7 January 2013 Mechanically compensated type for midwave infrared zoom system with a large zoom ratio
Author Affiliations +
Optical Engineering, 52(1), 013002 (2013). doi: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.
Hao, Ying, Qiang, Chun, Xiaolong, and Jianbo: Mechanically compensated type for midwave infrared zoom system with a large zoom ratio

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.12.3 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.34.5.6.7.8.9.10.11.12 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).1,2,1415.16.17.18.19.20.21 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.

Fig. 1

Four-component mechanically compensated zoom system.

OE_52_1_013002_f001.png

Fig. 2

Six-component mechanically compensated zoom system.

OE_52_1_013002_f002.png

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,

(1)

1L21L2=1F2,
where F2 is the focal length of the component.

Fig. 3

General six-component zoom system.

OE_52_1_013002_f003.png

This becomes

(2)

L2=F2L2F2+L2,
and

(3)

L2=F1D1.

Likewise, for the third component,

(4)

L3=F3L3F3+L3,
but

(5)

L3=L2D2.

For the fifth component,

(6)

1L51L5=1F5.

This becomes

(7)

L5=F5L5F5L5,
and

(8)

L5=D5+L6.

Likewise for the sixth component,

(9)

L6=F6D6F6D6.

Likewise, for the fourth component,

(10)

L4=F4L4F4L4,
but

(11)

L4=D4+L5,
and

(12)

L3L4=D3.

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

(13)

F3[F2(F1D1)F2+F1D1D2]F3+F2(F1D1)F2+F1D1D2F4[D4+F5(D5+F6D6F6D6)F5D5F6D6F6D6]F4D4F5(D5+F6D6F6D6)F5D5F6D6F6D6=D3.

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:

(14)

F3[F2(F1d1)F2+F1d1d2]F3+F2(F1d1)F2+F1d1d2F4[d4+F5(d5+F6d6F6d6)F5d5F6d6F6d6]F4d4F5(d5+F6d6F6d6)F5d5F6d6F6d6=d3.

Small letters indicate the values at the new zoom position. As previously defined, Z1 is the axial distance moved by the second and fourth components from the long-zoom position, and Z2 is the axial distance moved by the third and fifth components at zoom setting Z1 from the long-zoom position. Thus,

(15)

d1=D1+Z1.

(16)

d2=D2+Z2Z1.

(17)

d3=D3Z2+Z1.

(18)

d4=D4+Z2Z1.

(19)

d5=D5Z2.

(20)

d6=D6.

And by combining Eqs. (14)–(20),

(21)

Z2+a1Z22+b1Z2+c1Z22b2Z2c2+b3Z22Z2c4c5=0,
where
a1=F4
b1=F4(D4D5F6D6F6D6Z1),
b2=F4D4+Z1+D5+F6D6F6D6
b3=F3
c1=F4(F5D5F6D6F6D6)(D4Z1)+F4F5(D5+F6D6F6D6)
c2=(F5D5F6D6F6D6)(F4D4+Z1)F5(D5+F6D6F6D6)
c3=F3[F2(F1D1Z1)F2+F1D1Z1D2+Z1]
c4=F3+F2(F1D1Z1)F2+F1D1Z1D2+Z1
c5=D3+Z1

At the zoom setting Z1, as the long-zoom values are known, the above values of a1, b1, b2, b3, c1, c2, c3, c4 and c5 can then be evaluated. After rearrangement, Eq. (21) becomes

(22)

Z24+k3Z23+k2Z22+k1Z2+k0=0,
where
k3=a1b2+b3c4c5
k2=b1c2c3a1c4b2b3+b2c4+b2c5+c4c5
k1=c1b1c4+b2c3c2b3+c2c4+c2c5b2c4c5
k0=c1c4+c2c3c2c4c5

Using Eq. (22), the motion of the third and fifth components Z2 at the the zoom setting Z1 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 320×240 staring focal plane array, and the dimension of detector pixel is 30×30μm2. The characteristics of the design are shown in Table 1.

Table 1

Characteristics of the system.

Zoom range45
Focal length range−10 to −450  mm
F/number4
Image plane diagonal12 mm
Spectral band3.7 to 4.8 μm

3.1.

Thin-Lens Results

The initial data of the thin-lens system at the long-zoom position (EFL=450mm) are listed in Table 2. Using Eqs. (22) and (14)–(19), setting three different values of Z1, 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.

F1135.646D179.188
F2−36.930D29.528
F371.126D3105.237
F4−38.214D415.312
F550.702D5172.529
F627.052D651.512

NOTE: Unit: mm.

Table 3

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

Long EFLMid EFLShort EFL
Z10−17.121−53.587
Z2030.70843.004
d179.18862.06725.601
d29.52857.357106.119
d3105.23757.4088.646
d415.31263.141111.903
d5172.529141.821129.525
d651.51251.51251.512
EFL−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.

OE_52_1_013002_f004.png

Fig. 5

The zoom paths of the actual system.

OE_52_1_013002_f005.png

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 (16lp/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.

OE_52_1_013002_f006.png

Fig. 7

Spot diagrams of the system at three zoom positions.

OE_52_1_013002_f007.png

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.

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Biography

OE_52_1_013002_d001.png

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.

OE_52_1_013002_d002.png

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.

OE_52_1_013002_d003.png

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.

OE_52_1_013002_d004.png

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.

OE_52_1_013002_d005.png

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.

OE_52_1_013002_d006.png

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.

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

Mid-IR

Optical engineering

Image quality

Modulation transfer functions

Physics

Mechanics

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