RU2506616C1 - High-speed infrared lens - Google Patents

Abstract

FIELD: physics, optics.

SUBSTANCE: lens can be used for operation in the infrared range in thermal imaging devices. The lens has four components. The first component a single positive meniscus whose concave surface faces the image, the second component is a single meniscus whose convex side faces the image, the third component is a single meniscus and the fourth component is a single positive meniscus whose concave surface faces the image. The first and fourth components are made of germanium, while the second and third components are made of zinc selenide. The second component is positive and the third component is in form of a negative meniscus whose concave surface faces the image. Focal distances of the components satisfy the following conditions: F₁/F₀=1.2÷1.5; F₀/F₂=0÷0.05; |F₃|/F₀=1.6÷1.9; F₄/F₀=0.4÷0.6; where F₁, F₂, F₃, F₄, F₀ are focal distances of the first, second, third, fourth components and the lens, respectively.

EFFECT: high image quality of the lens with a high aperture ratio and large field of view.

3 cl, 4 dwg, 1 tbl

Description

The present invention relates to optical instrumentation, namely to special lenses operating in the infrared wavelength range, and can be used in thermal imaging devices.

Known fast lens [RF Patent No. 2183340; IPC ⁸ G02B 13/14, 9/34; 2002], containing four components along the rays, the first of which is a positive meniscus facing concavity to the image, the second is a flat-concave lens, the third is a flat-convex lens, and the fourth is a positive meniscus convex to a subject.

This lens works in the infrared wavelength range and all lenses are made of germanium.

Lens Specifications:

- focal length 51.14 mm;

- relative aperture 1: 1.65;

- angular field of view 18 degrees;

- back focal segment of 45.23 mm.

The specified lens has a small relative aperture.

The closest analogue to the claimed technical solution is a fast lens [RF Patent No. 2348059; IPC ⁸ G02B 9/34, 13/14; 2009], containing four components, the first of which is a single positive meniscus facing concavity to the image, the second is a single negative meniscus convex to the image, the third is a single positive meniscus convex to the image; the fourth is a single positive meniscus facing concavity to the image; moreover, the first and fourth components are made of germanium, the second and third components are made of zinc selenide, and the conditions are satisfied:

| R ₂ | = | R ₅ |

| R ₃ | = | R ₆ | = | R ₈ |

| R ₅ |> | R ₆ |,

where R ₂ , R ₃ , R ₅ , R ₆ , R ₈ are the radii of the second, third, fifth, sixth, eighth optical surfaces, respectively.

This lens is close in design to the claimed, but it does not have a large enough relative aperture and not high enough image quality.

The patent description describes the design parameters of the lens, calculated for the infrared wavelength range.

Characteristics of this lens:

- focal length 130.03 mm;

- relative aperture 1: 1.08;

- angular field of view 5 deg 20 min;

- lens length 200 mm;

- back focal segment of 45 mm.

Modern thermal imaging devices use microbolometric matrices of infrared sensitive elements, which have mainly a pixel size of 25 microns and 17 microns. For such microbolometric matrices, the relative aperture of the lens should be at least at least 1: 1.

The image quality of thermal imaging lenses is usually estimated by the diameter of the scattering circle, in which 80% of the energy is concentrated, or by the value of the frequency-contrast characteristic at the critical spatial frequency. For a pixel size of 17 μm, the critical spatial frequency is 30 mm ⁻¹ .

As the calculation shows, a lens whose design parameters are given in the description has a scattering circle diameter of 24 μm in the center of the field of view and 31 μm at the edge of the field of view (the diffraction diameter of the scattering circle is 19 μm).

The indicated values of the relative aperture and the diameter of the scattering circle are insufficient in those cases when extremely high values of aperture and resolution are required from the lens.

The lens in accordance with the characteristics indicated in the claims can be recalculated for the required values of the wavelength range, relative aperture and focal length.

The following characteristics can be obtained:

- relative aperture 1: 1;

- focal length 40 mm;

- angular field of view 12 degrees;

- lens length 62 mm;

- back focal segment 8.5 mm;

- the diameter of the scattering circle in the center and on the edge of the field of view is 22 μm and 25 μm, respectively (the diffraction diameter of the scattering circle is 20 μm).

