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

Hao Zhou; Liu Ying; Sun Qiang; Li Chun; Xiaolong Zhang; Huang Jianbo

doi:10.1117/1.OE.52.1.013002

7 January 2013 Mechanically compensated type for midwave infrared zoom system with a large zoom ratio

Hao Zhou, Liu Ying, Sun Qiang, Li Chun, Xiaolong Zhang, Huang Jianbo

Author Affiliations +

Optical Engineering, Vol. 52, Issue 1, 013002 (January 2013). https://doi.org/10.1117/1.OE.52.1.013002

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.

1. 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^–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^–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^–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.

Fig. 1

Four-component mechanically compensated zoom system.

Fig. 2

Six-component mechanically compensated zoom system.

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,

Eq. (1)

\frac{1}{L_{2}^{'}} - \frac{1}{L_{2}} = \frac{1}{F_{2}},

where

F_{2}

is the focal length of the component.

Fig. 3

General six-component zoom system.

This becomes

Eq. (2)

L_{2}^{'} = \frac{F_{2} \cdot L_{2}}{F_{2} + L_{2}},

and

Eq. (3)

L_{2} = F_{1} - D_{1} .

Likewise, for the third component,

Eq. (4)

L_{3}^{'} = \frac{F_{3} \cdot L_{3}}{F_{3} + L_{3}},

but

Eq. (5)

L_{3} = L_{2}^{'} - D_{2} .

For the fifth component,

Eq. (6)

\frac{1}{L_{5}^{'}} - \frac{1}{L_{5}} = \frac{1}{F_{5}} .

This becomes

Eq. (7)

L_{5} = \frac{F_{5} \cdot L_{5}^{'}}{F_{5} - L_{5}^{'}},

and

Eq. (8)

L_{5}^{'} = D_{5} + L_{6} .

Likewise for the sixth component,

Eq. (9)

L_{6} = \frac{F_{6} \cdot D_{6}}{F_{6} - D_{6}} .

Likewise, for the fourth component,

Eq. (10)

L_{4} = \frac{F_{4} \cdot L_{4}^{'}}{F_{4} - L_{4}^{'}},

but

Eq. (11)

L_{4}^{'} = D_{4} + L_{5},

and

Eq. (12)

L_{3}^{'} - L_{4} = D_{3} .

Therefore, using Eqs. (2)–(5) and (7)–(11) in Eq. (12),

Eq. (13)

\frac{F_{3} \cdot [\frac{F_{2} \cdot (F_{1} - D_{1})}{F_{2} + F_{1} - D_{1}} - D_{2}]}{F_{3} + \frac{F_{2} \cdot (F_{1} - D_{1})}{F_{2} + F_{1} - D_{1}} - D_{2}} - \frac{F_{4} \cdot [D_{4} + \frac{F_{5} \cdot (D_{5} + \frac{F_{6} \cdot D_{6}}{F_{6} - D_{6}})}{F_{5} - D_{5} - \frac{F_{6} \cdot D_{6}}{F_{6} - D_{6}}}]}{F_{4} - D_{4} - \frac{F_{5} \cdot (D_{5} + \frac{F_{6} \cdot D_{6}}{F_{6} - D_{6}})}{F_{5} - D_{5} - \frac{F_{6} \cdot D_{6}}{F_{6} - D_{6}}}} = D_{3} .

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:

Eq. (14)

\frac{F_{3} \cdot [\frac{F_{2} \cdot (F_{1} - d_{1})}{F_{2} + F_{1} - d_{1}} - d_{2}]}{F_{3} + \frac{F_{2} \cdot (F_{1} - d_{1})}{F_{2} + F_{1} - d_{1}} - d_{2}} - \frac{F_{4} \cdot [d_{4} + \frac{F_{5} \cdot (d_{5} + \frac{F_{6} \cdot d_{6}}{F_{6} - d_{6}})}{F_{5} - d_{5} - \frac{F_{6} \cdot d_{6}}{F_{6} - d_{6}}}]}{F_{4} - d_{4} - \frac{F_{5} \cdot (d_{5} + \frac{F_{6} \cdot d_{6}}{F_{6} - d_{6}})}{F_{5} - d_{5} - \frac{F_{6} \cdot d_{6}}{F_{6} - d_{6}}}} = d_{3} .

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,

Eq. (15)

d_{1} = D_{1} + Z_{1} .

Eq. (16)

d_{2} = D_{2} + Z_{2} - Z_{1} .

Eq. (17)

d_{3} = D_{3} - Z_{2} + Z_{1} .

