RU170736U1 - LIGHT LIGHT FOR INFRARED SPECTRUM - Google Patents

Abstract

The lens can be used in thermal imaging devices. The lens contains four menisci, the first of which is the positive meniscus, convex to the image, the second is the negative meniscus, convex to the image, the fourth is the positive meniscus, convex to the object. Radii of curvature of the surfaces the lenses and the distances between the lenses satisfy the conditions: where dd are the distances between the first and second and second and third lenses, respectively; R ÷ R are the radii of curvature of the lenses, numbered along the beam; f 'is the equivalent focal length of the entire lens. The technical result is an increase in the relative aperture and angular field of view while ensuring high image quality throughout the field. 4 ill., 1 app.

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 spectral region of 8-14 microns allow observing objects whose radiation temperature is -50 ÷ 50 ° C, 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: 0.8 ÷ 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 °.

3. High, close to diffractive, image quality. The size of the matrix element is Q = 25 × 25 μm. For infrared thermal imaging devices, it is necessary that in the pixel size the energy concentration η be at least 75%, while an aberration-free ideal system gives η = 89 ÷ 90%.

For thermal imagers that form an image 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 = 20 lin / mm be at least 0.6.

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.

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 material commonly used in IR lenses - optical germanium - 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. 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 '} ≥0.5 ÷ 1,0 f '

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, the lens [1] has a relatively small aperture (D: F <1: 1.65), an angular field of view of 2ω = 10 ° with low image quality (the transverse aberration of a wide inclined beam in the meridional section for ω = 5 ° is 0.49 mm, astigmatism - 0.22 mm).

In lenses with spherical lens surfaces, in order to ensure the required level of image quality, it is usually necessary to reduce the relative aperture or introduce vignetting for field field points [2, 3 and 4].

Closest to the claimed technical solution is a fast IR lens [5]. 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 of the image, the third lens is a convex plane 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 segment S '= 45.23,

- the transverse aberration of a wide inclined beam in the meridional section for an angular field ω = 9 ° is 0.081 mm, astigmatism is 0.014 mm, 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 small values of the energy concentration in the square corresponding to the pixel of the receiver.

The main task, which the proposed utility model is aimed at, is to increase the relative aperture and angular field of view while ensuring high image quality throughout the field.

To solve this problem, a fast lens for the infrared region of the spectrum is proposed, which, like the prototype, consists of four lenses, the first of which is made in the form of a positive meniscus facing the concavity to the image, the second lens is negative, the third lens is positive, and the fourth is positive meniscus convex to the subject.

Unlike the prototype, in the proposed high-aperture lens for the IR region of the spectrum, the second negative lens is made in the form of a meniscus convex to the image, the third positive lens is made in the form of a meniscus convex to the image, while the radii of curvature (R) of the lens surfaces and the distance (d) between the lenses satisfy the conditions:

where d ₁ , d ₂ are the distances between the first and second and second and third lenses, respectively;

R ₁ ÷ R ₈ are the radii of curvature of the lenses, numbered along the beam;

f is the equivalent focal length of the entire lens.

The essence of the proposed utility model is as follows. The aberrations of the optical system are characterized by third-order aberration coefficients - 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 implementation of the second lens in the form of a positive meniscus, convex to the image, and providing conditions (1) and (3) leads to the fact that the first and second lenses are concave surfaces facing each other. In this case, the spherical aberration of the first two lenses S _{I (1,2)} has a value less than (-2.0), coma S _{II (1,2) is} positive and equal to ≈ (0.1 ÷ 0.15), the magnitude of astigmatism S _{III (1,2)} and the curvature of the image S _{IV (1,2) of} different signs and do not exceed values (-0.05), the distortion S _{V (1,2) is} negative and no more (0.05 ÷ 0.11).

The implementation of the third lens in the form of a positive meniscus, convex to the image, and ensuring relations (2) and (4) leads to the fact that the third and fourth lenses are convex surfaces facing each other. In this case, the aberration coefficients of the last two lenses take values close in magnitude and opposite in signs of the first and second lenses: S _{I (3,4)} = + (2 ÷ 2.5), S _{II (3,4)} = - (0.1 ÷ 0.15 ), S _{III (3.4)} = -S _{IV (3.4)} = -0.15, S _{V (3.4)} = + (0.05 ÷ 0.1).

Thus, the aberrations of the first two lenses are compensated by the aberrations of the third and fourth lenses. In addition, the obtained ratios of the radii of the lenses and air gaps made it possible to minimize aberrations of higher orders and, thereby, ensure a high relative aperture.

