RU2578661C1 - Infrared lens with smoothly varying focal distance - Google Patents

Abstract

FIELD: optics.

SUBSTANCE: lens comprises five components. First component comprises a convex-concave meniscus made from silicon and a biconcave lens from fluorite. Second component is biconcave lens made of silicon. Third component comprises a convex-concave positive meniscus made from silicon and negative convex-concave meniscus made from fluorite. Fourth - a positive convex-concave meniscus from silicon. Fifth component includes doubly-concave lens from fluorite and doubly-convex lens from silicon. First surface of the lens of the second component, the first surface of the first lens of the third component and the second surface of the second lens of the fifth component are tapered. Second and fourth components and the second lens of the fifth component are movable. Fourth component at changing the focal distance of the lens in the whole range of its values moves at first in one direction with movement of the second component, and then in reverse. Relative optical power of the second and fourth components satisfy conditions given in the patent claim.

EFFECT: wider range of changing the focal distance lens with simultaneous reduction of relative length of the lens and longitudinal movements of movable elements, high spectral transmission and thermal stability of the lens.

1 cl, 6 dwg, 2 tbl

Description

The invention relates to the field of infrared optics and can be used in thermal imagers with photodetectors made in the form of a microbolometric matrix of sensitive elements. The spectral range of the lens is 3.7-4.8 microns.

A well-known infrared lens operating in the spectral region of 3.7-4.8 μm [Design of a Dual Field-of-View Optical System for Infra-Red Focal-Plane Arrays, SPIE, vol. 4767, p. 13-23. FIG. 2, 3]. The lens contains a sequentially mounted convex-concave meniscus, a movable negative lens, a positive lens and a three-lens image transfer optics. As a photodetector, a device containing a protective glass, a cooled aperture diaphragm and a rectangular matrix of sensitive elements is used. The image transfer optics serves for optical conjugation of the aperture diaphragm with the first lens along the beam to reduce its light diameter. At the same time, image transfer optics are used to athermalize the lens when operating in various temperature conditions due to its small (± 1 mm) movement along the optical axis, and also to focus the lens at a final distance of up to 25 m. For a 3.5-fold change in the focal length, the negative lens is discrete moves along the optical axis by 30 mm with a change in focal length f from 220 to 63 mm. The lens has a high relative aperture (1: 2.75) and a small length l (200 mm), i.e. l / f ratio = 0.91.

The lens has the following disadvantages.

1. Discreteness of the change of field of view, in which there is a short-term overlap of the field of view.

2. A small difference in magnifications (3.5 times).

3. The technological complexity of manufacturing, and therefore the cost of the lens due to the presence of diffractive aspherical surfaces.

The closest in technical essence - the prototype - is an infrared lens with a continuously variable focal length according to the patent of the Russian Federation No. 2310217 from 01/10/2006, IPC G02B 13/14, G02B 15/16. The lens contains a fixed first component sequentially located along the optical axis, consisting of a positive convex-concave lens, a second negative convex-concave lens, a third negative biconcave lens, a fourth positive biconvex lens and a fifth positive convex-concave lens, movable along the optical axis of the second component, consisting of a negative convex-concave lens, the third component moving along the optical axis, consisting of a negative concave-convex lens, is not a movable fourth component consisting of a positive concave-convex lens, and a stationary fifth component consisting of a first positive convex-concave lens and a second positive convex-concave lens. The second surface of the first lens of the fifth component is made aspherical.

Like the proposed lens, the specified lens contains five sequentially located optically coupled components. The first lens of the first component is a convex-concave meniscus. The second component is mounted to move along the optical axis.

The specified lens has the following disadvantages.

1. Insufficient magnification of 8.5 times.

2. A large relative length of the lens, equal to 1: 1.6.

3. A large number of lenses (10 lenses), which reduces the transmission of the lens.

4. The lens provides good image quality when operating in normal climatic conditions (+ 20 ° C). But when working in the temperature range from minus 50 ° C to 50 ° C, the lens gives a great defocusing of the image, especially when using germanium lenses as the material. Germanium has a significant change in the refractive index of temperature, which leads to a significant decrease in image quality.

5. One of the movable lenses has a large range of motion (310 mm). This leads to a significant loss of the line of sight, especially when the guides are curved when exposed to temperature factors.

