RU193226U1 - ATHERMALIZED LENS FOR THE INFRARED SPECTRUM - Google Patents

Abstract

The lens can be used in thermal imaging devices, the receivers of which are sensitive in the infrared (IR) region of the spectrum, in particular in the spectrum range of 8-12 microns. The lens contains four components. The first is the positive meniscus facing the concave surface to the image plane of oxygen-free glass IKS-25, the second is the negative meniscus facing the concave surface to the image plane of zinc selenide, the third is the negative meniscus facing Germany, facing the image plane with a convex surface made aspherical in in the form of a flattened hyperboloid with a conical constant within the limits: e = - (6 ÷ 10), the fourth component is a positive meniscus from germany, facing a convex surface to the image plane zheniya. The relative optical powers of the first ϕ, second ϕ, third ϕ and fourth ϕ components are made with the relation: ϕ: ϕ = (1.44 ÷ 1.8) :-( 0.8 ÷ 1.18), ϕ: ϕ = - (0, 38 ÷ 0.5) :( 1.2 ÷ 1.5). The technical result is an increase in the relative aperture to 1: 1, an increase in the angular field and compensation for defocusing when the temperature changes in the range from -40 ° C to + 50 ° C. 6 ill.

Description

The proposed utility model relates to the field of optical instrumentation and can be used in thermal imaging devices, the receivers of which are sensitive in the infrared (IR) region of the spectrum, in particular in the spectrum range of 8-12 μm.

For lenses operating in the spectrum range of 8-12 microns, the following requirements are met:

1. An ultrahigh relative aperture comprising D: f '= 0.75 ÷ 1.1.

2. High, close to diffractive, image quality. For thermal imagers forming images of objects of finite sizes, it is necessary that the contrast value of the image of the sinusoidal world at the Nyquist frequency υ = 1 / 2Q = 30 ÷ 15 l / mm be at least 0.4 ÷ 0.6.

3. Image quality characteristics should be constant throughout the image field of the lenses, which implies the absence of field rays vignetting, the value of the energy concentration in the receiver pixel should be at least 60–70%.

4. The minimum number of lens elements to reduce the weight, size, cost and transmission of the lens.

5. The design of the optical scheme, as well as the choice of optical materials and the optical forces of the lens components should minimize the effect of temperature changes on the aberrations of the system, of which defocusing has the greatest impact on image quality. To compensate for thermal defocusing, if it is impossible to manually move the lens relative to the receiver, either remote-controlled motors are used, which complicates the design and increases the mass-dimensional characteristics, or the materials of the components of the optical system are selected in such a way as to compensate for thermal defocusing of the lens.

Known athermalized fast lens for the infrared region of the spectrum [1], consisting of the first positive, second negative and third positive menisci facing concave surfaces to the image plane. The first lens is made of oxygen-free glass IKS-25, the second - from zinc selenide, and the third - from Germany.

The lens has a high 1: 1 relative aperture, an angular field of view of 13.8 °, but the image quality is not high enough: the modulation transmission coefficient (KPM) at a spatial frequency of 20 lines / mm is 0.6 for a point on the axis and 0.4 for the edge angular field.

Closest to the claimed technical solution is an athermalized lens for the infrared region of the spectrum [2], consisting of four lenses, the first of which is a positive meniscus from oxygen-free glass IKS-25, the second is a negative meniscus from zinc selenide, the third is a negative meniscus from Germany . Menisci face with concave surfaces to the image plane. The concave surface of the third meniscus is aspherical with a conical constant ranging from 0.32 to 0.46. The fourth component is a positive germanium lens with a convex first surface facing the plane of objects and a flat or concave second surface. The ratio of the optical power of the components:

ϕ ₁ : ϕ ₂ : ϕ ₃ : ϕ ₄ = (1.44 ÷ 1.8) :-( 0.8 ÷ 1.18) :-( 0.62 ÷ 1.0) :( 1.38 ÷ 1 , 56)

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

The lens has high image quality: KPM at a spatial frequency of 20 lines / mm is 0.7 for a point on the axis and 0.6 for the edge of an angular field. However, the relative aperture is less than required (1: 1).

