RU2604112C2 - Objective lens for infrared spectrum - Google Patents

Abstract

FIELD: electronics.

SUBSTANCE: invention can be used in thermal imagers sensitive within the spectral range from 8 to 12 µm. Lens has four menisci, from which first, second and fourth menisci on beam path are positive, and third one is negative. All meniscus by concave surfaces face image plane. First and fourth menisci are made from germanium, and second and third are from zinc selenide. Third meniscus is made movable along the optical axis. Following relationships are satisfied: φ₁:φ₂:φ₃:φ₄=(0.70÷0.78):(0.18÷0.57):-(1.0÷1.78):(1.9÷2.5), where φ₁, φ₂, φ₃, φ₄ are relative optical powers of first, second, third and fourth menisci, respectively; D2/f′_e=0.6÷0.7, where D2 is air gap between first and second menisci, f′_e is lens equivalent focal distance.

EFFECT: technical result is increase in manufacture ability, image quality with lens extended production and assembly tolerances, invariance of lens focal distance with lens thermal compensation in temperature range from minus 40 °C to +50 °C.

1 cl, 4 dwg, 2 tbl

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.

Known lens for the infrared region of the spectrum according to the patent of the Russian Federation No. 140705 from 12/11/2013, IPC G02B 13/14. The lens contains four menisci, of which the first and fourth menisci along the ray are positive, with their concave surfaces facing the image plane, and the second is negative, the first and fourth menisci being made of germanium, and the third of zinc selenide, characterized in that the second meniscus is made of germanium and faces with a concave surface to the image plane, the third meniscus is negative and faces with a concave surface to the image plane, and the following Relations:

φ ₁ : φ ₂ : φ ₃ : φ ₄ = (1.07 ÷ 1.31) :-( 0.57 ÷ 1.5) :-( 1.41 ÷ 0.68) :( 1.31 ÷ 2 , 08),

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

Focusing the lens to a finite distance and compensating for the displacement of the image plane in the operating temperature range from minus 40 ° C to 50 ° C is achieved when moving along the optical axis either the entire lens, or the second and third menisci together, or the fourth meniscus.

The specified lens is not technologically advanced, because It has tight tolerances and, therefore, is suitable for production in small batches. In addition, focusing the lens at a finite distance and compensating for the displacement of the image plane in the range of operating temperatures leads to a change in focal length, which, in turn, will lead to a change in the scale of the scale.

Closest to the claimed technical solution - the prototype - is the lens for the infrared region of the spectrum according to the patent of the Russian Federation No. 115514 of 01/11/2012, IPC G02B 13/14. The lens contains four menisci, of which the second is negative, the rest are positive, the first and 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 the fourth menisci are installed with the possibility of simultaneous movement along the optical axis, between the relative optical forces of the menisci, the 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 the} relative optical forces of the first, second, third and fourth menisci, respectively.

For comparison, the specified lens is calculated at a focal length of 75 mm. The lens has the following disadvantages.

1. To maintain the design characteristics of the lens, it is necessary to manufacture optical parts and the lens as a whole (relative position of the lens components) with high accuracy. Image quality is ensured within small tolerances for meniscus manufacturing and lens assembly. The equivalent focal length of the moving menisci is ~ 22 mm. To prevent the line of sight from moving away by one angular minute when the image is focused at a final distance of 15 m, the decentration of the moving menisci should not exceed 0.02 mm.

2. With thermal compensation in the operating temperature range from minus 40 ° C to 50 ° C and focusing on a finite distance by the joint movement of the third and fourth menisci, a significant change in the focal length occurs, which leads to a change in image scale to unacceptable values for lenses with high requirements for focal constancy distances in the operating temperature range. Since the movable component has a positive optical power, the change in the focal length of the lens is 1.19 mm.

3. The joint movement of the two components of the lens is a technological challenge.

The objective of the invention is to achieve the following technical results, and without increasing the mass and dimensions of the lens: increasing the manufacturability of the lens by expanding the manufacturing tolerances of optical parts and assembling the lens, improving image quality with extended manufacturing and assembly tolerances of the lens, as well as ensuring the focal length of the lens remains constant during thermal compensation lens in the range of working temperatures.

