- Email: [email protected]
Design of dual-band shared-aperture Co-zoom optical system
Infrared Physics & Technology 64 (2014) 40–46
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Design of dual-band shared-aperture Co-zoom optical system Gao Ming, Chen Yang ⇑, Liu Jun, Lv Hong School of Optoelectronic Engineering, Xi’an Technological University, Xi’an 710021, China
h i g h l i g h t s Optical system, fusing visible/mid-IR light in the same light path, is designed. A method is obtained to compensate zoom ratio difference between visible and mid-IR. Volume of dual-band continuous zoom optical system is reduced. This optical system can conduct synchronous observation, tracking and measurement with dual-band.
a r t i c l e
i n f o
Article history: Received 15 August 2013 Available online 15 February 2014 Keywords: Dual band Shared aperture Co-zoom Zoom ratio difference compensation Athermalization
a b s t r a c t An optical system that features visible plus mid-infrared light, shared aperture, synchronous and continuous zoom is designed with a 10 zoom ratio. Analysis is performed to differentiate visible plus midinfrared light focal length and zoom ratio during zooming, and the change law of this difference. Upon combination with two-group and three-group zoom theories and upon derivation of the conditions for compensating zoom ratio difference, a method has been obtained to directly compensate this difference. The focal length and the zoom ratio for visible/mid-infrared light at any zoom location are similar with this method, thereby conducting synchronous observation, tracking, and measurement on the target. Design results have shown that the system is small, has a fast response, has an excellent in overall image quality, and is athermal for temperatures between 40 °C and 60 °C. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction A dual-band continuous zoom optical system incorporates the characteristics of a dual-band system (e.g., round-the-clock detection and accurate and comprehensive access to target information) with the advantages of a continuous zoom system (e.g., wide detection range, rapid search, and continuous observation on target). This system has been gained research interest worldwide [1,2]. Vizgaitis [3] has designed an 11.7 military MWIR/LWIR continuous zoom system in 2010 that adopts the reflective-refractive mixed light path and the infrared dichroic focal plane array in concurrent imaging at medium/long-wave infrared band. In his article, Petrushevsky and Tsur [4] has analyzed a Goodrich DB-110 aerial camera having mutually independent visible light and infrared optical systems, which realizes round-the-clock detection and accurate target observation. Han et al. [5] have respectively folded both visible light and infrared dual-band systems via multiple reflections to reduce the system volumes, which is the main problem for these systems. An optical system that fully adopts the ⇑ Corresponding author. Tel.: +86 15129398622. E-mail address: [email protected] (G. Ming). http://dx.doi.org/10.1016/j.infrared.2014.01.014 1350-4495/Ó 2014 Elsevier B.V. All rights reserved.
refraction form to fuse visible light/infrared in the same light path and realizes synchronized and continuous zoom is yet to be reported. The standards for optical system performance have increased with the rapid development of social demand and the increasingly complicated environmental applications [6–10]. However, most of the existing visible light/infrared dual-band zoom systems comprise two separate systems with a large system volume and complex reconnaissance equipment structure. Target search and focal length readjustment prior to another target observation are necessary because of external environment changes, such as target obscurity, smoke interference, alternation of day and night, and change in light path, resulting in a long change process and a slow system response. Furthermore, the target may be lost provided that the target being tracked and observed moves fast. This design obtains continuous zoom for dual bands (visible light/mid-infrared) in the same optical path. Both systems have the same focal length and zoom ratio at any location upon compensating the zoom difference for dual bands. Changing the optical path is unnecessary and can be observed directly upon observation with different bands. This system yields synchronous observation, tracking and measurement on the target with dual bands,
41
G. Ming et al. / Infrared Physics & Technology 64 (2014) 40–46 Table 1 Optical design specifications. Visible
Mid-infrared
Wave
0.48–0.64 lm
Wave
3.7–4.5 lm
Zoom ratio F/# Field of view Focal length
10 5 18–1.8° 6.5–65 mm
Zoom ratio F/# Field of view Focal length
10 3.5 18–1.8° 6.5–65 mm
improves the response rate of the optical system, and avoids the target loss during optical path change. 2. Indicator and structural design of optical system 2.1. Optical system design indicator An optical system is required to implement round-the-clock observation and yield excellent resolution in infrared band, thereby selecting dual bands. The total zoom ratio is designed at 10 to ensure that the system has a larger scope of observation. The overall length of this system is less than 300 mm because of small size and lightweight requirements. Table 1 lists the specific design indicators. The visible-light detector is a 1=400 CCD having a 6.5 lm 6.25 lm pixel size; the mid-infrared detector is a 1=400 uncooled focal plane array having a 25 lm 25 lm pixel size. 2.2. Optical system structure design This system is required to realize synchronous continuous zoom at dual bands. However, a traditional zoom system cannot meet structural design requirements, thereby redesigning this structure comprising five parts: public zoom group, dispersion prism, zoom difference compensation group, visible-light fixed rear lens group, and mid-infrared fixed rear lens group (Fig. 1). (1) The public zoom group comprises three units: front fixation group, zoom group, and compensation group. This portion goes through the visible and infrared light simultaneously, and realizes the synchronous zoom at dual bands.
