RU2365952C1 - Infrared objective - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: objective can use in thermal imagers with micro bolometric matrixes of sensitive elements. The objective contains four components: positive meniscus lens 1 turned to an object by its convex, negative lens 2, positive lens 3 and positive meniscus lens 4 turned to an image by its concavity. Meniscuses 1, 4 are executed from germanium, lenses 2, 3 - from oxygen-free glass. Ratios are carried out: f'₁/f'=1.1÷1.3, f'₂/f'=-0.69÷0.74, f'₃/f'=0.84÷0.98, f'₄/f'=0.93÷1.6, where f'₁, f'₂, f'₃, f'₄ - focal lengths of components 1, 2, 3, 4, accordingly, f' - an equivalent focal length of the whole objective. Lens 2 can be mounted with possibility of moving along an axis for alignment of an objective, focusing on final distance, shift thermoindemnification of an adjustment plane.

EFFECT: objective image enhancement at the big relative aperture and field of vision, reduction of its length and increase in a back piece for convenient interface of its plane of the image to a plane of sensitive elements of a matrix.

3 cl, 4 dwg

Description

The invention relates to the field of optics and can be used in thermal imagers with photodetectors made in the form of a microbolometric matrix (MBM) of sensitive elements that do not require cooling to cryogenic temperatures.

Modern MBMs have a size of 10 × 13 mm, which imposes certain requirements on infrared lenses for viewing angles. The main problem in designing an IR lens in the 8-12 μm spectrum region is the very small pixel size of the MBM matrix (sensitive element). In modern MBM, the pixel diameter is 0.025 mm. For such MBM, the relative aperture calculated by the formula 3.12 [1] should be not lower than 1: 1. Currently, the scattering circle is determined at 80% energy concentration (written as "D 80%"), which meets the Rayleigh criterion. Another criterion for image quality is the frequency-contrast characteristic (Polychromatic diffraction MTF) - frequency response. For MBM with a small pixel diameter, the contrast T of the lens image should be within 0.70–0.75 at a spatial frequency of 15 mm ⁻¹ . The diffraction limit is T = 0.8.

Due to the great importance of image quality characteristics, we analyzed each presented lens according to the declared design parameters. The calculations were performed on the Optical Design Program "ZEMAX", Focus Software, Incorporated. Version: May 16, 2002.

A known infrared lens (8-15 μm) containing four components, the first of which is a positive lens, the second is a negative meniscus, the third is a positive lens, and the fourth is a concave-flat lens [2]. The third lens is made from germanium, the rest from zinc sulfide (ZnS). This choice of materials is due to the need to compensate for defocusing the image when the ambient temperature changes. The lens has an angular field of view of 12 °, a relative aperture of 1: 2, with a focus of 100 mm, the length along the axis is 152 mm.

The lens has the following disadvantages.

1. Small relative aperture. With an ideal correction of geometric aberrations, the scattering circle at the level of 80% energy concentration (D 80%) will be 0.04 mm.

2. The image plane coincides with the back surface of the last objective lens, the rear segment is equal to zero, which leads to the difficulty of optical pairing of the lens with a photodetector - MBM.

3. Analysis of image quality showed that even with a zero field of view, D 80% = 0.072 mm, which leads to a significant deterioration in image quality.

A known lens for the infrared region of the spectrum (8-12 microns), made according to the Petzval scheme [3]. The lens contains two components, the first of which consists of a positive and negative meniscus, and the second of a positive and negative lens. The positive meniscus and the positive lens are made of BSA material from the English company Barr & STROUD (Russian analogue of IKS-25 glass), the second from zinc selenide (ZnSe), and the fourth from Germany. This choice of materials is due to the need to compensate for defocusing the image when the ambient temperature changes, but at the same time it leads to a large increase in chromaticity even for small angles of the field of view. The lens has a field of view angle of 5 °, relative aperture 1: 1.5.

This lens has a sufficient rear segment (27.5 mm) and a small length (144 mm), however, a verification calculation showed the following.