The indicated values of the relative aperture and the diameter of the scattering circle are also insufficient in those cases when the lens requires extremely high values of aperture and resolution.

The objective of the invention is to increase the relative aperture and improve the image quality of the lens.

The problem is solved in that in the fast lens of the infrared region containing four components, the first of which is a single positive meniscus facing the concavity to the image, the second is a single meniscus facing the convex to the image, the third is a single meniscus and the fourth is a single positive meniscus facing the image with the first and fourth components made of germanium, the second and third components made of zinc selenide, new is that the second component is made in the form of a positive Nogo meniscus third component formed as a negative meniscus facing to the image is concave, while the focal lengths of the components satisfy the following conditions:

F ₁ / F ₀ = 1.2 ÷ 1.5;

F ₀ / F ₂ = 0 ÷ 0.05;

| F ₃ | / F ₀ = 1.6 ÷ 1.9;

F ₄ / F ₀ = 0.4 ÷ 0.6;

where F ₁ , F ₂ , F ₃ , F ₄ , F ₀ are the focal lengths of the first, second, third, fourth components and the lens, respectively.

In a particular case, the following relationships are made in the lens:

| R ₃ | = | R ₈ |;

| R ₄ | = | R ₅ |;

where R ₃ , R ₄ , R ₅ , R ₈ are the radii of the third, fourth, fifth and eighth surfaces, respectively.

In the particular case of the lens, the third component is mounted with the possibility of movement along the optical axis.

The presented design of the lens with the specified conditions for the focal lengths of the components allows you to simultaneously increase the relative aperture and improve image quality.

In the particular case, the ratios on the radii of the surfaces make the lens design more technologically advanced and reduce the range of test glasses used in the manufacture.

In the particular case, the installation of the second component with the ability to move along the optical axis can be used: 1) to align the lens with a fixed microbolometric matrix during the assembly of the thermal imaging device, 2) to compensate for the displacement of the plane of the best installation when the ambient temperature changes, 3) to focus the lens to a finite distance.

Figure 1 shows the optical diagram of the lens with the actual path of the rays for three points of the field of view, figure 2 is a function of energy concentration, figure 3 is a frequency-contrast characteristic, figure 4 is a graph of transverse aberrations.

The lens contains four optically coupled components in series: the first is the positive meniscus 1 facing the image, the second is the positive meniscus 2 facing the image, the third is the negative meniscus facing the image, the fourth is the positive meniscus 4 facing the image.

Components 1 and 4 are made of germanium, components 2 and 3 are made of zinc selenide.

The focal lengths of the components satisfy the following conditions:

F ₁ / F ₀ = 1.2 ÷ 1.5;

F ₀ / F ₂ = 0 ÷ 0.05;

| F ₃ | / F ₀ = 1.6 ÷ 1.9;

F ₄ / F ₀ = 0.4 ÷ 0.6;

where F ₁ , F ₂ , F ₃ , F ₄ , F ₀ are the focal lengths of the first, second, third, fourth components and the lens, respectively.

The entrance pupil is located on the first surface of the lens.

Figure 1 also shows a protective glass 5 that protects the microbolometric matrix of sensitive elements.

In a particular case, the following relations are satisfied:

| R ₃ | = | R ₈ |;

| R ₄ | = | R ₅ |;

where R ₃ , R ₄ , R ₅ , R ₈ are the radii of the third, fourth, fifth and eighth surfaces, respectively.

In the particular case, the second component is mounted for movement along the optical axis.

The following is an example of a specific implementation of the lens.

Table 1 shows the design parameters of the lens — the radii of the surfaces, the thickness of the lenses and the air gaps between them, the light and full diameters and the material of the lenses.

