RU2586394C1 - Objective lens for infrared spectrum - Google Patents

Abstract

FIELD: optics.

SUBSTANCE: lens can be used in thermal imagers sensitive in spectral range from 8 to 12 mqm. Lens has four menisci, from which first, second and fourth menisci on beam path are positive, and third one is negative. All meniscus by concave surfaces face image plane. First and fourth menisci are made from germanium, and second and third are from zinc selenide. Third meniscus is made movable along optical axis. Following ratios are implemented: φ₁:φ₂:φ₃:φ₄=(0.75÷0.83):(0.3÷0.64):-(0.69÷1.5):(1.30÷1.77), where φ₁, φ₂, φ₃, φ₄ is relative optical power of first, second, third and fourth menisci, D2/f′=0.18÷0.42; D6/f=0.34÷0.60, where D2 is air gap between first and second menisci, f′ is equivalent focal distance of lens; D6 is air gap between third and fourth menisci.

EFFECT: high aperture ratio of lens while maintaining high image contrast and constant focal distance in range of temperatures from -40 to +50 °C.

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Description

The invention relates to the field of optical instrumentation, namely to lenses for the infrared (IR) region of the spectrum, and can be used in thermal imagers built on the basis of matrix photodetector devices sensitive in the spectral range from 8 to 12 microns.

где

- фокусные расстояния первого, второго, третьего и четвертого компонентов соответственно, f′ - эквивалентное фокусное расстояние всего объектива. Фокусировка объектива на конечное расстояние и компенсация смещения плоскости изображения в диапазоне рабочих температур от -40°C до +50°C достигается при выполнении отрицательной линзы, подвижной вдоль оптической оси.A known infrared lens according to RF patent No. 2365952 of February 13, 2008, contains four components, the first of which is a positive meniscus convex to the object, the second is a negative lens, the third is a positive lens, and the fourth is a positive meniscus, facing concavity to the image. The menisci are made of germanium, the negative and positive lenses are made of oxygen-free glass. The focal lengths of the components satisfy the following conditions:

Where

are the focal lengths of the first, second, third, and fourth components, respectively, f ′ is the equivalent focal length of the entire lens. Focusing the lens at a finite distance and compensating for the displacement of the image plane in the operating temperature range from -40 ° C to + 50 ° C is achieved by performing a negative lens moving along the optical axis.

The specified lens has a high relative aperture of 1: 1 and good image quality at a temperature of 20 ° C. However, in the temperature range from -40 ° C to + 50 ° C, the image contrast is significantly reduced. The lens with f = 100 mm has a large length along the optical axis, equal to 145 mm. In addition, focusing the lens at a finite distance and compensating for the displacement of the image plane in the operating temperature range leads to a change in the focal length, which in turn will lead to a change in the scale of the electronic grid scale on the photodetector matrix.

Closest to the claimed technical solution - the prototype - is a lens for the infrared region of the spectrum according to the patent of the Russian Federation No. 115514 of 01/11/2012, IPC G02B 13/14. The lens contains four menisci, of which the second is negative, the rest are positive, the first and fourth menisci are made of germanium and face the image plane with their concave surfaces, the second and third menisci are made of zinc selenide and face the image plane with their convex surfaces, the third and the fourth menisci are installed with the possibility of simultaneous movement along the optical axis, between the relative optical forces of the menisci, the following relations hold: φ ₁ : φ ₂ : φ ₃ : φ ₄ = (0.75 ÷ 0.85) :-( 2.0 ÷ 2.5) :-( 1.40 ÷ 1.66) :( 1.7 ÷ 1.9), where φ ₁ , φ ₂ , φ ₃ , φ _{4 are the} relative optical powers of the first, second, third, and fourth menisci, respectively.

For comparison, the specified lens is calculated at a focal length of 75 mm. The lens has good image contrast over the entire temperature range from -40 ° C to + 50 ° C. Moreover, the lens has the following disadvantages:

1. Low relative aperture equal to 1: 1.3.

2. With thermal compensation in the operating temperature range from -40 ° C to + 50 ° C and focusing at a finite distance by moving the third and fourth menisci together, the lens focal length changes by 1.7%.

The objective of the invention is to achieve the following technical results: increasing the relative aperture of the lens while maintaining a high image contrast and ensuring the invariability of the focal length of the lens in the temperature range from -40 ° C to + 50 ° C.

The specified technical results are achieved as follows.