1. The scattering diffraction circle D 80% is 0.028 mm for the axial point of the field of view and 0.050 mm for the edge of the field of view, which will lead to illumination of neighboring elements of the matrix and degrade image quality.

2. The lens has a small field of view, which will not allow to illuminate the matrix over the entire working surface.

3. The lens has an insufficiently high relative aperture, which reduces image contrast.

The closest in technical essence to the claimed invention - the prototype - is a lens [4] for the infrared region of the spectrum (8-14 microns). The lens contains four components, the first of which is the positive meniscus, convex to the object, the second is the negative lens, the third is the positive lens, the fourth is the positive meniscus, the concavity to the image. All components are made from Germany. The focal lengths of the components listed in the Lens view database, U.S. Patent bibliographic data, satisfy the following requirements:

f ′ ₁ /f′=1.47,

f ′ ₂ /f′=-1.33,

f ′ ₃ /f′=0.81,

f ′ ₄ /f′=0.57.

Common with the claimed invention are the signs: 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, the fourth is a positive meniscus facing concavity to the image, the negative and positive lenses are made from Germany.

The specified lens has a field of view angle of 16.5 ° with a relative aperture of 1: 0.59. However, a verification calculation by design parameters (radii of curvature of the lens surfaces, thicknesses and air gaps) showed the following lens disadvantages:

1. Low image quality: already for the central points of the field of view, the diameter of the scattering circle is D 80% = 0.060-0.100 mm.

2. The length of the lens with a focal length of 100 mm is 176 mm, which indicates its large dimensions.

3. The size of the rear segment is close to zero, which creates certain difficulties when installing a photodetector, the sensitive elements of which are located inside the mechanical case (usually at a distance of 15-20 mm from the end).

4. All lens elements of the lens are made of temperature-sensitive germanium, which, when the temperature changes, leads to defocusing of the image and further deterioration of image quality.

The objective of the invention is to improve the image quality of the lens with a large relative aperture and field of view, reducing its length and increasing the rear segment for convenient pairing of its image plane with the plane of the sensitive elements of the matrix.

The problem is solved in that in the lens for the PC region of the spectrum containing four components, the first of which is the positive meniscus convex to the object, the second is the negative lens, the third is the positive lens, the fourth is the positive meniscus facing the concavity to the image, moreover, the menisci are made of germanium, the following is true: the negative and positive lenses are made of oxygen-free glass, and the focal lengths of the components satisfy the following conditions:

f ′ ₁ /f′=1.1rd1.3,

f ′ ₂ /f′=-0.69rd-0.74,

f ′ ₃ /f′=0.84 faith0.98,

f ′ ₄ /f′=0.93rd1.6,

where f ₁ , f ₂ , f ₃ , f ₄ are the focal lengths of the first, second, third and fourth components, respectively, f is the equivalent focal length of the entire lens.

As the material of the negative and positive lenses, oxygen-free glass of the IKS-25, IKS-28, IKS-31, IKS-32 (OST3-3441-83) grades can be used.

In special cases of implementation, for example, to focus the lens to a finite distance, to thermally compensate for the displacement of the lens mounting plane when the temperature is changed, to align the lens with a stationary photodetector, a negative lens can be mounted with the ability to move along the optical axis of the lens.

An example of a specific implementation of the lens is shown in the drawings. Figure 1 shows the optical diagram of the lens with the actual path of the rays for the axial and field points of the field of view, figure 2 - graphs of transverse aberrations, figure 3 - circles of scattering (for the center, zone and edge of the field of view), Fig. 4 - contrast image CCK.

The lens (Fig. 1) contains four optically connected components in series: the first is the positive meniscus 1, convex to the object, the second is the negative lens 2, the third is the positive lens 3, the fourth is the positive meniscus 4, facing the concavity to the image. Menisci 1 and 4 are made of Germany, lenses 2 and 3 are made of oxygen-free glass IKS-25. The entrance pupil is located on the first surface of the lens. The design elements of the lens are shown in the table.

