÷ 3.8) :( 1.5 ÷ 2.7) is accompanied by a change in the air gaps D2 and D6 in the intervals: D2 = (0.15 ÷ 0.35) f ′, D6 = (0.25 ÷ 0.45) f ′.

Let us consider the characteristics of the image quality of a telephoto lens, namely the point scattering function (PSF), which clearly demonstrates the topology of scattering spots in the geometric approximation (Fig. 2), image contrast (TSC) and the energy concentration function (PCE), which allows to determine the diffraction scattering circle (Fig. . 3).

In FIG. 2 in the first column gives the topology of the scattering circles for 20 ° C, in the second column for minus 40 ° C, and in the third for 50 ° C. The first line contains scattering circles for the axial point of the field of view, the second for the zone, and the third for the edge of the field of view, i.e. 6.3 ° × 4.7 ° diagonal. The square size is 100 microns. In addition, a diffraction (non-aberrational) Airy circle, 32.3 μm in diameter for a relative aperture of 1: 1.25, is imprinted on each scattering spot. This circle contains 83.4% of energy. As can be seen from FIG. 2, all scattering spots practically fit into the Airy circle, which clearly demonstrates the high image quality in the geometric approximation.

In FIG. 3. On the left is the image contrast at a frequency of 20 mm ^-1 , and on the right is a function of energy concentration for the entire temperature range. The temperature values are printed in the field of the corresponding graphs. According to the Nyquist criterion for the expected scattering circles of 0.025 mm, the image contrast at a frequency of 20 mm ^-1 should be at least 0.6, which follows from FIG. 3. A detailed examination of the FEC graphs made it possible to determine the diameter of the scattering circles at 80% energy concentration, the values of which are imprinted in the upper part of the squares in FIG. 2. An examination of these values allows us to conclude that the proposed telephoto lens has diffractive image quality with its length equal to the focal length, i.e. at L = f ′ = 100 mm.

Due to the reduction in the length of the telephoto lens, the length of its body and, consequently, the mass of the telephoto lens are accordingly reduced.

In the prototype for the athermalization of the lens and its focus on a finite distance, movement along the axis of two positive menisci is used. The change in focal length is 1.7%. In the invention, a meniscus 3 having negative optical power moves along the axis. This helps prevent a change in focal length when moving it. The change in focal length is 0.02%, i.e. focal length is almost constant.

At a temperature of 50 ° C, meniscus 3 moves 0.15 mm to the left along arrow A (Fig. 1), and at minus 40 ° C - to 0.33 mm to the right along arrow B. When focusing a telephoto lens to a final distance of 15 m in normal climatic conditions, i.e. at 20 ° C, meniscus 3 moves 0.21 mm to the right.

The calculations show that the claimed technical results are achieved with the ratios:

φ ₁ : φ ₂ : φ ₃ : φ ₄ = (1.15 ÷ 1.27) :( 0.9 ÷ 1.15) :-( 2.8 ÷ 3.8) :( 1.5 ÷ 2, 7);

D2 / f ′ = (0.15 ÷ 0.35);

D6 / f ′ = (0.25 ÷ 0.45).

Thus, the invention compared with the prototype allows to reduce the longitudinal dimensions and weight of the telephoto lens while maintaining a high contrast image and to ensure the invariability of the focal length in the temperature range from -40 ° C to + 50 ° C.

Claims (1)

A telephoto lens for the infrared region of the spectrum containing four menisci, of which the first and fourth along the meniscus ray are positive, with their concave surfaces facing the image plane, the first meniscus made of germanium and the second and third of zinc selenide, characterized in that the second meniscus is made positive and faces with a concave surface to the image plane, the third meniscus is made negative, faces with a concave surface and an image plane and is mounted with the ability to move along the optical and a fourth meniscus formed of zinc selenide, wherein the following relations:
φ ₁ : φ ₂ : φ ₃ : φ ₄ = (1.15 ÷ 1.27): (0.9 ÷ 1.15): - (2.8 ÷ 3.8): (1.5 ÷ 2, 7)
where φ ₁ , φ ₂ , φ ₃ , φ ₄ are the relative optical forces of the first, second, third and fourth menisci, respectively;
D2 / f ′ = 0.15 ÷ 0.35;
D6 / f ′ = 0.25 ÷ 0.45;
where D2 is the air gap between the first and second menisci;
D6 - air gap between the third and fourth menisci;
f ′ is the equivalent focal length of a telephoto lens.

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