The technical result of the invention is to increase the interval for changing the focal length of the lens (magnification factor) while reducing the relative length of the lens and the longitudinal movements of the moving elements, increasing the spectral transmittance and increasing the thermal stability of the lens (athermalization) - preserving image quality in the temperature range from minus 50 ° C to 50 ° C

The specified technical result is achieved as follows. An infrared lens with a smoothly changing focal length, like the prototype, contains five optically connected components in series, the first lens of the first component made in the form of a convex-concave meniscus, and the second component mounted for movement along the optical axis. In contrast to the prototype, the following was performed in the lens: the first component contains two lenses, of which the first lens is made of silicon, and the second is a biconcave lens made of fluorite, the second component is a biconcave negative lens mounted with the possibility of movement along the optical axis, made of silicon, the first surface of which is conical, with a conical constant ranging from (-9) to (-13), the third component contains a convex-concave positive meniscus made of silicon, the first surface of which go - conical, with a conical constant ranging from (-4) to (-7), and a negative convex-concave meniscus made of fluorite, the fourth component is a convex-concave meniscus made of silicon, mounted for movement along the optical axis in the process of changing the focal length of the lens in the entire range of focal length values, first in one direction with the second component moving, and then in the opposite direction, the fifth component contains a biconcave negative lens made of fluorite and a biconcave bent positive lens made of silicon and mounted to move along the optical axis regardless of the movement of the second and fourth components, while the second surface of the second lens of the fifth component is made conical with a conical constant in the range from (-4) to (-8), and the relative optical powers of the second and fourth components satisfy the following conditions:

, $$\frac{f_{II}}{f_{\max }}=-\left(0.03÷0.04\right)$$

,

, $$\frac{f_{IV}}{f_{\max }}=0.15÷0.20$$

,

where: f _II is the focal length of the second component;

f _IV is the focal length of the fourth component;

f _max - the maximum value of the focal length of the lens.

Giving the second and fourth movable components a certain optical power and arranging a fixed two-lens third component between them makes it possible to reduce the longitudinal dimension of the lens to 175 mm at a focal length of 300 mm, and also to reduce the range of linear movement of the moving components to 30 mm.

The conical shape of the three surfaces of the lenses made of non-toxic material (silicon) allows for good image quality with a reduced number of lenses and a difference in focal lengths of 10 times

The choice of the two-lens design of the fifth component (image transfer optics) with a significant gap between them provides, due to the longitudinal movement of the second positive lens of this component, temperature compensation of image defocusing (athermalization) in the range of ± 50 ° C when the lens is located in an aluminum body. This was also facilitated by the exclusion of germanium from the optical scheme of the lens and the use of three lenses made of fluorite. In addition, the use of silicon in combination with fluorite can correct chromatic aberrations.

The image transfer optics allows optical coupling of the cooled diaphragm of the photodetector with the frame of the first lens of the lens, thereby minimizing its light diameter in a narrow field of view, which reduces the weight of the lens and eliminates the appearance of undesirable shadow effects (vignetting or cutting off of field beams of rays).

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

In FIG. 1 is an optical diagram of a lens with a smoothly changing focal length from 300 mm to 30 mm with a lens arrangement for a focal length of 300 mm.

In FIG. Figure 2 shows the movement graphs of the second and fourth components with a change in air gaps d4 and d10, with a change in focal length.

In FIG. Figure 3 shows the process of changing the course of the rays when changing the focal length from 300 mm to 30 mm for various values of focal lengths (configurations).

In FIG. 4 in the upper left corner are the scattering circles, i.e. dot scattering function in five configurations, and in the remaining sections - image contrast.

In FIG. Figure 5 shows scattering circles when the lens operates in a temperature range of ± 50 ° C.

In FIG. Figure 6 shows scattering circles without lens athermalization (lens 8 is stationary).

The lens contains sequentially mounted and optically conjugated first component I, made in the form of a convex-concave meniscus 1, and a biconcave lens 2, the second component II, made in the form of a biconcave lens 2, the third component III, containing a convex-concave positive meniscus 4 and a negative convex -concave meniscus 5, the fourth component IV, made in the form of a positive convex-concave meniscus 6, and the fifth component V, made in the form of a negative biconcave lens 7 and a positive biconvex lens 8. In the rear section of the lens is a photodetector 9, comprising a protective glass 10, a cooled aperture diaphragm 11, and a photosensitive matrix 12. Between the fourth and fifth components, there is a plane of the intermediate image 13.

