RU2449327C1 - High-aperture lens for infrared region - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: lens can be used in thermal imaging devices whose receivers are sensitive in the infrared region, particularly in the λ=8-14 mcm region. The lens comprises three meniscus lenses and a further biconvex lens between the first and second meniscus lenses. The first lens is a diverging lens and faces the object with its concave surface. The second and third lenses are converging lenses, face the object with their convex surfaces and are spaced apart by an air space, the value of which is not less than 0.8 times the equivalent focal distance of the lens. The lens satisfies the following relationships: 0.3 < |φ₁/φ_eq| < 0.4, 0.3 < φ₂/φ_eq < 0.4, 0.7 < φ₃/φ_eq ^< 0.8, 0.4 < φ₄/φ_eq < 0.5, where φ₁ φ₂, φ₃ denote focal power of the 1^st, 2^nd and 3^rd meniscus lenses, φ₄ is the focal power of the additional biconvex lens and φ_eq is the optical power of the lens.

EFFECT: high aperture ratio while ensuring high quality of the image on the entire field.

1 dwg, 1 app

Description

The invention 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 λ = 8-14 μm.

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

The following requirements are imposed on lenses operating in the spectrum range λ = 8-14 μm:

1. Ultrahigh relative aperture constituting

D: F = 1: 0.75 ÷ 1: 1.

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

2. High, close to diffractive, image quality. The size of the matrix element is Q = 0.035 × 0.035 mm. For IR devices, it is necessary that the energy concentration η in the pixel size be at least 75%, while an aberration-free ideal system yields 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 = 15 lines / mm be at least 0.6.

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.

3. The minimum number of lens elements

The lenses of IR lenses are made mainly from optical single-crystal germanium having a large refractive index (n = 4), a large specific gravity (γ = 5.33 g / cm ³ ) and high cost. Minimizing the number of lens elements reduces the weight of the lens and its cost.

5. Dimensional parameters include the requirement determined by the design of the receiver - the back focal segment S '_f' must satisfy the condition S '_f' ≥0.35f '. In addition, it is desirable that the length of the lens is minimal. It is important to keep in mind that in the design of a number of IR receivers the bolometric matrix is “recessed”, i.e. located in the back of the receiver, the hole in which limits the diameter of the light beams incident on the matrix.

6. An important problem in creating IR lenses using germanium is a significant change in its refractive index with temperature, which leads to defocusing and, consequently, to a deterioration in image quality. 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 for a fast IR lens is an urgent task.

Known designs of optical circuits of IR lenses that satisfy the above requirements. Many of them consist of two or three lenses with a significant aperture ratio (aperture value close to 1), often containing aspherical surfaces [1, 2]. In lenses that do not have aspherical surfaces, in order to ensure the required high image quality, it is necessary to reduce the relative aperture and angular field of view, for example [3]. In this regard, the task of creating a lens for IR systems [4], which would ensure high image quality with a large relative aperture and angular field of view without the use of technologically complex aspherical surfaces, becomes urgent.

Closest to the claimed technical solution is a fast IR lens [5], consisting of three meniscus lenses, the first of which with negative optical power, with a concave surface facing the object, the second and third lenses with positive optical forces, with convex surfaces facing the object, separated by an air gap, the value of which is not less than 0.8 equivalent focal length of the lens. The optical power of the lenses satisfy the conditions:

0.15 <| φ ₁ / φ _equiv | <0.2,

0.6 <φ ₂ / φ _equiv <0.7,

0.9 <φ ₃ / φ _equiv <1.1,

0.4 <φ _1.2 / φ _equiv <0.6,

0.8≤dφ _equiv ≤0.9,

where φ ₁ , φ ₂ , φ ₃ , are the optical powers of 1, 2, 3 lenses,

φ _1,2, - the optical power of 1 and 2 lenses,

φ _equiv - the optical power of the lens,

d is the distance between 2 and 3 lenses.

The disadvantages of this lens include a low relative aperture - D: F '= 1: 1.07.

The main task, the solution of which the invention is directed, is to increase the relative aperture while ensuring high image quality throughout the field.

To solve this problem, a fast lens for the IR spectral region is proposed, which, like the prototype, consists of three meniscus lenses, of which the first lens has a negative optical power φ ₁ , with a concave surface facing the object, and the second meniscus lens has a positive optical power φ _2, and the third meniscus lens - with positive optical power of the lens φ _3, with convex surfaces facing the object, separated by an air gap, the value of which is not less than 0.8 equivalent fokusnog of the lens.

In contrast to the prototype, an additional biconvex lens with a positive optical power of φ _{4 is} installed in the proposed high-aperture lens for the IR spectral region between the first and second meniscus lenses. The following relationships are true:

0.3 <| φ ₁ / φ _equiv | <0.4,

0.3 <φ ₂ / φ _equiv <0.4,

0.7 <φ ₃ / φ _equiv <0.8,

0.4 <φ ₄ / φ _equiv <0.5,

where φ ₁ , φ ₂ , φ ₃ , are the optical powers of the first, second and third meniscus lenses,

φ ₄ - the optical power of an additional biconvex lens,

φ _equiv - the optical power of the lens.

