The infrared optical system is an important part of the infrared imager, which is used to concentrate the infrared radiation energy, zoom, heat dissipation, control the imaging quality and focus the infrared radiation energy to the focal plane. Due to the single function of the single-field infrared optical system, the practical use is limited, and it is difficult to meet the development needs of modern infrared optical systems.

The dual-field infrared optical system has two fields of view of different sizes. The large field of view can be used to search for suspected targets in a wide range, and the small field of view can be used to identify and track suspected targets. It has a simple structure, a short field of view switching time, and imaging. It has the advantages of high quality, simplicity, and practicality, so it has been widely used in modern infrared optical systems.

The zoom infrared optical system used for the cooling detector mostly adopts the secondary imaging method to match the exit pupil of the optical system with the cold screen of the detector, and can effectively control the aperture of the objective lens, which usually does not cause the variety of intermediate image plane during the zooming process.

In this article, a novel dual-field infrared optical system with zoom form is introduced, which uses the zoom group to move axially in front of and behind the primary image to achieve zoom, focus, and temperature compensation, which not only changes the focal length of the system but also changes the intermediate image. The position of the surface has the advantages of fewer system components and a very compact structure.

In this article, for the 320×256 long-wave staring focal plane detector, a novel zooming form is used, and a secondary imaging dual-field optical system is designed with only 5 lenses. The wide and narrow fields of view are at the Nyquist frequency (17lp/mm). The MTFs are close to the diffraction limit, and the imaging quality is excellent.

There are a few types of zoom modes of the dual-field infrared optical system. At present, switching mode or two-component optical compensation zoom mode are usually adopted. These two zoom modes do not cause image plane changes, and each has advantages and disadvantages. The switching type mainly changes the focal length of the optical system by cutting in/out of the zoom group in the system.

When the zoom group cuts into the optical path, the focal length of the system becomes shorter, forming a wide field of view mode; when the zoom group cuts out of the optical path, the focal length of the system changes. long, forming a narrow field of view mode, as shown in Figure 1.

                                                                                        Fig.1 Rotate-Zoom optical system

The two-component optical compensation zoom mode mainly changes the focal length of the optical system by axially moving the zoom group in the system. The zoom group usually uses a negative lens group. When moving forward, the focal length of the system becomes shorter, forming a wide field of view mode; when the zoom group moves backward, the focal length becomes longer, forming a narrow field of view mode, as shown in Figure 2.

                                                                                               Fig.2 Step-Zoom optical system

This article introduces another novel zoom method, which is also achieved by moving the zoom group axially to achieve system zoom. Different from the two-component optical compensation zoom method, this zoom method is ingeniously combined with the second imaging system.

The zoom group moves back and forth in the primary image plane position, which not only changes the focal length of the system but also changes the position of the intermediate image plane. Ensure that the final image plane of the system is stable and the image quality is good.

As shown in Figure 3, the system consists of the front fixed group 1, the zoom group 2, and the rear fixed group 3. When the zoom group moves in the direction of the front fixed group, it forms an objective lens group with the front fixed group to converge the radiation rays of the scene target and a primary image plane is formed.

At this time, the rear fixed group forms a relay group to perform secondary imaging on the primary image plane to form a narrow field of view mode; when the zoom group moves toward the rear fixed group, it forms a relay group with the rear fixed group. , perform secondary imaging on the primary image plane formed by the front fixed group, and move the position of the primary image plane forward to form a wide field of view mode.

The dual field of view system has a novel zooming method. The objective lens group and the relay group are closely combined through the zoom group, which effectively shortens the interval between the objective lens group and the relay group, making the structure of the whole system very compact and greatly shortening the total length of the system. Compared with the two-component optical compensation zoom method, the zoom method has one less lens group, fewer lenses, higher transmittance, lower cost, and a more compact structure.

                                                                                 Fig.3 Novel Step-Zoom optical system

Considering various constraints such as volume, weight, and performance, the main design technical indicators of the long-wave dual-field athermalized infrared optical system are shown in Table 1.

                                                                                      Table 1 Parameters of Optical design

For the 320×256 element long-wave cooled infrared focal plane detector, an appropriate initial structure was selected, and the ZEMAX optical aided design software was used to optimize the design of the dual-field infrared optical system. The design result is shown in Figure 4. The dual-field infrared optical system has a total of 5 lenses. It consists of anterior fixation group (2 pieces), zoom group (1 piece), and rear fixation group (2 pieces), and adopts +, -, -, +, + optical focus structure.