Component II (lens 3) is mounted with the ability to move along the optical axis, and component IV (lens 6) is mounted with the ability to move in the process of changing the focal length of the lens in the entire range of focal lengths, first in one direction with the movement of component II, and then in the opposite. The lens 8 of the component V is mounted to move along the optical axis regardless of the movement of components II and IV. To obtain a high image contrast and reduce the length of the lens, the first surface of the lens 3, the first surface of the lens 6 and the second surface of the lens 8 are made conical.

The optical characteristics of the lens have the following meanings.

1. The range of the focal length, mm 300 ÷ 30 2. Linear field of view (diagonal), mm 13.6 3. The increase, krat 10 4. Relative hole 1: 4 5. The spectral range, microns 3.7 ÷ 4.8 6. Length, mm 175 7. Number of lenses 8 8. The number of conical surfaces 3 9. Temperature range of work ± 50 ° C

The design parameters of the inventive lens with the location of the lenses for a focal length of 300 mm are presented in table 1, which gives the radii of curvature "r" of the surfaces, thicknesses and air gaps "d", as well as the material from which the lenses are made.

The relative optical powers of components II and IV are respectively equal: (-0.0383) and 0.17 excluding image wrapping, i.e. with f _max = 300 mm.

Moving the lenses 3, 6 serves to change the focal length of the lens, and moving the lens 8 - for its athermalization.

An exception to the optical germanium scheme allowed us to reduce the thermo-optical aberrations of the lens, and due to the independent movement of the lens 8 of the fifth component, completely compensate for the defocusing of the image caused by a change in ambient temperature when using the lens body made of a common material - aluminum, and the use of silicon in combination with fluorite - correct chromatic aberration.

The lens works as follows: a parallel beam of infrared rays passes through all the lenses of the lens, including the plane of the intermediate image 13, refracted on each surface in accordance with the radii of curvature of each surface and lens materials, and focuses in the plane of the photosensitive matrix 12. The diagonal of the rectangular photosensitive matrix 12 located in the image plane is 13.6 mm.

The main beam of an inclined beam of rays (in reverse), passing through the center of the aperture diaphragm 11, goes approximately to the center of the light diameter of the meniscus 1, which ensures its minimum dimensions.

The diameter of the beam of rays is determined by the diameter of the aperture diaphragm, which is a cooled diaphragm 11 with a diameter of 5.1 mm. In this photodetector 9, the configuration of the parts is such that the photosensitive matrix 12 is in the air gap d20 = 20.47 mm from the diaphragm 11. Therefore, the distance from the rear surface of the lens 8 to the photosensitive matrix 12 (back segment) must be at least 30 mm the gap value d17, since the lens 8 is movable. The large rear segment is a factor limiting the achievement of higher lens characteristics, such as its length and image contrast.

Components II, III, and IV are a zoom lens group, with which you can change the focal length of the lens. To this end, components II and IV move along the optical axis according to a certain law with a change in variable air gaps d4, d6 and d10, d12, with the component III stationary. The values of these gaps for the five values of the focal length of the lens f are shown in Table 2, and the process of change is shown in FIG. 2.

As can be seen from FIG. 2, component II (the curve from above) moves according to a hyperbolic law, and component IV makes a reciprocating motion during the period of changing the focal length from 300 mm to 30 mm.

The process of changing the path of the rays when changing the focal length from 300 to 30 mm is shown in FIG. 3 for the following focal lengths (configurations): 300 mm, 200 mm, 130 mm, 70 mm and 30 mm. The corresponding focal length of the lens is imprinted in the field of each configuration.

In the upper left corner, there are graphs of astigmatism and distortion equal to 5% for an average focal length of 130 mm.

From table 2 it is seen that the ratio of the maximum value of the focal length f to the minimum is 10 times.