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

φ ₁ : φ ₂ : φ ₃ : φ ₄ = (0.70 ÷ 0.78) :( 0.18 ÷ 0.57) :-( 1.0 ÷ 1.78) :( 1.9 ÷ 2, 5),

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 ′ _e = 0.6 ÷ 0.7,

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

f ′ _e is the equivalent focal length of the lens.

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

In FIG. Figure 1 shows an optical diagram of a lens with real ray paths for the axial and field points of the field of view.

In FIG. Figure 2 shows the point spread function (PSF).

In FIG. Figure 3 shows the contrast of the image and the function of energy concentration (FFE).

In FIG. Figure 4 shows a graph of the energy concentration function (FFE) under the influence of tolerances by the Monte Carlo method.

The lens for the infrared region of the spectrum (Fig. 1) contains four menisci located in the housing (not shown in the figure). 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 Germany. All menisci face a concave surface to the image plane. The 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 minus 40 ° C to 50 ° C.

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

Optical characteristics of the lens:

1. Focal length 75 mm 2. Relative bore 1: 1.25 3. Field of view 7.3 ° × 5.5 ° 4. Spectral range 8-12 microns 5. Lens Length 99.4 mm 6. The back section 13.02 mm

The lens body can be made of steel, aluminum and other materials. The calculation of the lens was carried out taking into account the use of aluminum, which has a higher coefficient of linear expansion compared to steel. The design of the lens housing of aluminum is preferable to provide a small mass of the lens.

The design elements of the lens, shown in table 1, provide the following relationships:

φ ₁ : φ ₂ : φ ₃ : φ ₄ = 0.746: 0.493: (- 1.548): 2.186;

D2 / f ′ _e = 0.645;

where φ ₁ , φ ₂ , φ ₃ , φ ₄ are the relative optical forces of the menisci 1, 2, 3 and 4, respectively;

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

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

f ′ ₁ , f ′ ₂ , f ′ ₃ , f ′ ₄ are the focal lengths of menisci 1, 2, 3, and 4, respectively;

D2 - air gap between menisci 1 and 2.

The lens works as follows. Beams of rays from an object sequentially pass through menisci 1, 2, 3, 4 and converge in the image plane 5. In the temperature range, to obtain a high quality image, meniscus 3 moves along the axis, while the distances between menisci 2 and 3, 3 and 4 change. those. respectively, segments D4, D6. The same movement is used to focus the lens at a finite distance.

The main idea of the constructive implementation of the lens is to reduce the optical power of the lens elements.

The lens thermalization and its focusing on a finite distance in the present invention is carried out in a manner different from the prototype. In the prototype for this purpose, movement along the axis of two positive lenses is used, the equivalent optical power of which is large. In the present invention, the movable lens 3 has an optical power (in absolute value) two times less. This, on the one hand, makes it possible to double the allowance for its decentration with the retraction of the line of sight of not more than one angular minute, and on the other hand, to prevent a change in focal length when moving it. This is also due to the fact that thermal compensation and focusing of the lens to a finite distance in the inventive lens is a negative component, and not positive, as in the prototype. The change in focal length is 0.025%, i.e. focal length is almost constant. The prototype has a change in focal length of 1.7%.

At a temperature of 50 ° C, meniscus 3 moves 0.34 mm to the left, and at minus 40 ° C - 0.68 mm to the right. When focusing the lens at a final distance of 15 m under normal climatic conditions, i.e. at 20 ° C, meniscus 3 moves 0.42 mm to the right, i.e. Included in the range of shifts during thermal compensation.

The values of the air gap D2 influence the lens parameters in two ways. With an increase in this gap, the image quality of the lens decreases, and with a decrease, the light diameters of the lenses and, accordingly, their frames increase, which leads to a deterioration in the overall dimensions of the lens.

With an increase in the air gap D2, the curvature of the concave surface of the meniscus 2 increases and the curvature of the convex surface of the meniscus 3 decreases, which leads to a decrease in the air gap between the edges of these menisci. This, in turn, can lead to the contact of the frames of these menisci with temperature compensation in the operating temperature range from minus 40 ° C to 50 ° C. In addition, there is an increase in the length of the lens by an amount equal to the increase in the air gap D2. When the air gap D2 decreases, the opposite effect takes place, but at the same time, the light diameters of menisci 2 and 3 and, consequently, the mass of the lens increase.

