RU184257U1 - LIGHT LIGHT FOR 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 8-14 microns. The high-aperture lens for the infrared region of the spectrum contains four lenses, the first a positive meniscus facing a convex image, the second a negative meniscus convex to the image, and the fourth a positive meniscus having a convex surface in the form of an elongated ellipsoid of rotation of a higher order and facing concavity to the image. The optical power of the lenses satisfy the conditions: 0.5 <ϕ / ϕ <0.7; 0.1 <| ϕ / ϕ | <0.3; | ϕ / ϕ | <5⋅10; 1.0 <ϕ / ϕ <1.3; where ϕ, ϕ, ϕ, ϕ, ϕ are the optical forces of the first, second, third, fourth lenses, respectively, of the entire lens. The first, third and fourth lenses are made of optical germanium, and the second lens is made of zinc selenide. The technical result is an increase in the relative aperture while ensuring high image quality throughout the field. 4 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 8-14 microns.

Systems operating in the spectrum region of 8-14 microns allow observing objects whose radiation temperature is -50 ÷ + 50 ° С, which corresponds to radiation in the range of 8-14 microns.

The following requirements are imposed on lenses operating in the spectrum range of 8-14 microns:

1. An ultrahigh relative aperture comprising D: F = 1: 0.75 ÷ 1: 1.

This requirement is due to the fact that the brightness of the emitting natural objects is low, and the sensitivity of the used modern receivers (bolometric matrices) is low.

2. The angular field must be at least (20 ÷ 25) °.

3. High, close to diffractive image quality. The size of the matrix element is Q = (17 × 17) μm. For infrared thermal imaging devices, it is necessary that in the pixel size the energy concentration η be at least (65–70)%, while the non-aberrational ideal system gives η≈89%. 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 mm ^-1 be at least 0.5.

4. Image quality specifications should be consistent across the entire image field of the lens. This requirement is especially important for devices for detecting and tracking remote objects of small sizes. This requirement also implies:

- lack of vignetting of field rays;

- close to the telecentric course of the main rays in the space of images.

The fulfillment of these requirements ensures the constancy of the energy characteristics of the lens throughout the image field.

5. The minimum number of lens elements.

The materials commonly used in IR lenses are zinc selenide and optical germanium, which has a large specific gravity of 5.33 g / cm ³ . In addition, germanium is a rather expensive material. Minimizing the number of lens elements reduces the weight of the lens and its cost.

6. The overall parameters include the requirement, determined by the design of the receiver, for the rear focal length S '_{f' to} satisfy the condition S '_f' ≥10 mm, which for small focal lengths f 'of the lenses is S' _{f '} ≥0.3f' . In addition, it is desirable that the length of the lens does not exceed 2 ÷ 2.5 f '.

7. An important problem of creating IR lenses using germanium is a significant change in its refractive index with temperature. This causes image quality deterioration. The design of the optical scheme, as well as the choice of the optical powers of the lens components should minimize the effect of temperature changes on image quality.

The creation of an optical system of a fast lens for IR thermal imagers is becoming an urgent task.

Known designs of optical circuits of lenses that satisfy many of the above requirements and consisting of 4 lenses.

For example, a fast IR lens [1]. The lens consists of 4 lenses, the first of which is a positive meniscus facing concavity to the image, the second is a flat-concave negative lens facing a flat surface to the image, the third lens is flat convex facing a subject, and the fourth lens is a positive meniscus convex to a subject .

The lens has the following characteristics:

- Focal length f '= 51 mm

- Relative hole D: F = 1: 1.65

- Angular field of view 2ω = 18 °

- Back focal length S '= 45.23

- The transverse aberration of a wide oblique beam in the meridional section for the angular field ω = 9 ° is 0.081 mm, astigmatism is 0.014 mm, and distortion is 5%.

- All lenses are made of Germany.

The disadvantages of this lens include:

- Low relative aperture (1: 1.65);

- Small angular field of view 2ω = 18 °;

- Low image quality in the field: the above aberrations will lead to unacceptably low values of the energy concentration in the size corresponding to the pixel of the receiver.

Closest to the claimed technical solution is a fast IR lens [2]. The lens consists of 4 sequentially mounted lenses, the first of which is the positive meniscus facing the convexity of the image, the second is the negative meniscus convex to the image, the third is the meniscus convex to the image, and the fourth is the positive meniscus convex to the object.

The lens has the following characteristics:

- Focal length -F '= 28 mm

- Relative hole D: F = 1: 0.8

- Angular field of view 2ω = 25 °

- Back focal length S '_F' = 0.75f '

- The telecentric path of the main rays in the image space

- The length of the lens (L) is the distance from the first surface of the first lens to the focal plane L = 2.2f '

- Image quality:

- The concentration of energy in the size of (17 × 17) microns

in the center of the field - 67%

on the edge of the role - 47%

- KPM at a frequency of 30 mm ^-1

in the center of the field - 0.50

at the edge of the field - 0.27.