The main task, which the proposed utility model is aimed at, is to create an athermalized lens for the infrared region of the spectrum with an increase in the relative aperture to 1: 1, an increase in the angular field and compensation for defocusing when the temperature changes over a wide range from -40 ° С to + 50 ° С due to the optical design of the lens.

To solve this problem, a fast athermalized lens for the infrared region of the spectrum is proposed, which, like the prototype, contains four components sequentially installed, the first of which is a positive meniscus facing a concave surface to the image plane and made of oxygen-free glass IKS-25, the second negative a meniscus facing a concave surface to the image plane and made of zinc selenide, a third is a negative meniscus made of germanium, and a fourth exponentials - positive lens made of germanium, the relative optical powers of the first φ ₁ and φ ₂ of the second component are made to the relationship:

ϕ ₁ : ϕ ₂ = (1.44 ÷ 1.8) :-( 0.8 ÷ 1.18).

In contrast to the prototype, the negative meniscus of the third component faces the image plane with a convex surface made aspherical in the form of a flattened hyperboloid with a conical constant e ² in the range: e ² = - (6 ÷ 10), and the positive lens of the fourth component is made in the form of a meniscus facing a convex surface to the image plane, while the following ratio holds:

ϕ ₃ : ϕ ₄ = - (0.38 ÷ 0.5) :( 1.2 ÷ 1.5),

where ϕ ₃ , ϕ ₄ are the relative optical powers of the third and fourth components, respectively.

The essence of the proposed utility model is as follows.

The main contribution to the aberration characteristic of the lens is made by III order aberrations. In a fairly good approximation, they can be described by the Seidel coefficients: S _I - spherical aberration, S _II - coma, S _III - astigmatism, S _IV - image curvature, S _V - distortion. For a adjusted system, the total values of the coefficients should not exceed a value of 0.1.

The fulfillment of the first positive and second negative menisci with turned concave surfaces to the image plane, with optical forces satisfying the condition ϕ ₁ : ϕ ₂ = (1.44 ÷ 1.8) :-( 0.8 ÷ 1.18), introduces the total positive spherical aberration S _{I (1 + 2 lenses)} = 0.74, positive coma S _{II (1 + 2 lenses)} = 0.17, positive astigmatism S _{III (1 + 2 lenses)} = 0.38.

Execution of the third negative meniscus with a convex surface facing the image plane and with a convex aspherical surface in the form of a flattened hyperboloid with e ² = - (6 ÷ 10) and the fourth component in the form of a meniscus facing a convex surface to the image plane, with optical forces satisfying the condition ϕ ₃ : ϕ ₄ = - (0.38 ÷ 0.5) :( 1.2 ÷ 1.5), allowed to obtain the values of the coefficients:

S _{I (3 lens)} = -2.22, S _{II (3 lens)} = -0.7, S _{III (3 lens)} = -0.5;

S _{I (4 lens)} = 1.53, S _{II (4 lens)} = 0.6, S _{III (4 lens)} = 0.07.

The Seidel coefficients for the entire system are:

S _I = -0.03, S _{II (4} lenses ₎ = -0.035, S _{III (4} lenses ₎ = -0.058.

That is, monochromatic aberration corrected. The implementation of the relative optical forces ϕ _i in these ratios made it possible to correct the position chromatism equal

where υ _i is the dispersion of the optical material in the working range of the spectrum;

and provide athermalization, defined by the ratio

- термооптическая постоянная,

- приращение относительных значений показателя преломления при изменении температуры, α - коэффициент линейного расширения.Where

- thermo-optical constant,

- increment of the relative values of the refractive index with temperature, α is the coefficient of linear expansion.