The essence of the invention lies in the fact that the resulting ratio of the optical power of the lenses allows for the correction of axial and field aberrations. An additional biconvex lens introduces positive spherical aberration, astigmatism, field curvature and negative coma, compensating for negative spherical aberration, astigmatism, field curvature and positive coma of the first meniscus lens. In addition, the first group of lenses, consisting of two meniscus lenses and a biconvex lens, acts as a single force-correction element that simultaneously compensates for higher order aberrations of wide beams when the relative aperture is increased to 1: 0.75.

Another advantage of the proposed scheme is that, despite a significant increase in the output aperture, the size of the posterior segment in it is at least 0.35f ' _eq .

Thus, the proposed scheme of the IR lens with a relative aperture D: F '= 1: 0.75 and an angular field of 2ω = 25 ° ensures high image quality over the entire field.

The essence of the claimed invention is illustrated by the drawing, where Fig. 1 shows the optical scheme of the lens, and the Appendix, which shows the design parameters and optical characteristics of a particular sample.

The high-aperture lens for the infrared region of the spectrum consists of four lenses, the first of which 1 is negative and has a concave surface facing the object, the second 2 and third 3 are positive meniscus lenses with convex surfaces facing the object, between the first 1 and second 2 meniscus lenses there is an additional biconvex positive lens 4, while the following relations are true:

0.3 <| φ ₁ / φ _equiv | <0.4,

0.3 <φ ₂ / φ _equiv <0.4,

0.7 <φ ₃ / φ _equiv <0.8,

0.4 <φ ₄ / φ _equiv <0.5,

where φ ₁ , φ ₂ , φ ₃ , are the optical powers of the 1st, 2nd, and 3rd meniscus lenses,

φ ₄ - the optical power of an additional biconvex lens,

φ _equiv - the optical power of the lens.

The aperture diaphragm 5 is located near the first surface of the positive meniscus lens 3, providing it with a small light diameter.

Behind the positive meniscus lens 3 is a receiver 6.

The lens is as follows.

A parallel beam of radiation from an infinitely distant object is focused in the imaginary rear focus of the meniscus lens 1. A positive additional biconvex lens 4 transfers the imaginary image to the real plane, i.e. the beam diverging after the meniscus lens 1 propagates after the positive additional biconvex lens 4 converges, falling on the positive meniscus lenses 2 and 3 with the smallest angles to the normals with the surfaces of these lenses, and forms an image in the plane coinciding with the rear focus F 'of the entire lens.

The Appendix contains a lens with the following optical powers of the lenses:

φ ₁ = -0.346,

φ ₂ = 0.311,

φ ₃ = 0.776,

φ ₄ = 0.487.

Lens Options:

- relative aperture D: F '= 1: 0.75,

- angle of view 2ω = 25 °,

- focal length f ' _eq = 38.55 mm,

- back focal segment S'F '= 14.96 mm,

- light diameter of the lens 3 ⌀ = 25 mm,

- the energy concentration in the spot corresponding to the pixel size (0.035 × 0.035), more than 75% throughout the field,

- modulation transmission coefficient for the spatial frequency N = 15 mm ^-1 over 50% throughout the field,

- when calculating the protective glass of the receiver 6 was taken into account.

Thus, in the proposed lens, an increase in the relative aperture to 1: 0.75 is achieved with high image quality, a large rear segment and a small diameter of the lens closest to the image plane.

If the prototype works with a receiver having a pixel size of 0.045 × 0.045 mm, the proposed lens is designed for a new bolometric matrix with a pixel size of 0.035 × 0.035 mm, and accordingly, the image quality characteristics (energy concentration in the pixel and PDA) of the proposed lens are higher than at the prototype.

Information sources

1. US patent No. 6292293, IPC G02 1/00, from 18.09.2001.

2. US Patent No. 623501, IPC G02 1/00, dated 05.22.2002.

3. RF patent No. 2187135, IPC G02 13/34, dated 10.08.2002.

4. RF patent No. 2183340, IPC G02 13/34, dated 10.06.2002.

5. RF patent №52488, IPC G02 13/34, dated 19.10.2005, - prototype.

Claims (1)

A fast lens for the infrared region of the spectrum, consisting of three meniscus lenses, the first of which is a lens with negative optical power with a concave surface facing the object, the second and third lenses with positive optical forces, with convex surfaces facing the object, separated by an air gap, the magnitude of which is at least 0.8 equivalent focal lengths of the lens, characterized in that an additional biconvex polo is installed between the first and second meniscus lenses itelnaya lens, wherein the following relations:
0.3 <| φ ₁ / φ _eq | <0.4,
0.3 <φ ₂ / φ _equiv <0.4,
0.7 <φ ₃ / φ _equiv <0.8,
0.4 <φ ₄ / φ _equiv <0.5,
where φ ₁ , φ ₂ , φ ₃ , are the optical powers of the 1st, 2nd, and 3rd meniscus lenses;
φ ₄ - the optical power of an additional biconvex lens;
φ _equiv - the optical power of the lens.

Images