The disadvantages of this lens include:

- Reduced relative aperture - (1: 0.8)

- Low image quality - (energy concentration over the field in a pixel of size (17 × 17) microns).

The main task, which the proposed utility model is aimed at, is to increase the relative aperture to 1: 0.75 while ensuring high image quality throughout the field.

To solve this problem, a high-aperture lens is proposed for the IR spectral region, which, like the prototype, consists of 4 lenses.

The first lens is the positive meniscus facing the concavity of the image, the second lens is the negative meniscus convex to the image, the third lens is the meniscus convex to the image, and the fourth lens is the positive meniscus.

In contrast to the prototype, in the proposed high-aperture lens for the infrared region of the spectrum, the third meniscus convex to the image is negative, and the fourth positive meniscus is made with a convex surface in the form of an elongated ellipsoid of revolution of higher order and facing concavity to the image, while optical lens powers satisfy the conditions:

where ϕ ₁ , ϕ ₂ , ϕ ₃ , ϕ ₄ , ϕ are the optical forces of the first, second, third, fourth lenses and the lens as a whole, in addition, the first, third and fourth lenses are made of optical germanium, and the second

The essence of the proposed utility model is as follows.

The aberrations of the optical system are primarily determined by the third-order aberrations, which are characterized by the third-order Seidel coefficients:

S _I - spherical aberration, S _II - coma, S _III - astigmatism, S _IV - image curvature, S _V - distortion. For high-quality lenses, the coefficient values are close to zero.

The fulfillment of the first and fourth lenses by positive menisci facing the concavity to the image, with optical forces satisfying the conditions: 0.5 <ϕ ₁ /ϕ<0.7 and 1.0 <ϕ ₄ /ϕ<1.3, introduces the total positive spherical aberration S _I (1 + 4 lenses ) = 1.78, negative coma S _II (1 + 4 lenses) = - 0.37, positive astigmatism S _III (1 lenses) = 0.4.

To correct them, the second and third lenses are made with negative menisci, convex to the image with optical forces that satisfy the conditions:

In this case, the spherical aberration S _I (2 + 3 lenses) = - 1.64, coma S _II (2 + 3 lenses) = 0.2. To correct astigmatism, the convex surface of the fourth lens (fourth positive meniscus) is made elliptical: S _III (7th surface) = - 0.57.

Higher orders of the elliptical surface correct coma and higher order astigmatism in the field.

и 1.0<ϕ₄/ϕ<1.3, из германия с дисперсией ν=727 в диапазоне спектра (8÷12) мкм для первой, третьей и четвертой линз и второй

- из селенида цинка с дисперсией ν=33.8, дает возможность исправить кривизну изображения объектива S_IV=0.095, хроматизим положения S_хр.пол.=0.006 и хроматизм увеличения S_хр.пол.=-0.001.The performance of lenses with optical powers ϕ ₁ , ϕ ₂ , ϕ ₄ , ϕ ₃ that satisfy the conditions:

and 1.0 <ϕ ₄ /ϕ<1.3, from Germany with a dispersion ν = 727 in the spectrum range (8 ÷ 12) μm for the first, third and fourth lenses and the second

- from zinc selenide with a dispersion of ν = 33.8, makes it possible to correct the curvature of the image of the lens S _IV = 0.095, the chromatism of the position of S _hr. = 0.006 and the chromatism of an increase in S _h.pol. = -0.001.

The implementation of the first and fourth lenses in the form of positive menisci, facing concavity to the image, and the second and third in the form of negative menisci, convex to the image, minimized higher-order aberrations and, thus, ensured a high relative aperture.

Important advantages of the proposed lens are:

- High relative aperture 1: 0.75

- High image quality throughout the field:

- The concentration of energy in the pixel size (17 × 17) microns

in the center of the field of view ≥70%

at the edge of the field of view ≥65%

- Modulation transmission coefficient (KPM) at a frequency of 30 mm ^-1 :

in the center of the field of view ≥50%

at the edge of the field of view ≥47%

- Large posterior segment S '_F' ≥0.5f ', obtained by performing the second and third in the form of negative menisci, convex to the image

- Ensuring close to the telecentric path of the main rays in the image space and, thereby, ensuring uniform illumination throughout the image field

- Minimizing the ambient light of the receiver,

due to reflection from the last surface of the radiation, the source of which is the self-radiation of the receiver matrix, due to the radius of the last surface (R ₈ ) exceeding the rear segment of the lens: R8 / S '_F' = 5.4.