. По сравнению с прототипом относительная длина объектива (0.58) в 2.75 раза меньше (у прототипа 1.6). Диапазон линейного перемещения подвижных компонентов сокращен до 30 мм, что в 10 раз меньше, чем в прототипе.With the claimed design, the lens length l (distance from the first surface to the image plane) is 175 mm and does not exceed the maximum focal length more than 0.58 times $$(\frac{l}{f_{\max }}=0.58)$$

. Compared with the prototype, the relative length of the lens (0.58) is 2.75 times less (the prototype 1.6). The range of linear movement of the moving components is reduced to 30 mm, which is 10 times less than in the prototype.

In FIG. 4 in the upper left corner are given scattering circles, i.e. point spread function (PSF) for the five above configurations. The first line is for the axis, the second is for zone (0.7) and the third is for the edge of the field of view. In the remaining sections, the contrast of the image at a frequency of 20 strokes per millimeter is also given for five configurations. As can be seen from the figure, the scattering spots fit into the Erie circle, imprinted on each spot, amounting to 42.5 μm, and the image contrast is constant for all configurations and approaches the diffraction limit.

In FIG. Figure 5 shows the scattering circles when the lens operates in a temperature range of ± 50 ° C, while the side of the square is 200 μm. On the left, for comparison, scattering circles are given in the range of focal lengths from 300 to 30 mm when the lens operates at + 20 ° C. In the most severe mode, i.e. at a focal length of 300 mm, the lens 8 of component V moves 1.25 mm along the axis to the right (towards the photodetector 9) for a temperature of minus 50 ° C and 0.5 mm to the left for a temperature of + 50 ° C due to a change in segments d15 and d17. The calculations are based on the fact that the lens body is made of aluminum.

In FIG. Figure 6 shows scattering circles without lens athermalization (lens 8 is stationary), while the side of the square is 1000 μm. As can be seen from the figure, there is a complete degradation of image quality.

Active lens thermalization with an independent optical element (lens 8) has the following advantages. Usually athermalization of pan-optical lenses is carried out by installing temperature sensors on the lens body, the signals from which enter the processor to adjust the control of the law of motion of the moving zoom lenses with temperature. This, on the one hand, complicates the electronic system of the lens, and on the other hand, it does not allow a sufficient degree of accuracy to combine the image plane with the photosensitive matrix, especially at high relative apertures of the lens and with the accuracy of the zoom lens movement of the order of 10 μm. This is due to the fact that the configuration features of the lens body and lens mount frames, as well as fluctuations in the temperature environment, do not allow creating an accurate mathematical model for adjusting the law of motion of moving components.

On the other hand, the operator’s work in real time allows a simple way to eliminate these disadvantages by independently controlling the position of the lens 8 on the optical axis. The same movement can be used to focus the lens at a finite distance.

Thus, the present invention allows to increase the interval of variation of the focal length of the lens (magnification factor) while reducing the relative length of the lens and the longitudinal displacements of the moving elements, to increase the spectral transmission and thermal stability of the lens in the temperature range of ± 50 ° C.

Claims (1)

An infrared lens with a smoothly changing focal length containing five optically connected components in series, the first lens of the first component made in the form of a convex-concave meniscus, and the second component mounted for movement along the optical axis, characterized in that the first component contains two lenses, of which the first lens in the form of a convex-concave meniscus is made of silicon, and the second is a biconcave lens made of fluorite, the second component is a biconcave a solid lens made of silicon, the first surface of which is conical, with a conical constant ranging from minus 9 to minus 13, the third component contains a convex-concave positive meniscus made of silicon, the first surface of which is conical, with a conical constant ranging from minus 4 to minus 7, and a negative convex-concave meniscus made of fluorite, the fourth component is a convex-concave meniscus made of silicon, mounted to move along the optical axis in the process of changing the focal length of the lens in the entire range of focal length values, first in one direction with the second component moving, and then in the opposite direction, and the fifth component contains a biconcave negative lens made of fluorite and a biconvex positive lens made of silicon, the second surface of which is conical, with a conical constant ranging from minus 4 to minus 8, installed with the ability to move along the optical axis regardless of the movement of the second and fourth components, and about in relative optical power of the second and fourth components satisfy the following conditions:

Where:
f _II is the focal length of the second component;
f _IV is the focal length of the fourth component;
f _max - the maximum value of the focal length of the lens.

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