RU2583338C1 - Athermalised high-aperture infrared lens - Google Patents

Abstract

FIELD: instrument making.

SUBSTANCE: invention can be used in imaging devices. Lens comprises four single menisci whose concave side faces image. First meniscus is positive, second is positive, made from material with negative temperature coefficient of refraction index, third is negative, made from material with positive temperature coefficient of refraction index and low value of dispersion, fourth is positive. Between focus distances of lenses, following relationships are satisfied: F₁/F₀ = 1.7÷2.2; F₀/F₂ = 0.15÷0.55; |F₃|/F₀ = 1.6÷3.0; F₄/F₀ = 0.6÷1.0; where F₁, F₂, F₃, F₄, F₀ - focal distances of first, second, third, fourth components and lens respectively.

EFFECT: technical result is lens image quality improvement over a wide temperature range with a large relative aperture and field of view angle.

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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 athermalized fast IR lens (EU Patent EP 2687889 A1, G02B 13/14) containing three components along the rays, which are positive menisci facing concavity to the image.

This lens operates in the infrared wavelength range, with the first element made of oxygen-free chalcogenide glass of the Vitron IG6 brand, the second element made of germanium, the third made of zinc selenide.

Lens Specifications:

- focal length 90 mm;

- relative aperture 1: 1;

- angular field of view ± 2.7 deg;

- back focal length 6.5 mm.

Said lens is atermalizovannym by applying oxygen-free chalcogenide glass and kinoform element formed on the second surface of the first element, and an aspherical surface 12 ^th order on the second surface of the third element. The disadvantages of athermalization by this method include the energy loss of radiation on diffraction elements, the high cost of manufacturing a kinoform element. In addition, the use of diffraction elements requires high accuracy of installation in the optical system due to the large dependence of the characteristics on the displacement of the element relative to the optical axis of the system, which also increases the cost of manufacturing the lens.

A well-known athermalized four-component infrared lens (Patent of Ukraine No. UA 88915 U, IPC (2014.01) G02B 13/00) containing four components along the rays, the first of which is the negative meniscus, and the second, third and fourth components are positive menisci, facing concavity to the image. The first, second and fourth components of the lens are made of Germany, and the third is made of KRS-5 (thallium bromide - thallium iodide, TlBr - TlI).

Lens Specifications:

- focal length 50.8 mm;

- relative aperture 1: 1;

- angular field of view ± 12.5 degrees;

- rear focal segment 4.05 mm.

The presented lens according to the description has the stability of characteristics in the temperature range from 0 ° C to ± 60 ° C. The specified operating temperature range is insufficient. In addition, the lenses have a significant thickness, which increases the mass of the lens and reduces the transmittance. In addition, this lens has large dimensions (the ratio of the length of the optical system to the focal length is 3.2).

The closest analogue to the claimed technical solution is an athermalized lens for the infrared region of the spectrum (Ukrainian Patent No. UA 81919 U, IPC (2013.01) G02B 13/00), consisting of four components along the rays, which are positive menisci facing concavity to the image, moreover, the first and fourth components are made of Germany, the second and third components are made of cattle-5.

Lens Specifications:

- focal length 60 mm;

- relative aperture 1: 1.3;

- angular field of view ± 10 degrees;

- back focal segment of 6.9 mm.

This lens is close in design to the claimed one, however, it does not provide high enough image quality when the temperature changes (the function of energy concentration and the frequency-contrast characteristic are presented in Fig. 1, Fig. 2 (a - at minus 50 ° C, b - at +50 ° C). In addition, this first lens element has a non-spherical surface, which increases the manufacturing cost of the lens.

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, the lens has a diameter of the scattering circle in terms of energy concentration of 80% at minus 50 ° C - 72 μm, and at + 50 ° C - 40 μm, which is far from the maximum possible diffraction limit (the diffraction diameter of the Erie scattering circle is 32 μm )

As can be seen from the frequency response curve, 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 0.13 and 0.17 at minus 50 ° C, and 0.34 and 0.18 at +50 ° C, respectively (value of the diffraction frequency-contrast characteristic 0.52).

In addition, the lens has a large length and a significant mass of elements.

The indicated values of the relative aperture, frequency response, and diameter of the scattering circle, as well as the stability of the characteristics when the temperature changes, are insufficient in those cases when the lens requires extremely high values of aperture and resolution when working in a wide temperature range.

The objective of the invention is to increase the relative aperture and improve the image quality of the lens when working in a wide range of ambient temperatures.

The problem is solved in that in an athermalized fast IR lens containing four components, the first of which is a single positive meniscus facing concavity to the image, the second is a single positive meniscus facing concavity to the image made of material with a negative temperature coefficient of the indicator refraction, the third is a single meniscus facing concavity to the image, the fourth is a single positive meniscus facing concavity to the image, new is that the third component is made in the form of a negative meniscus from a material with a positive temperature coefficient of refractive index and a small value of the dispersion index, while the following conditions for focal lengths are fulfilled:

F ₁ / F ₀ = 1.7 ÷ 2.2;

F ₀ / F ₂ = 0.15 ÷ 0.55;

| F ₃ | / F ₀ = 1.6 ÷ 3.0;

F ₄ / F ₀ = 0.6 ÷ 1.0;

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 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 when the temperature changes over a wide range. The ratio of the length of the optical system to the focal length lies in the range from 0.50 to 0.80.

In the particular case, the materials of the optical elements are selected in compliance with the following ratios:

n ₁ , n ₄ ≥3; n ₂ , n ₃ ≤3;

υ ₁ , υ ₄ ≥100; υ ₃ ≤100;

where n ₁ , n ₂ , n ₃ , n ₄ are the refractive indices of the materials of the first, second, third and fourth components, respectively, and υ ₁ , υ ₃ , υ ₄ , are the dispersion coefficients of materials in the range of 8-12 μm for the first, third and the fourth component, respectively.