The lens for the infrared region of the spectrum, like the prototype, contains four menisci, of which the first and fourth menisci along the ray are positive, with their concave surfaces facing the image plane, the first and fourth menisci made of germanium, and the second and third of zinc selenide . In contrast to the prototype in the inventive lens, the following is fulfilled. The second meniscus is made positive and faces with a concave surface toward the image plane; the third meniscus is made negative and faces with a concave surface toward the image plane. The third meniscus is mounted to move along the optical axis. The following relationships are true:

φ ₁ : φ ₂ : φ ₃ : φ ₄ = (0.75 ÷ 0.83) :( 0.3 ÷ 0.64) :-( 0.69 ÷ 1.5) :( 1.30 ÷ 1.77) ,

where φ ₁ , φ ₂ , φ ₃ , φ ₄ are the relative optical forces of the first, second, third and fourth menisci, respectively;

D2 / f ′ = 0.18 ÷ 0.42,

D6 / f ′ = 0.34 ÷ 0.60,

where D2 is the air gap between the first and second menisci;

f ′ is the equivalent focal length of the lens;

D6 - air gap between the third and fourth menisci.

An example of a specific implementation of the lens is shown in the drawings.

In FIG. 1 shows an optical diagram of a lens with a real path of rays for the axial point of the field of view.

In FIG. Figure 2 shows the point scattering function.

In FIG. Figure 3 shows the contrast of the image and the function of energy concentration.

The lens for the infrared region of the spectrum (Fig. 1) contains four meniscus. Meniscus 1 - positive, made of Germany. Meniscus 2 - positive, made of zinc selenide (ZnSe). Meniscus 3 - negative, made of zinc selenide. Meniscus 4 - positive, made of Germany. All menisci face a concave surface to the image plane. The meniscus 3 is arranged to move along the optical axis to focus the lens at a finite distance and compensate for the displacement of the image plane 5 in the range of operating temperatures from -40 ° C to + 50 ° C.

Table 1 shows the characteristics of the lens, considered as an example of the implementation of the inventive lens. The entrance pupil is located on the first surface of the lens.

Optical characteristics of the lens:

1. Focal length 75 mm 2. Relative bore 1: 1 3. Field of view 7.3 ° × 5.5 ° 4. Spectral range 8-12 microns 5. Lens Length 96.5 mm 6. The back section 13.14 mm

The lens structural elements shown in Table 1 provide the following relationships between the relative optical powers of menisci 1, 2, 3, 4: φ ₁ : φ ₂ : φ ₃ : φ ₄ = 0.79: 0.37: -0.78: 1.36

Where

f ′ is the equivalent focal length of the entire lens;

- фокусные расстояния соответственно менисков 1, 2, 3, 4. При этом воздушные промежутки D2 и D6 соответственно между менисками 1 и 2, 3 и 4 равны: D2=0,25f′, D6=0,52f′.

- focal lengths respectively of menisci 1, 2, 3, 4. Moreover, the air gaps D2 and D6 respectively between menisci 1 and 2, 3 and 4 are equal: D2 = 0.25f ′, D6 = 0.52f ′.

The lens works as follows.

Beams of rays from an object sequentially pass through menisci 1, 2, 3, 4 and build an image in the image plane 5. To obtain a high quality image in the temperature range, meniscus 3 moves along the axis, while the air gaps D4 change - between menisci 2 and 3, and D6 - between menisci 3 and 4. The same movement is used to focus the lens at a finite distance.

The main idea of the constructive implementation of the lens is to reduce the meniscus optical forces, their steepness and location. This is necessary in order to reduce the angle of incidence of the rays with respect to the normal to the surfaces of the menisci. The lens calculations were performed under the assumption that the lens body is made of aluminum.

Changes in the optical forces of the menisci φ ₁ , φ ₂ , φ ₃ , φ ₄ in the ranges respectively (0.75 ÷ 0.83), (0.3 ÷ 0.64), - (0.69 ÷ 1.5), ( 1.30 ÷ 1.77) is accompanied by a change in the air gaps D2, D6 in the intervals: D2 = (0.18 ÷ 0.42) f ′, D6 = (0.34 ÷ 0.60) f ′.

The value of the air gap D4 between menisci 2 and 3 affects the lens parameters as follows.

By increasing D4, the image quality of the lens decreases, and by decreasing it is possible to “zoom in” the meniscus frames 2 and 3 on top of each other. Therefore, the value of the air gap D4 is selected for structural reasons and lies within the limits determined from the ratio D4 / f ′ = 0.08 ± 10%.