The essence of the claimed utility model is illustrated by drawings, where in FIG. 1 is an optical diagram of a lens; FIG. 2. - presents graphs of transverse aberrations, in FIG. 3 shows graphs of aberrations of the main rays, in FIG. 4 - plots of energy concentration in a square of size (17 × 17) microns are presented.

The fast aperture lens for the infrared region of the spectrum consists of four meniscus lenses 1, 2, 3 and 4, the first of which 1 is made in the form of a positive meniscus facing the concavity to the image, the second negative lens 2 is made in the form of a meniscus facing the convexity of the image, the third negative the lens 3 is made in the form of a meniscus, convex to the image, and the fourth lens 4 is made in the form of a positive meniscus, turned concavity to the image. In this case, the optical powers of the lenses satisfy the conditions:

The optical power of the lenses satisfy the conditions:

where ϕ ₁ , ϕ ₂ , ϕ ₃ , ϕ ₄ , ϕ are the optical forces of the first, second, third, fourth lenses, and the lens as a whole, respectively.

Aperture diaphragm 5 is located on the first surface of the lens.

The fourth positive meniscus 4 is made with a convex surface in the form of an elongated ellipsoid of revolution of higher order.

The first, third and fourth lenses are made of optical germanium, and the second lens is made of zinc selenide.

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 lenses 2 and 3 alternately redesigns it into its image plane, and the last 4th lens redesigns the intermediate image after the 3rd lens in its image plane, which coincides with the focal plane of the entire lens.

An illustration of the proposed utility model is a fast lens for the IR spectral region (Δλ = 8 ÷ 12.5) μm with the following parameters:

- Focal length - f '= 30.66 mm

- Relative hole D: F = 1: 0.75

- Angular field of view 2ω = 25 °

- Back section S '_F' = 0.5f '

- The length of the lens (L) is the distance from the first surface of the first lens to the focal plane L = 2f '

ϕ ₁ /ϕ=0.58

ϕ ₂ /ϕ=-0.18

ϕ ₃ /ϕ=-2.45.10 ^-3

1ϕ ₄ /ϕ=1.14

where ϕ ₁ , ϕ ₂ , ϕ ₃ , ϕ ₄ , ϕ are the optical forces of the first, second, third, fourth lenses, and the lens as a whole, respectively.

- Image quality:

- The concentration of energy in the pixel size (17 × 17) microns

in the center of the field of view - 72%

at the edge of the field of view - 65%

- Modulation transmission coefficient (KPM) at a frequency of 30 mm ^-1 :

in the center of the field of view - 54%

on the edge of the field of view (mer / sag) - (51/47)%

The equation of surface 7 (the convex surface of the meniscus 4) describes an even curve of higher orders (rotation ellipsoid):

,

,

- кривизна поверхности (величина, обратная радиусу при вершине); k=-е² - коническая постоянная (эксцентриситет), a ₁, a ₂, a ₃, a ₄ - коэффициенты, значения которых приведены ниже.Where

- surface curvature (the reciprocal of the radius at the vertex); k = -e ² is the conical constant (eccentricity), a ₁ , a ₂ , a ₃ , a ₄ are the coefficients, the values of which are given below.

e ² = 0.2885, R ₀ = 42.3778, R bl.s.s. = 43.046, maximum asphericity 0.0169 mm

a ₁ = 0, and ₂ = -4.3514 E-007, and ₃ = 5.6487 E-009, and ₄ = -3.2318 E-011, and ₅ = 8.4776 E-014, and ₆ = -8.5932 E-017, and ₇ = 0

Thus, in the proposed lens, an increase in the relative aperture to 1: 0.75 is achieved while ensuring high image quality throughout the field.

INFORMATION SOURCES

1. Russian Federation, copyright certificate No. 1714562, IPC: G02B 13/34, 1992

2. Russian Federation, patent for utility model No. 170736, IPC: G02B 13/34, 2006.01. - prototype.

Claims (6)

A fast lens for the infrared region of the spectrum containing four sequentially mounted lenses, the first of which is the positive meniscus facing the convexity of the image, the second is the negative meniscus convex to the image, the third is the meniscus convex to the image, and the fourth is the positive meniscus the fact that the third meniscus, convex to the image, is negative, and the fourth positive meniscus, made with a convex surface in the form of an elongated ellipse psoida higher order of rotation and facing concavity to the image, and the optical power of lenses satisfy the conditions: 0.5 <ϕ ₁ / ϕ <0.7 0.1 <| ϕ ₂ / ϕ | <0.3 | ϕ ₃ / ϕ | <5⋅10 ^-3 1.0 <ϕ ₄ / ϕ <1.3, where ϕ ₁ , ϕ ₂ , ϕ ₃ , ϕ ₄ , ϕ are the optical forces of the first, second, third, fourth lenses and the lens as a whole, in addition, the first, third and fourth lenses are made of optical germanium, and the second lens from zinc selenide.

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