Important advantages of the proposed lens are:

- Large posterior segment S '_F' ≥ (0.6 ÷ 0.7) f ', obtained due to the selected air gaps and the second lens in the form of a negative meniscus, convex towards the image.

- Ensuring close to the telecentric course of the main rays in the image space, and thereby, obtaining uniform illumination throughout the field, by combining the entrance pupil with the front focal plane of the lens.

- Minimization of the scattered illumination of the receiver due to reflection from the last surface of the lens (radius R ₈ ) of the radiation, the source of which is the self-radiation of the receiver matrix, due to the value of the radius R ₈ exceeding the rear segment S '_F' .

An illustration of the proposed utility model is a fast lens for the IR spectral region, consisting of four lenses made of germanium, operating in the spectrum range λ = 8 ÷ 12.5 μm with the following parameters:

- Relative hole D: F = 1: 0.8,

- Angular field of view 2ω = 25 °,

- Back focal length S '_F' ≥0.75f ',

- The telecentric course 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 pixel size (25 × 25) microns

in center field ≥0.8 on the edge of the field ≥0.7

- KPM at a frequency of 20 mm ^-1

in center field ≥70% on the edge of the field ≥52%

- Transverse aberration of a wide inclined beam at the edge of the field

in the meridional section ≤0.03 mm in sagittal section ≤0.02 mm

Thus, the proposed lens with the same number of lenses as the prototype provides significantly better lens parameters (relative aperture, angular field, rear focal length) with high image quality.

The essence of the claimed utility model is illustrated by the drawing, where in FIG. 1 - presents an optical diagram of the lens, and the Appendix, which shows the design parameters and optical characteristics of a particular sample and drawings, where in 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 25 × 25 microns in size 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 positive the lens 3 is made in the form of a meniscus, convex to the image, and the fourth positive lens 4 is made in the form of a meniscus, convex to the object.

Lenses 3 and 4 are made with positive optical forces and are convex to each other, in addition, lens 2 is installed at a distance d ₁ from lens 1, and lens 3 at a distance d ₂ from lens 2, the following relations are true:

where d ₁ , d ₂ are the distances between the first and second and second and third lenses, respectively,

f 'is the equivalent focal length of the entire lens.

The ratio of the radii of curvature of the surfaces R ₁ R ₂ for lens 1, R ₃ , R ₄ for the second, R ₅ , R ₆ for the third, R ₇ , R ₈ for the fourth lens satisfies the relations:

Aperture diaphragm 5, located on the second surface of the first lens, is located near the front focus of the lens.

The lens operation is as follows:

A parallel beam of radiation from a distant object is focused at the rear focus of meniscus 1, a negative meniscus 2 transfers it to the space of the meniscus 3 object, and then menisci 3 and 4 redesign it into the image plane coinciding with the back focus F 'of the entire lens.

The Appendix shows a lens with the following parameters:

- relative hole D: F = 1: 0.8, - Angular field of view 2ω = 25 °,

- Focal length f '= 28.0

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Thus, the proposed lens provides significantly better lens parameters - an increase in the relative aperture and angular field of view while ensuring high image quality throughout the field.

INFORMATION SOURCES

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

2. United States Patent No. 6292293, IPC: G02B 1/00, 2001

3. United States Patent No. 6236501, IPC: G02B 1/00, 2002

4. Russian Federation patent No. 2187135, IPC: G02B 13/34, 2002

5. Russian Federation, patent No. 2183340, IPC: G02B 13/34, 2002 - prototype.

APPENDIX

Focal Length, mm: F '= 28.00 Aperture Number: F '/ D = 0.80 Diameter of the entrance pupil, mm: Dp = 35.00 The position of the entrance pupil, mm: Sp = 0.80 Diameter of exit pupil, mm: D'p = 106.84 The position of the exit pupil, mm: S'p = 92.25 The linear value of the image, mm: 2y '= 12.00 Angular field of view: 2ω = 24.50 ° Spectral range, microns: λ = 8.0 ÷ 12.5

Claims (8)

Fast lens for the infrared region of the spectrum, containing four sequentially mounted lenses, the first of which is a positive meniscus facing concavity to the image, the second is a negative lens, the third is a positive lens, the fourth is a positive meniscus convex to the subject, characterized in that the second negative the lens is made in the form of a meniscus, convex to the image, the third positive lens is made in the form of a meniscus, convex to the image, with the radius curvature (R) lens surface and the distance (d) between the lenses satisfy the following conditions:

where d ₁ , d ₂ are the distances between the first and second and second and third lenses, respectively; R ₁ ÷ R ₈ are the radii of curvature of the lenses, numbered along the beam; f 'is the equivalent focal length of the entire lens.

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