Regarding the length of the lens L and the rear segment S ′ (Fig. 1), the following is true. The larger L and less S ′, the higher the image quality and more potential for further increasing the relative aperture. But in the first case, the dimensions and weight of the lens increase, and in the second, problems arise with the placement of the mechanical body of the photodetector in the rear segment of the lens.

Let us consider the characteristics of the image quality of the lens, namely the point scattering function, which clearly demonstrates the topology of scattering spots in the geometric approximation (Fig. 2), image contrast and the energy concentration function (PCE), which makes it possible to determine the scattering diffraction circle (Fig. 3).

In FIG. 2, the first column gives the topology of the scattering circles for 20 ° C, in the second column for -40 ° C, and in the third for + 50 ° C. The first line contains scattering circles for the axial point of the field of view, the second for the zone, and the third for the edge of the field of view, i.e. diagonally 7.3 ° × 5.5 °. The square size is 100 microns. In addition, a diffraction (non-aberrational) Airy circle with a diameter of 26 μm for a relative aperture of 1: 1 (for a relative aperture of 1: 1.3 this circle is 33.4 μm) is imprinted on each scattering spot. This circle contains 83.4% of energy. As can be seen from FIG. 2, all scattering spots practically fit into the Airy circle, which clearly demonstrates the high image quality in the geometric approximation.

In FIG. 3. On the left is the image contrast at a frequency of 20 mm ^-1 , and on the right is a function of energy concentration for the entire temperature range. The temperature values are printed in the field of the corresponding graphs. According to the Nyquist criterion for the expected scattering circles of 0.025 mm, the image contrast at a frequency of 20 mm ^-1 should be at least 0.6, which follows from FIG. 3. A detailed examination of the FEC graphs made it possible to determine the diameter of the scattering circles at 80% energy concentration, the values of which are imprinted in the upper part of the squares in FIG. 2. Consideration of these values allows us to conclude that the presented lens has a diffractive image quality with a relative aperture of 1: 1.

An essential circumstance is the method of athermalization of the lens and its focusing to a finite distance. In the prototype for this purpose, movement along the axis of two positive lenses is used, the equivalent optical power of which is large. In the present invention, the movable lens 3 has an optical power (in absolute value) of about eight times less. On the one hand, this allows one to increase ten times the tolerance for its decentration with the retraction of the line of sight of not more than one angular minute, and on the other, to prevent a change in focal length when moving it. This is also due to the fact that athermalization and focusing of the lens at a finite distance is carried out not by the positive, but by the negative component. The change in focal length is 0.05%, i.e. almost constantly. In the prototype, this change is 1.7%.

At a temperature of 50 ° C, meniscus 3 moves 0.35 mm toward meniscus 2, and at -40 ° C it moves 0.71 mm toward meniscus 4. When the lens is focused at a final distance of 15 m under normal climatic conditions, t. e. at 20 ° C, meniscus 3 moves 1 mm towards meniscus 4.

The calculations show that the technical results are achieved for the ratios φ ₁ : φ ₂ : φ ₃ : φ ₄ , D2 / f ′ and D6 / f ′ within all of the claimed ranges.

Thus, the invention in comparison with the prototype allows to increase the relative aperture of the lens while maintaining a high image contrast and to ensure the invariability of the focal length in the temperature range from -40 ° C to + 50 ° C.

Claims (1)

A lens for the infrared region of the spectrum containing four menisci, of which the first and fourth menisci along the beam are positive, with their concave surfaces facing the image plane, the first and fourth menisci made of germanium, and the second and third of zinc selenide, characterized in that the second meniscus is made positive and faces with a concave surface to the image plane, the third meniscus is negative, faces with a concave surface and the image plane and is mounted with the possibility of moving along the opt axis, while the following relationships are true:
φ_one: φ₂: φ₃: φ_four= (0.75 ÷ 0.83) :( 0.3 ÷ 0.64) :-( 0.69 ÷ 1.5) :( 1.30 ÷ 1.77),
where φ_one, φ₂, φ₃, φ_four - relative optical powers of the first, second, third and fourth menisci, respectively;
D2 / f ′ = 0.18 ÷ 0.42,
D6 / f ′ = 0.34 ÷ 0.60,
where D2 is the air gap between the first and second menisci;
f ′ is the equivalent focal length of the lens;
D6 - air gap between the third and fourth menisci.

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