Eq. (18)

d_{4} = D_{4} + Z_{2} - Z_{1} .

Eq. (19)

d_{5} = D_{5} - Z_{2} .

Eq. (20)

d_{6} = D_{6} .

And by combining Eqs. (14)–(20),

Eq. (21)

Z_{2} + \frac{a_{1} \cdot Z_{2}^{2} + b_{1} \cdot Z_{2} + c_{1}}{Z_{2}^{2} - b_{2} \cdot Z_{2} - c_{2}} + \frac{b_{3} \cdot Z_{2} - 2}{Z_{2} - c_{4}} - c_{5} = 0,

where

a_{1} = F_{4}

b_{1} = F_{4} \cdot (D_{4} - D_{5} - \frac{F_{6} \cdot D_{6}}{F_{6} - D_{6}} - Z_{1}),

b_{2} = F_{4} - D_{4} + Z_{1} + D_{5} + \frac{F_{6} \cdot D_{6}}{F_{6} - D_{6}}

b_{3} = F_{3}

c_{1} = F_{4} \cdot (F_{5} - D_{5} - \frac{F_{6} \cdot D_{6}}{F_{6} - D_{6}}) \cdot (D_{4} - Z_{1}) + F_{4} \cdot F_{5} \cdot (D_{5} + \frac{F_{6} \cdot D_{6}}{F_{6} - D_{6}})

c_{2} = (F_{5} - D_{5} - \frac{F_{6} \cdot D_{6}}{F_{6} - D_{6}}) \cdot (F_{4} - D_{4} + Z_{1}) - F_{5} \cdot (D_{5} + \frac{F_{6} \cdot D_{6}}{F_{6} - D_{6}})

c_{3} = F_{3} \cdot [\frac{F_{2} \cdot (F_{1} - D_{1} - Z_{1})}{F_{2} + F_{1} - D_{1} - Z_{1}} - D_{2} + Z_{1}]

c_{4} = F_{3} + \frac{F_{2} \cdot (F_{1} - D_{1} - Z_{1})}{F_{2} + F_{1} - D_{1} - Z_{1}} - D_{2} + Z_{1}

c_{5} = D_{3} + Z_{1}

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

Eq. (22)

Z_{2}^{4} + k_{3} \cdot Z_{2}^{3} + k_{2} \cdot Z_{2}^{2} + k_{1} \cdot Z_{2} + k_{0} = 0,

where

k_{3} = a_{1} - b_{2} + b_{3} - c_{4} - c_{5}

k_{2} = b_{1} - c_{2} - c_{3} - a_{1} \cdot c_{4} - b_{2} \cdot b_{3} + b_{2} \cdot c_{4} + b_{2} \cdot c_{5} + c_{4} \cdot c_{5}

k_{1} = c_{1} - b_{1} \cdot c_{4} + b_{2} \cdot c_{3} - c_{2} \cdot b_{3} + c_{2} \cdot c_{4} + c_{2} \cdot c_{5} - b_{2} \cdot c_{4} \cdot c_{5}

k_{0} = - c_{1} \cdot c_{4} + c_{2} \cdot c_{3} - c_{2} \cdot c_{4} \cdot c_{5}

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 corresponding effective focal length (EFL) of the system is obtained.

3. Example of the Design

A MWIR zoom system designed with a $320 \times 240$ staring focal plane array, and the dimension of detector pixel is $30 \times 30 {μ m}^{2}$ . The characteristics of the design are shown in Table 1.

Table 1

Characteristics of the system.

Zoom range	45
Focal length range	$- 10$ to $- 450 mm$
$F / number$	4
Image plane diagonal	12 mm
Spectral band	3.7 to 4.8 μm

3.1.

Thin-Lens Results

The initial data of the thin-lens system at the long-zoom position ( $EFL = - 450 mm$ ) 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.

Table 2

The initial data of the thin-lens zoom system.

$F_{1}$	135.646	$D_{1}$	79.188
$F_{2}$	$- 36.930$	$D_{2}$	9.528
$F_{3}$	71.126	$D_{3}$	105.237
$F_{4}$	$- 38.214$	$D_{4}$	15.312
$F_{5}$	50.702	$D_{5}$	172.529
$F_{6}$	27.052	$D_{6}$	51.512

NOTE: Unit: mm.

Table 3

Thin-lens data of the system at three zoom positions.