Table 1 No. Radii, mm Thickness mm Material Light diameters, mm Full diameters, mm one 61.81 5,0 Germanium 47.7 50,0 2 91.62 27.12 Air 45.9 48.0 3 -30.48 8.0 Zinc Selenide 31.5 34.0 four -34.51 11.91 Air 34.8 37.0 5 34.51 2.7 Zinc Selenide 22.8 25.0 6 24.21 1,0 Air 20.7 23.0 7 21.68 3 Germanium 20.3 22.0 8 30.48 5,0 Air 18.7 22.0 9 ∞ 1,0 Germanium 12.3 14.0 10 ∞ 1.36 Air 12.3 14.0 eleven ∞ - - 10.0 13.6

The aperture diaphragm is located on the first surface of the lens. The diameter of the aperture diaphragm is 47.7 mm.

For the given lens, the ratio between the focal lengths of the lens components is:

F ₁ / F ₀ = 1.34;

F ₀ / F ₂ = 0.03;

| F ₃ | / F ₀ = 1.71;

F ₄ / F ₀ = 0.50.

To improve the manufacturability of this design, two convex surfaces are made with the same radii of curvature (| R ₄ | = | R ₅ |), two concave surfaces are also made with the same radii of curvature (| R ₃ | = | R ₈ |).

To align the lens with a fixed microbolometric matrix during the assembly of the thermal imaging device, the second component is made with the ability to move along the optical axis within ± 1.0 mm and fix it in a position that provides the best image quality.

The IR lens has the following characteristics:

- relative aperture 1: 0.84;

- focal length 40 mm;

- angular field of view 12 degrees;

- lens length 66 mm;

- back focal length of 6.7 mm.

Figure 2 shows the function of the concentration of energy of the lens. As can be seen from the graph, the diameter of the scattering circle in the center and at the edge of the field of view is 18 μm and 20 μm, respectively (the diffraction diameter of the scattering circle is 17 μm).

Figure 3 shows the frequency-contrast characteristic of the lens. As can be seen from the graph, at a spatial frequency of 30 mm ^{-1, the} value of the frequency-contrast characteristic in the center and at the edge of the field of view is 0.60 and 0.56 μm, respectively. (value of diffraction frequency-contrast characteristic 0.64).

Figure 4 shows characteristic plots of the transverse aberrations of the lens. As can be seen in the graphs, geometric aberrations do not exceed 16 microns and 30 microns in the center and on the edge of the field of view, respectively.

The developed lens has high image quality with a large relative aperture and the required field of view, while the lens has a small length and sufficient rear focal length.

The lens is easy to manufacture. It does not contain aspherical surfaces. Two convex surfaces are made with the same radii of curvature, two concave surfaces are also made with the same radii of curvature. This allows you to make the design of the lens more technologically advanced and reduce the range of test glasses used in the manufacture.

The second component of the lens is mounted to move along the optical axis. This movement can be used: 1) to align the lens with a fixed microbolometric matrix during the assembly of the thermal imaging device, 2) to compensate for the displacement of the plane of the best setting when the ambient temperature changes, 3) to focus the lens to a finite distance.

Thus, the proposed invention improves the image quality of the lens with a large relative aperture and field of view.

Claims (3)

1. Aperture lens of the infrared region containing four components, the first of which is a single positive meniscus with a concavity to the image, the second is a single meniscus convex to the image, the third is a single meniscus, and the fourth is a single positive meniscus with a concavity to the image wherein the first and fourth components are made of germanium, the second and third components are made of zinc selenide, characterized in that the second component is made in the form of a positive meniscus, the third component is made a negative meniscus facing to the image is concave, while the focal lengths of the components satisfy the following conditions:
F ₁ / F ₀ = 1.2 ÷ 1.5;
F ₀ / F ₂ = 0 ÷ 0.05;
| F ₃ | / F ₀ = 1.6 ÷ 1.9;
F ₄ / F ₀ = 0.4 ÷ 0.6;
where F ₁ , F ₂ , F ₃ , F ₄ , F ₀ are the focal lengths of the first, second, third, fourth components and the lens, respectively. 2. A fast lens according to claim 1, characterized in that the optical surfaces are made in compliance with the following ratios:
| R ₃ | = | R ₈ |;
| R ₄ | = | R ₅ |;
where R ₃ , R ₄ , R ₅ , R ₈ are the radii of the third, fourth, fifth and eighth optical surfaces, respectively. 3. The fast lens according to claim 1, characterized in that the second component is mounted for movement along the optical axis.

Images