In special cases, the first and fourth components are made of germanium, the second component is from KRS-5, the third component is from zinc selenide; or the first and fourth components are made of gallium arsenide, the second component is from KRS-5, the third component is from zinc sulfide; or the first and fourth components are made of gallium arsenide, the second component is from cesium bromide, the third component is from zinc sulfide.

In a particular case, the following relationship can be fulfilled in the lens:

R ₂ = R ₄ = R ₆ ;

where R ₂ , R ₄ , R ₆ are the radii of the second, fourth and sixth surfaces, respectively. 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.

The presented invention is illustrated by the graphic materials of FIG. 3 - FIG. 6. In FIG. 3 shows a variant of the optical scheme of the lens with the actual path of the rays for three points of the field of view (0 deg, 4 deg, 5.35 deg), and its characteristics, illustrated in FIG. 4 is a function of energy concentration, in FIG. 5 is a frequency-contrast characteristic and in FIG. 6 - graphs of transverse aberrations are shown for two lens states - at minus 50 ° C and at + 50 ° C.

The lens contains four serially mounted optically coupled components: the first is a positive meniscus 1 facing concavity to the image, the second is a positive meniscus 2 facing concavity to the image, the third is a negative meniscus facing concavity to the image, and the fourth is a positive meniscus 4 facing concavity to image. The entrance pupil is located on the first surface of the lens.

In the presented embodiment, components 1 and 4 are made of germanium, component 2 is made from KRS-5, component 3 is made of zinc selenide.

In the table. Figure 1 shows the design parameters of the presented lens variant — 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.

The aperture diaphragm is located on the first surface of the lens. The diameter of the aperture diaphragm is 60 mm.

For the given lens, the ratio between the focal lengths of the lens components and the ratio of the focus of the lens to its length is:

F ₁ / F ₀ = 2.03;

F ₀ / F ₂ = 0.46;

| F ₃ | / F ₀ = 1.91;

F ₄ / F ₀ = 0.76;

F ₀ / L = 0.68.

The IR lens has the following characteristics:

- relative aperture 1: 1;

- focal length 60 mm;

- field of view angle of 10.7 degrees;

- lens length 88.7 mm;

- back focal length 9.7 mm.

In FIG. 4 shows the function of energy concentration in the lens scattering circle. As can be seen from the graph, the diameter of the scattering circle (at a level of 80%) is 18 μm in the center of the field of view in the entire temperature range, for the edge of the field of view - 27 μm in the entire temperature range (the diffraction diameter of the Erie scattering circle is 25 μm).

In FIG. 5 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 0.59 and 0.43 at minus 50 ° C, and 0.58 and 0.38 at + 50 ° C, respectively (the value of the diffraction frequency-contrast characteristic 0.61).

In FIG. Figure 6 (a, b) shows characteristic plots of the transverse aberrations of the lens at minus 50 ° C and at + 50 ° C, respectively. As can be seen from the graphs, the geometric aberrations do not exceed 3 μm and 7 μm in the center and on the edge of the field of view, respectively, over the entire temperature range.

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, does not contain aspherical surfaces and maintains stability in the temperature range from minus 50 ° C to + 50 ° C.

Three concave surfaces can be made with the same radii of curvature, which makes the lens design more technological and reduce the range of test glasses used in the manufacture.

Thus, the proposed invention improves the image quality of the lens when working in a wide temperature range with a large relative aperture and the angle of the field of view of the optical system.

Claims (6)

1. The infrared aperture lens of the infrared range, containing four components, the first of which is a single positive meniscus facing concavity to the image, the second is a single positive meniscus facing concavity to the image made of material with a negative temperature coefficient of refractive index, the third is single meniscus facing concavity to the image, the fourth is a single positive meniscus facing concavity to the image, characterized in that the third component nen in the form of a negative meniscus made of a material with a positive temperature coefficient of refractive index and a small value of the dispersion index, while the focal lengths of the components satisfy the following conditions:
F ₁ / F ₀ = 1.7 ÷ 2.2;
F ₀ / F ₂ = 0.15 ÷ 0.55;
| F ₃ | / F ₀ = 1.6 ÷ 3.0;
F ₄ / F ₀ = 0.6 ÷ 1.0;
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 materials of the optical elements are selected in compliance with the following ratios:
n ₁ , n ₄ ≥3; n ₂ , n ₃ ≤3;
υ ₁ , υ ₄ ≥100; υ ₃ ≤100;
where n ₁ , n ₂ , n ₃ , n ₄ are the refractive indices of the materials of the first, second, third and fourth components, respectively, and υ ₁ , υ ₃ , υ ₄ are the dispersion coefficients of materials in the range of 8-12 μm for the first, third, and fourth components, respectively. 3. A fast lens according to claim 1, characterized in that the first and fourth components are made of germanium, the second component is from KRS-5, the third component is from zinc selenide. 4. A fast lens according to claim 1, characterized in that the first and fourth components are made of gallium arsenide, the second component is from KRS-5, the third component is from zinc sulfide. 5. A fast lens according to claim 1, characterized in that the first and fourth components are made of gallium arsenide, the second component is cesium bromide, the third component is zinc sulfide. 6. A fast lens according to claim 1, characterized in that the optical surfaces are made in compliance with the following ratios:
R ₂ = R ₄ = R ₆ ;
where R ₂ , R ₄ , R ₆ are the radii of the second, fourth and sixth optical surfaces, respectively.

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