	Long EFL	Mid EFL	Short EFL
$Z_{1}$	0	$- 17.121$	$- 53.587$
$Z_{2}$	0	30.708	43.004
$d_{1}$	79.188	62.067	25.601
$d_{2}$	9.528	57.357	106.119
$d_{3}$	105.237	57.408	8.646
$d_{4}$	15.312	63.141	111.903
$d_{5}$	172.529	141.821	129.525
$d_{6}$	51.512	51.512	51.512
$E F L$	$- 450$	$- 63.2$	$- 10$

NOTE: Unit: mm.

3.2.

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.

Fig. 4

Layouts of the actual system at three zoom positions.

Fig. 5

The zoom paths of the actual system.

3.3.

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 ( $16 l p / mm$ ) 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. 6

MTF curves of the system at three zoom positions.

Fig. 7

Spot diagrams of the system at three zoom positions.

4. Conclusions

Using six-component mechanically compensated type, a MWIR zoom system is designed in this paper. The results present this zoom lens achieving good image quality, with specifications of 45 zoom ratio and $F / 4$ , by utilizing the layout of interlaced mechanical linkage to perform the role of variator and compensator. The six-component mechanically compensated type system can achieve a large zoom ratio with short overall length and moving lengths. The continuous motions of the zoom elements are smooth. Image qualities are good over the entire zoom range. The six-component mechanically compensated type has advantages in the design of MWIR zoom system with a large zoom ratio.

References

1.

R. L. Sinclair, “High magnification zoom lenses for 3–5 μm applications,” Proc. SPIE, 3429 11 –18 (1998). http://dx.doi.org/10.1117/12.328533 PSISDG 0277-786X Google Scholar

2.

C. W. KuoJ. M. MiaoC. H. Tai, “Midwave infrared optical zooming design and kinoform degrading evaluation methods,” Appl. Opt., 50 (18), 3043 –3049 (2011). http://dx.doi.org/10.1364/AO.50.003043 APOPAI 0003-6935 Google Scholar

3.

M. N. Akram, “Design of a multiple-field-of-view optical system for 3- to 5-μm infrared focal-plane arrays,” Opt. Eng., 42 (6), 1704 –1714 (2003). http://dx.doi.org/10.1117/1.1572892 OPEGAR 0091-3286 Google Scholar

4.

H. S. KimC. W. KimS. M. Hong, “Compact mid-wavelength infrared zoom camera with $20 ∶ 1$ zoom range and automatic athermalization,” Opt. Eng., 41 (7), 1661 –1667 (2002). http://dx.doi.org/10.1117/1.1481048 OPEGAR 0091-3286 Google Scholar

5.

H. S. Kimet al., “Compact MWIR camera with $\times 20$ zoom optics,” Proc. SPIE, 4369 673 –679 (2001). http://dx.doi.org/10.1117/12.445329 PSISDG 0277-786X Google Scholar

6.

M. W. Chang, “Design of a compact zoom system for 3–5 micron wave band thermal imager,” Proc. SPIE, 3129 144 –155 (1997). http://dx.doi.org/10.1117/12.284236 PSISDG 0277-786X Google Scholar

7.

Z. Y. Fanet al., “Design of high ratio middle infrared continuous zoom optical system,” Proc. SPIE, 8193 81931R (2011). http://dx.doi.org/10.1117/12.900176 PSISDG 0277-786X Google Scholar

8.

M. N. AkramM. H. Asghar, “Step-zoom dual-field-of-view infrared telescope,” Appl. Opt., 42 (13), 2312 –2316 (2003). http://dx.doi.org/10.1364/AO.42.002312 APOPAI 0003-6935 Google Scholar

9.

Z. D. XuX. LiuZ. Wei, “Airborne IR system design using $640 \times 512$ element detector,” J. Nanjing Univ. Aeronaut. Astronaut., 39 (4), 524 –529 (2007). Google Scholar

10.

J. Q. Meng, “Dual field zoom ( $\times 6$ ) infrared imaging optical system,” Infrared Laser Eng., 37 (1), 89 –92 (2008). 1007-2276 Google Scholar

11.

L. Zhang, “Optical design for middle infrared zoom system,” J. Appl. Opt., 27 (1), 32 –34 (2006). JOAOF8 1464-4258 Google Scholar

12.

H. Y. GaoT. Xiong, “Mid-wavelength infrared dual field-of-view optical system,” Opt. Precision Eng., 16 (10), 1891 –1894 (2008). Google Scholar

13.

A. Mann, Infrared Optics and Zoom Lenses, SPIE Press, Bellingham, WA (2009). Google Scholar

14.