The proposed utility model is illustrated by drawings, where

in FIG. 1 shows the optical circuit of the lens,

in FIG. 2 - graphs of transverse aberrations (t = + 20 ° С),

in FIG. 3 - aberration of the main rays (t = + 20 ° C),

in FIG. 4 - graphs of the energy concentration in a square of size (17 × 17) microns and KPM at a spatial frequency of 20 lines / mm at an ambient temperature of t = + 20 ° C,

in FIG. 5 - graphs of the energy concentration squared (17 × 17) μm in size and KPM at a spatial frequency of 20 lines / mm at an ambient temperature of t = -40 ° С

in FIG. 6 - graphs of the energy concentration in a square of size (17 × 17) microns and KPM at a spatial frequency of 20 lines / mm at an ambient temperature of t = + 50 ° C.

The inventive athermalized lens for the infrared region of the spectrum consists of four meniscus lenses 1, 2, 3 and 4 and a protective glass 5. The positive meniscus 1 and the negative meniscus 2 face the concave surfaces to the image plane 6, and the negative meniscus 3 and the positive meniscus 4 face the convex surfaces to the image plane 6.

The first lens of the lens is made of oxygen-free glass IKS-25, the second lens is made of zinc selenide, and the third and fourth lenses are made of germanium.

Between the optical forces of the components the following relations are satisfied:

ϕ ₁ : ϕ ₂ = (1.44 ÷ 1.8) :-( 0.8 ÷ 1.18),

ϕ ₃ : ϕ ₄ = - (0.38 ÷ 0.5) :( 1.2 ÷ 1.5),

where ϕ ₁ , ϕ ₂ , ϕ ₃ , ϕ ₄ are the relative optical powers of the first, second, third and fourth components, respectively. The aperture diaphragm is located on the first surface of the lens.

The lens operates as follows: a parallel beam of radiation from a distant object is focused in the rear focus of meniscus 1, and then each of menisci 2 and 3 alternately projects into its image plane, and the last meniscus 4 re-projects the intermediate image after the third meniscus into its image plane, which coincides with the focal plane of the entire lens.

An illustration of the proposed utility model is a fast athermalized infrared lens with the following parameters:

- Focal length 30 mm;

- Angular field 2ω = 24.8 °;

- Relative aperture 1: 1.

Important advantages of the proposed lens are:

- High relative aperture;

- High image quality: the concentration of energy in a pixel measuring (17 × 17) microns in the center of the field of view is at least 70%, at the edge of the field of view at least 60%; KPM at a spatial frequency of 20 lines / mm: at the center of the field of view of at least 0.7, at the edge of the field of view of at least 0.5.

The use of IKS-25 glass, zinc selenide, and germanium as optical materials for the selected ratio of the optical forces of the lenses ensured the athermality of the lens. The result is achieved, which consists in increasing the relative aperture and ensuring high image quality when the temperature changes in the range from -40 ° C to + 50 ° C.

INFORMATION SOURCES

1. Russian Federation, utility model patent No. 156006, IPC: G02B 9/14, G02B 13/14, 2015.

2. Russian Federation, patent for invention No. 2613483, IPC: G02B 9/38, G02B 9/56, G02B 13/14, 2017 - prototype.

Claims (3)

An infrared lens for the infrared region of the spectrum containing four sequentially installed components, the first of which is the positive meniscus facing the concave surface to the image plane and made of oxygen-free glass IKS-25, the second is the negative meniscus facing the concave surface to the image plane and made of selenide zinc, the third is a negative meniscus made of germanium, and the fourth component is a positive lens made of germanium, with relative The physical forces of the first ϕ ₁ and second ϕ ₂ components are made with the ratio: ϕ ₁ : ϕ ₂ = (1.44 ÷ 1.8) :-( 0.8 ÷ 1.18), characterized in that the negative meniscus of the third component is inverted to the image plane with a convex surface made aspherical in the form of a flattened hyperboloid with a conical constant e ² in the range: e ² = - (6 ÷ 10), and the positive lens of the fourth component is made in the form of a meniscus facing the convex surface to the image plane, the following ratio: ϕ ₃ : ϕ ₄ = - (0.38 ÷ 0.5) :( 1.2 ÷ 1.5), where ϕ ₃ , ϕ ₄ are the relative optical powers of the third and fourth components, respectively.

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