High image quality of the lens is provided in the range D2 / f ′ _e = 0.6 ÷ 0.7. The most optimal value of the air gap D2 is the value D2 = 0.645f ′ _e .

Let us consider the characteristics of the image quality of the lens, namely the point scattering function (PSF), which clearly demonstrates the topology of scattering spots in the geometric approximation, image contrast (PTF) and the function of energy concentration (PCE), which makes it possible to determine the diffraction scattering circle.

The point spread function is shown in FIG. 2. The first column gives the topology of the scattering circles for 20 ° C, the second for minus 40 ° C, and the third for 50 ° C. The first line contains the 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. The square size is 100 microns. In addition, a diffraction (non-aberrational) Airy circle with a diameter of 33.5 μm is imprinted on each scattering spot. This circle contains 83.4% of energy.

As can be seen from FIG. 2, all scattering spots fit into the Airy circle, which clearly demonstrates the high image quality in the geometric approximation. In the prototype, scattering circles extend beyond the Airy disk, due to the presence of higher-order aberrations due to the steep incidence of rays on the surface of the lens relative to the surface normal.

For a more reliable assessment of image quality, the image contrast value and the energy concentration function are used. In FIG. Figure 3 on the left shows the contrast of the image at a frequency of 20 mm ^-1 , and on the right - the 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% of the energy concentration, the values of which are imprinted, in the upper part of the squares in FIG. 2. Consideration of these values allows us to conclude that the presented lens has diffractive image quality, and the image quality is 10-15% higher than that of the prototype.

The stability of the image quality of the lens depends on tolerances for the manufacture of optical parts and their assembly. The wider the tolerances, the lower the cost and higher quality of the finished products, especially in mass production.

Consider the sensitivity of the lens by the Monte Carlo method used in the analysis of tolerances in industrial production. Monte Carlo analysis models the effect of the simultaneous effects of tolerances on the entire system. For each Monte Carlo cycle, all parameters for which tolerances are set vary randomly in accordance with the law of normal distribution with a full width equal to four standard deviations.

Consider the FKE of the image when introducing tolerances on the lens for the axial point of the field of view. The tolerances for the manufacture of lenses and lens assembly are presented in table 2, and the FKE graph corresponding to these tolerances, in FIG. four.

As can be seen from the graph, the drop in the photomultiplier for twenty Monte Carlo cycles is insignificant, and this is with the tolerance range for the proposed lens 2 ÷ 5 times larger than that of the prototype.

The calculations show that the claimed technical results are achieved when the ratios of the optical forces of menisci 1-4 are equal to φ ₁ : φ ₂ : φ ₃ : φ ₄ = (0.70 ÷ 0.78) :( 0.18 ÷ 0.57) :-( 1.0 ÷ 1.78) :( 1.9 ÷ 2.5).

Thus, in comparison with the prototype, the present invention allows to increase the manufacturability of the lens by expanding the tolerances for the manufacture of optical parts and assembling the lens, to improve image quality with extended tolerances for manufacturing and assembling the lens, as well as to ensure the invariable focal length of the lens during thermal compensation of the lens in the operating temperature range from minus 40 ° C to 50 ° C. These results are achieved without increasing the mass and dimensions of the lens.

Claims (1)

A lens for the infrared region of the spectrum containing four menisci, of which the first and fourth menisci along the beam are positive, with their concave surfaces facing the image plane, the first and fourth menisci 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 negative, faces with a concave surface and the image plane and is mounted with the possibility of moving along the opt axis, while the following relationships are true:
φ ₁ : φ ₂ : φ ₃ : φ ₄ = (0.70 ÷ 0.78) :( 0.18 ÷ 0.57) :-( 1.0 ÷ 1.78) :( 1.9 ÷ 2, 5);
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 ′ _e = 0.6 ÷ 0.7;
where D2 is the air gap between the first and second menisci;
f ′ _e is the equivalent focal length of the lens.

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