No. Radius of curvature Axis thickness Material Light diameter one 129.82 8.5 Germanium 100.0 2 199.23 61.9 Air 97.44 3 -193.1 5.0 IKS-25 56.94 four 336.8 26.8 Air 56.46 5 -502.8 8.0 IKS-25 59.86 6 -129.5 1.3 Air 60.53 7 53.38 8.0 Germanium 54.01 8 58.48 25.678 Air 48.15 9 Pl. fig. 21.18

Optical characteristics of the lens

1. Focal length 100 mm 2. Relative bore 1: 1 3. Field of view 9.6 ° × 7.2 ° (diagonal - 12 °) 4. Spectral range 8-12 microns 5. Lens Length 145 mm 6. The back section 25.68 mm

For the given lens, the ratio between the focal lengths of the lens elements is:

f ′ ₁ / f′=1.298,

f ′ ₂ /f′=-0.690,

f ′ ₃ /f′=0.974,

f ′ ₄ /f′=0.937.

Consider the image quality indicators of the presented IR lens, using diagrams made using the ZE-MAX program (figure 2, 3, 4). The price of one division on the graphs of transverse aberrations (Fig. 2) is 0.01 mm and, as can be seen from the graphs, the geometric aberrations do not exceed 0.025 mm.

Figure 3 shows the circles of scattering (for the center, zone and edge of the field of view). In the upper left corner is a scale ruler measuring 0.1 mm. For each scattering circle, an Erie circle is imprinted. In addition, the “D 80%” size (0.023, 0.024 and 0.026 mm) is imprinted above each circle. In the lower left corner are the mean square (RMS) and geometric (GEO) scattering circles in the fields of view, as well as the diameter of the Erie circle. As can be seen from the diagram, the diameter of the scattering diffraction spot, taking into account geometric aberrations, does not exceed 0.026 mm (D 80% <0.026 mm) over the entire field of view.

The contrast of the frequency response image (Fig. 4) is given for all points of the field of view and is T = 0.75-0.77, which corresponds to the set requirements.

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 (145 mm) and a sufficient rear segment (25.68 mm).

The second negative lens can be mounted with the possibility of movement along the optical axis. This movement is used to focus the lens to a finite distance, to thermally compensate for the displacement of the lens mounting plane when the temperature is changed, and to align the lens with a stationary photodetector.

Thus, the proposed invention improves the image quality of the lens with a large relative aperture and field of view, reduce the length and increase the rear segment of the lens.

Bibliography

1. NN Michelson "Optical telescopes", M., "Science", 1976.

2. A.D. Kirkpatric. "Far infrared lens", US Pat. No. 3.439.969, cl. G02B 3/00, 1969.

3. I.A. Neil. "Infrared objective lens system", US Pat. No. 4.505.535, cl. G02B 9/34, 1985.

4. P.O. Rogers "Infra-red lenses", US Pat. No. 4.030.805, cl. G02B 1/00, 1977.

Claims (3)

1. The lens for the infrared region of the spectrum containing 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, the fourth is a positive meniscus facing concavity to the image, and the menisci are made of germanium , characterized in that the negative and positive lenses are made of oxygen-free glass, and the focal lengths of the components satisfy the following conditions:
f ′ ₁ / f ′ = 1.1 ÷ 1.3,
f ′ ₂ / f ′ = - 0.69 ÷ 0.74,
f ′ ₃ / f ′ = 0.84 ÷ 0.98,
f ′ ₄ / f ′ = 0.93 ÷ 1.6,
where f ′ ₁ , f ′ ₂ , f ′ ₃ , f ′ ₄ are the focal lengths of the first, second, third, and fourth components, respectively, f ′ is the equivalent focal length of the entire lens. 2. The lens according to claim 1, characterized in that the oxygen-free glass of the IKS-25 brand is used. 3. The lens according to claim 1, characterized in that the negative lens is mounted with the possibility of movement along the optical axis.

Images