(2) Dispersion prism [11] allows transmission of visible light and reflects mid-infrared light. (3) The zoom ratio difference compensation group is set in the reflected light path, which can compensate the difference in focal length and zoom ratio of dual bands via location movement. (4) The visible and mid-infrared light rear fixed lens groups can respectively correct image aberration and converge light in their respective detectors for imaging. 3. Initial structure design and analysis of the public zoom group 3.1. Initial structure design Based on design requirements, the zoom ratio is 10, and the zoom and compensation curves are smooth to subsequently compensate the zoom ratio difference. Thus, the mechanical positive group zoom is appropriate [12]. Zoom design theory states that the displacement relationship between zoom group and compensation group will meet formula (1) when the image plane is stable, q3 2f 0 f 0 q 3f 0 q2
dq2 ¼ dq1
2 3 1 3 1ffi q1 p1 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4 0 0 0 3
q1 4f2 f3 4f3 q1
f20 þ q1
þ
q21
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi q41 4f20 f30 q31
ð1Þ
2
2ðf20 þ q1 Þ
where q1 is displacement of zoom group along optical axis; q1 is displacement of compensation group along optical axis; f20 is focal length of zoom group; f30 is focal length of compensation group. The focal length of the zoom group is a standard value, which is f20 ¼ 1 in the initial structure design. The following results are obtained upon calculation coupled with design experience: focal length of front fixation group is f10 ¼ 6, that of the compensation group is f30 ¼ 1:15, and that of the rear fixed group is f40 ¼ 0:6. A combination is made at respective magnifications b1 = 1 and b2 = 1 in zoom compensation, and compensation groups in order to the spacing value of each group when the initial system is at the medium focal length; the, spacing value of the front fixed and zoom groups is d12 = 0.95, that of zoom and compensation groups is d23 = 1.95, and that of compensation group and rear fixed group is d34 = 0.55. The actual focal length f20 in zoom group is 42 mm following the analysis and calculation.
Fig. 1. Schematic diagram of the optical system. (1) front fixed group, (2) zoom group, (3) compensation group, (4) prism, (5) zoom ratio difference compensation group, (6) visible rear fixed group, and (7) mid-infrared rear fixed group.
Table 2 Visible, mid-infrared focal length focus and zoom ratio.
Visible Mid-infrared Focal length difference Zoom ratio difference
Short-focus (mm)
Mid short-focus (mm)
Mid long-focus (mm)
Long-focus (mm)
Zoom ratio
6.509 7.482 0.973
24.090 24.10 0.01
43.7376 38.2089 4.5287
65.0079 51.8958 13.1121
10.00 6.936 3.064
42
G. Ming et al. / Infrared Physics & Technology 64 (2014) 40–46 0
3.2. Analysis of initial structure design result The focal lengths of visible and mid-infrared light are significantly different in each group upon use of ZEMAX into data on initial structure, because the wavelength ranges are different in these light sources. Even in same lens structure, difference in refractive indices results in different focal lengths. The focal length and the spacing of visible light in each group are obtained by calculations based on zoom theory during initial structure design, but those of mid-infrared light are not obtained in this manner. Thus, the latter does not meet the system design based on zoom theory. This theory states that the zoom ratio C, the focal length f0 , and the corresponding image plane displacement D of the zoom system satisfies the following formula [13]:
C¼
b2L b3L b2 b3
f 0 ¼ fL0 C ¼ fL0
ð2Þ b2L b3L b2 b3
ð3Þ
D ¼ b23 ð1 b22 Þdq1 þ ð1 b22 Þdq2
ð4Þ
where b2L is the magnification of zoom group at long system focal lengths, b3L is the magnification of compensation group at long system focal lengths, b2 is the magnification of zoom group at current zoom location, b3 is the magnification of compensation group at
current zoom location, f is focal length of system at current zoom location, and fL0 is focal length at long system focal lengths. Table 2 lists the focal length and the zoom ratio at key points for visible and infrared light, in which a difference exists with the maximum focal length difference of 13.1121 mm is reached at long focal lengths. The maximum difference in zoom ratio is 3.064. 3.3. Principle and method of zoom difference compensation The direct method compensation of zoom ratio difference directly adds a zoom group in the zoom system to increase the total zoom ratio. This method is simple in structure and high in compensation accuracy with the scope of compensation suitable for the system at zoom ratio difference of 5 and below to meet the system design requirements. Table 2 shows that zoom ratio is lower in mid-infrared than in visible light, thereby requiring an addition of a zoom group in the mid-infrared light. The zoom form of this mid-infrared band is as a linkage form comprising three adjacent zoom groups [14–17]. The zoom ratio, the focal length, and the image plane displacement in mid-infrared satisfy Eqs. (5)–(7):
C¼
b2L b3L b4L b2 b3 b4L
f 0 ¼ fL0 C ¼ fL0
ð5Þ
b2L b3L b4L b2 b3 b4L
ð6Þ
(a)
(c)
(b)
(d)
Fig. 2. MTF diagrams of visible light (a) at 6.5 mm, (b) at 24.23 mm, (c) at 43.4 5 mm, and (d) at 65.1 mm.
