Mechanically compensated type for midwave infrared zoom system with a large zoom ratio

Abstract. In some circumstances, there is a need for a midwave infrared (MWIR) zoom system with a large zoom ratio. Using traditional four-component mechanically compensated types of MWIR zoom systems cannot achieve a large zoom ratio. To meet this demand, we describe a six-component mechanically compensated type. The thin-lens theory of this type is developed and equations are presented. Using the six-component mechanically compensated type, a MWIR continuous zoom system with a zoom ratio of 45 is designed, and it has high image quality over the entire zoom range.


Introduction
Infrared systems operating in midwave infrared (MWIR, from 3 to 5 μm) are used in civilian and military applications, such as law enforcement, life rescue, territorial surveillance, vehicle tracking, aerial surveillance, and stealth searching. [1][2][3] In recent years, the demand for MWIR zoom systems has increased. In these systems, the wide field of view (WFOV) is used for observing a large scene area for possible targets of interest, and the narrow field of view (NFOV) is used for close-up identification of the target of interest. [3][4][5][6][7][8][9][10][11][12] In some circumstances, such as aerial surveillance and life rescue, there is a need for wider observation of the scene. 3 There is a significant need to design a MWIR zoom system with a large zoom ratio.
There are two types of zoom systems: optically compensated and mechanically compensated. Almost all infrared zoom systems are mechanically compensated. 13 In a traditional mechanically compensated zoom system, the second component moves for changes in focal length, while the third component moves to eliminate image shift; the image stays in focus throughout the zoom range (refer to Fig. 1). 13 Using a traditional four-component mechanically compensated type, MWIR zoom systems cannot achieve a large zoom ratio (such as 45) with a general F/number (such as 4). 1,2,[14][15][16][17][18][19][20][21] In this paper, we examine the six-component mechanically compensated type, which can achieve a large zoom ratio. The design concept is shown in Fig. 2; the second and fourth components are linked and move together for changes in focal length, while the third and fifth components are linked and move together to eliminate image shift. This type still involves just two moving groups. It is likely that the zoom range could also be achieved with a third moving group, but there is additional expense and complexity to do so. Section 2 presents the thin-lens theory. In Sec. 3, a MWIR continuous zoom system with a zoom ratio of 45 is designed. Finally, in Sec. 4, we present our conclusions.
2 Thin-Lens Theory Figure 3 represents a general six-component zoom system, with the zoom components in their long-zoom positions.
Capital letters indicate the long-zoom values. Thin-lens theory is used to investigate the zooming properties. For the second component, where F 2 is the focal length of the component. This becomes and Likewise, for the third component, For the fifth component, This becomes and Likewise for the sixth component, Likewise, for the fourth component, and Therefore, using Eqs. (2)-(5) and (7)- (11) in Eq. (12), As the second, third, fourth, and fifth components move from their long-zoom positions, the image should stay in focus, and the following conditions must hold: Small letters indicate the values at the new zoom position. As previously defined, Z 1 is the axial distance moved by the second and fourth components from the long-zoom position, and Z 2 is the axial distance moved by the third and fifth components at zoom setting Z 1 from the long-zoom position. Thus, And by combining Eqs. (14)- (20), At the zoom setting Z 1 , as the long-zoom values are known, the above values of a 1 , b 1 , b 2 , b 3 , c 1 , c 2 , c 3 , c 4 and c 5 can then be evaluated. After rearrangement, Eq. (21) becomes Using Eq. (22), the motion of the third and fifth components Z 2 at the the zoom setting Z 1 may be evaluated. Using Eqs. (14)- (19), the distance values may be found. Then, the Spectral band 3.7 to 4.8 μm Table 2 The initial data of the thin-lens zoom system.  Optical Engineering corresponding effective focal length (EFL) of the system is obtained.
3 Example of the Design A MWIR zoom system designed with a 320 × 240 staring focal plane array, and the dimension of detector pixel is 30 × 30 μm 2 . The characteristics of the design are shown in Table 1.

Thin-Lens Results
The initial data of the thin-lens system at the long-zoom position (EFL ¼ −450 mm) are listed in Table 2. Using Eqs. (22) and (14)- (19), setting three different values of Z 1 , thin-lens data of the system at the three zoom positions are calculated and listed in Table 3.

Actual Results
According to the thin-lens design results, the original structure of the actual system was obtained. Computer optimization was initiated at three zoom positions and was expanded to nine and eventually to 34 zoom positions. The actual zoom system consists of eight elements, made from silicon and germanium to achieve achromatization. There are four aspheric surfaces utilized for compactness in order to achieve the desired optical performance. Layouts of the system at three zoom positions are shown in Fig. 4. Characteristics of the system are shown in Table 1. The overall length of the zoom system is 400 mm. The second and fourth elements are linked and move together; the length of the move is 53.45 mm over the entire zoom range. The third and fifth elements are also linked and move together; the length of the move is 41.40 mm over the entire zoom range. The overall length and moving lengths are short compared with those of the typical zoom systems. 1,15 Figure 5 shows the continuous motions of the zoom elements; the focal length of the system is smooth and continuous. To have 100% cold shielding efficiency, the cold shield of the detector and the exit pupil of the system have to be superposed.

Performance
The modulation transfer function (MTF) performances of the system at three zoom positions are shown in Fig. 6. The MTF value in Nyquist limit (16l p∕mm) is more than 0.3 over the entire zoom range. Figure 7 illustrates spot diagrams for three different zoom positions. Root mean square (RMS) radius of the spot is less than 30 μm over the full zoom range. The MWIR zoom system has high image quality. Fig. 5 The zoom paths of the actual system. Fig. 4 Layouts of the actual system at three zoom positions.