Infrared materials use Ge and ZnSe, germanium material has the characteristics of low dispersion and high refractive index, and has good dispersion performance at long wavelengths, but it is difficult to correct chromatic aberration for complex zoom systems with only one material, while ZnSe is mainly used for achromatic. In order to reduce the number of lenses, improve the system transmittance, effectively control the system aberration, and improve the image quality, three high-order aspheric surfaces are introduced.

The zoom group is driven by a DC motor to move forward and backward along the optical axis to realize the functions of the field of view switching, focusing, and temperature compensation. After optimized design, the system has stable image planes with wide and narrow fields of view, and the imaging quality is close to the diffraction limit. Its main features are:

(1) The form of zooming is novel, and the field of view switching is realized by moving the zooming group in front and back of an image, which reduces the number of elements of the system;

(2) The structure is more compact, the total length of the system is short, the distance from the first mirror to the focal plane of the infrared detector is less than 130mm, and the total length/focal length ratio is about 0.7;

(3) The number of lenses is small, only 5 pieces, which improves the transmittance of the system, effectively save infrared materials, and reduces costs;

(4) The secondary imaging method is adopted, which is conducive to the complete matching of the exit pupil of the optical system and the cold screen of the infrared detector, the cold screen efficiency can be guaranteed to be 100%, and the aperture of the objective lens can be effectively compressed;

(5) The axial moving zoom group can realize the functions of zoom, focus, and temperature compensation, which simplifies the structure and electronic design;

(6)Small size, lightweight, high image quality, low cost, and high-cost performance.

                                                                                   Fig.4 Dual-FOV Infrared Optical System

The transfer function is an important evaluation method for an optical system. After the optimized design, the system is stable in the narrow and wide field of view, and the image quality is excellent. Figure 5 and Figure 6 show the optical modulation transfer function diagrams of narrow and wide fields of view at room temperature, respectively. It can be seen from the figures that the MTF at the Nyquist frequency (17lp/mm) is close to the diffraction limit (the uppermost black solid line is the diffraction limit), which is enough to ensure the excellent imaging quality of the optical system.

                                                                                             Fig.6 The MTF curve of WFOV

The spot diagram is the geometric image spot formed by the optical system imaging the point target. Figure 7 and Figure 8 show the root mean square (RMS) of the diameter of the scattered spot in the narrow and wide fields of view at room temperature, which is less than one detection element size (30μm×30μm), which well meets the requirements of the system.

                                                                                                Fig.8 The Spot Diagram of WFOV

Since the refractive index of infrared optical materials changes significantly with temperature, which is about two orders of magnitude higher than that of visible light optical materials, temperature changes will cause the infrared optical system to defocus, resulting in deterioration of image quality. In order to ensure the stable performance of the infrared optical system in a wide temperature range. The design of heat dissipation difference should be considered in the design.

In order to be suitable for the axial moving double field of view zoom structure, and to reduce the difficulty of light and machine design. In this paper, the electromechanical active heat dissipation method is adopted, that is, at different temperatures, the variable magnification lens is driven to move slightly in the axial direction by controlling the DC motor to compensate for the drift of the image surface. Figures 9 to 12 show the optical transfer functions of narrow and wide fields of view at high and low temperatures, respectively.

                                                                                      Fig.12 The MTF curve of WFOV at +60℃

The dual-field long-wave infrared optical system plays an irreplaceable role in the modern military field. It has been widely used in various fields such as reconnaissance, detection, search, and tracking targets and has become an important means to obtain battlefield information.

Aiming at the long-wave infrared 320×256 CMT focal plane detector, this paper designs a dual-field infrared optical system by reasonably assigning the optical power and reasonably matching two common infrared materials, Ge and Znse, and realizes 183mm/61mm two-speed zoom, both wide and narrow fields of view have good imaging quality.

The zooming method of the system is novel, and the zoom group cleverly combines the objective lens group and the relay group, which effectively compresses the total length of the system and reduces the number of components in the system, which is especially suitable for cooling focal plane detectors.

It has the advantages of a small number of lenses, high transmittance, compact and reasonable structure, easy adjustment, lightweight, small volume, material saving, high-cost performance, and the imaging quality fully meets the application requirements, and meets the high performance, miniaturization and low cost of thermal imaging cameras.

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Authors: Chen Luji, Chen Jinjin, Li Ping

Journal source: Infrared Technology Vol.33 No.7 July 2011

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