R. G. Liet al., “Design of dual-field-of-view optical system for mid-wave infrared focal-plane arrays,” Laser Infrared, 39 (6), 640 –642 (2009). 1007-2276 Google Scholar

15.

L. J. ChenP. LiL. Ma, “Compact MWIR zoom system,” Infrared Technol., 32 (10), 562 –566 (2010). Google Scholar

16.

H. T. WangL. X. Guo, “Cooled thermal imaging mid-wavelength infrared zoom camera,” Infrared Technol., 29 (1), 8 –11 (2007). Google Scholar

17.

M. C. SansonJ. Cornell, “MWIR continuous zoom with large zoom range,” Proc. SPIE, 7660 76601X (2010). http://dx.doi.org/10.1117/12.850375 PSISDG 0277-786X Google Scholar

18.

M. C. Sansonet al., “Development of MWIR continuous zoom with large zoom range,” Proc. SPIE, 8012 80122F (2011). http://dx.doi.org/10.1117/12.886270 PSISDG 0277-786X Google Scholar

19.

Y. Aronet al., “Topaz: a novel design of a high magnification, athermalized $1 ∶ 30$ zoom in the MWIR,” Proc. SPIE, 5406 97 –106 (2004). http://dx.doi.org/10.1117/12.543907 PSISDG 0277-786X Google Scholar

20.

H. Y. GaoT. XiongC. C. Yang, “Middle infrared continuous zoom optical system,” Opt. Precision Eng., 15 (7), 1038 –1043 (2007). Google Scholar

21.

A. Portaet al., “ERICA: compact MWIR camera with $20 \times$ step zoom optics and advanced processing,” Proc. SPIE, 6737 673706 (2007). http://dx.doi.org/10.1117/12.737937 PSISDG 0277-786X Google Scholar

Biography

Zhou Hao received a BS degree in optical information science and technology from Hefei University of Technology in 2009. He is currently pursuing his PhD at the Graduate School of the Chinese Academy of Sciences. His research interests are infrared optical systems and system evaluation.

Liu Ying received a PhD degree in optical engineering from Graduate School of the Chinese Academy of Sciences in 2010. She is currently a research assistant at the Changchun Institute of Optics, Fine Mechanics, and Physics, Chinese Academy of Sciences. Her research interests are infrared optical systems and spectrometer.

Sun Qiang received a PhD degree in optical engineering from Nankai University in 2003. He is currently a professor at the Changchun Institute of Optics, Fine Mechanics, and Physics, Chinese Academy of Sciences. His research interests are infrared optical systems and spectrometer.

Li Chun received an MS degree in optical engineering from Graduate School of the Chinese Academy of Sciences in 2010. He is currently a research assistant at the Changchun Institute of Optics, Fine Mechanics, and Physics, Chinese Academy of Sciences. His research interests are image enhancement and image fusion.

Zhang Xiaolong received a BS degree in applied physics from Qingdao University of Science and Technology in 2009. He is currently pursuing his PhD at the Graduate School of the Chinese Academy of Sciences. His research interests are optics design and testing.

Huang Jianbo received an MS degree in mechanical engineering from Jilin University in 2008. He is currently a research assistant at the Changchun Institute of Optics, Fine Mechanics, and Physics, Chinese Academy of Sciences. His research interests are mechanical system design and testing.

CC BY: © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

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Hao Zhou, Liu Ying, Sun Qiang, Li Chun, Xiaolong Zhang, and Huang Jianbo "Mechanically compensated type for midwave infrared zoom system with a large zoom ratio," Optical Engineering 52(1), 013002 (7 January 2013). https://doi.org/10.1117/1.OE.52.1.013002

Published: 7 January 2013

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KEYWORDS

Zoom lenses

Mid-IR

Image quality

Optical engineering

Mechanics

Modulation transfer functions

Physics

1.

Introduction

Fig. 1

Fig. 2

2.

Thin-Lens Theory

Eq. (1)

Fig. 3

Eq. (2)

Eq. (3)

Eq. (4)

Eq. (5)

Eq. (6)

Eq. (7)

Eq. (8)

Eq. (9)

Eq. (10)

Eq. (11)

Eq. (12)

Eq. (13)

Eq. (14)

Eq. (15)

Eq. (16)

Eq. (17)

Eq. (18)

Eq. (19)

Eq. (20)

Eq. (21)

Eq. (22)

3.

Example of the Design

Table 1

3.1.

Thin-Lens Results

Table 2

Table 3

3.2.

Actual Results

Fig. 4

Fig. 5

3.3.

Performance

Fig. 6

Fig. 7

4.

Conclusions

References

Biography

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