43
G. Ming et al. / Infrared Physics & Technology 64 (2014) 40–46
D ¼ b24 b23 ð1 b22 Þdq1 þ b24 ð1 b22 Þdq2 þ ð1 b24 Þq3
ð7Þ
where b4L is the magnification of zoom ratio difference compensation group at long system focal lengths, b4 is the magnification of zoom ratio difference compensation group at current system focal length of system, and q3 is the displacement of zoom ratio difference compensation group. The following equations should be met to satisfy the requirement that focal length and zoom ratio of visible light and infrared in any zoom location are similar and that the image plane displacement is compensated in the mid-infrared band based on Eqs. (2), (5), (7):
C¼
b2L b3L b4L b2 b3 b4L
ð8Þ
DI ¼ b24I b23I ð1 b22I Þdq1I þ b24I ð1 b22I Þdq2I þ ð1 b24I Þq3I
ð9Þ
where the parameters with subscripts V and I represent visible and infrared light. Eq. (8) indicates that the zoom ratios of visible and mid-infrared light are similar, and Eq. (9) indicates that mid-infrared band meets the requirement for image plane stability. 4. Optical system design result and image quality evaluation 4.1. Optical system design The requirement of independent aberration correction in each group shall be met to reduce the burden of aberration correction
of rear fixed group and simplify the system structure. Because of large color difference in visible light plus mid-infrared bands as well limited choice of materials, the analysis and design shall focus on public zoom group. The material and the system structure meet the requirement, the materials used for front fixation group are CAF2 and IRG11, and the structure form is dual separation from calculations based on three-level aberration theory and achromatic equation. The materials used for zoom group are ALN and KCL, both with excellent optical performances, in which the lenses are designed into a thick meniscus shape and the two lenses are combined in the manner of dual separation. The materials used for zoom group are meniscus lenses CAF2 and ALN. The final structure of the system comprised 12 lenses and a group of dispersion prisms, with a total length of 281.432 mm in visible light, and a size of 254.379 mm 42.323 mm in mid-infrared band. One diffraction plane and four aspheric planes are added in the system to improve the image quality [18,19].
4.2. Optical system image quality evaluation Fig. 2 shows the visible light transfer function diagram. The transfer function value at each focal length is >0.5 for visible light at 80 lp/mm, and the imaging quality is at or near the diffraction limit. Fig. 3 shows the mid-infrared transfer function diagram. The transfer function value at each mid-infrared focal length is >0.45 at 20 lp/mm that indicates a good image quality of the system.
(a)
(c)
(b)
(d)
Fig. 3. MTF diagrams of mid-infrared light (a) at 6.5 mm, (b) at 24.23 mm, (c) at 43.45 mm, and (d) at 65.1 mm.
44
G. Ming et al. / Infrared Physics & Technology 64 (2014) 40–46
Table 3 Visible/mid-infrared focal length and zoom ratio.
Visible Mid-infrared Focal length difference
Short-focus (mm)
Mid short-focus (mm)
Mid long-focus (mm)
Long-focus (mm)
Zoom ratio
6.50093 6.50272 0.0179
24.2303 24.2344 0.0041
43.4517 43.4514 0.0003
65.1 65.099 0.0001
10.0 10.0
4.3. Zoom difference compensation result The focal length in the zoom ratio difference compensation group is 19.7 mm after calculation and optimization. Table 3 gives the compensation for the zoom ratio difference at each focal length, which shows that the maximum difference in focal length value at each focal length location for visible and mid-infrared is 0.0179 mm and the total zoom ratios for them are similar. The displacement of zoom group q1is taken as the x-coordinate and the focal length of visible and mid-infrared light as the y-coordinate. Two focal length curves were fitted using MATLAB, and Fig. 4(a) shows the results in the same coordinate system, which fully overlapped with each other. Subtraction is performed in the fitting formula for both light sources to obtain the difference between their focal lengths as it changes with the displacement in zoom group (Fig. 4(b)). The maximum difference of <0.02 mm at
short focal lengths exists for visible and mid-infrared light. The design requirements should have a focal depth of ±0.027 mm for visible light. 5. Analysis of optical system athermalization The public zoom group adopts materials that can both get through visible and infrared light [20]; some of these materials are greatly influenced by temperature, thereby considering athermalization for visible and infrared light. Analysis is conducted for the system at a temperature ranging from 40 °C to 60 °C. Table 4 shows specific data of focal lengths for these light sources at different temperatures that are affected by temperature and causes the production of focal shifts. The change of focal shift follows the similar law of change. This shift is relatively small at short focal lengths and maximum at long focal lengths. The maximum focal
Fig. 4. (a) The focal length of visible and mid-infrared light and (b) the focal length difference between these light against the displacement in zoom group.
45
G. Ming et al. / Infrared Physics & Technology 64 (2014) 40–46
Table 4 Visible, mid-infrared focal length change with temperature. Temperature (°C)
Short-focus (mm)
Mid short-focus (mm)
Mid long-focus (mm)
Long-focus (mm)
20 40 60
6.51093 6.52472 6.50181
24.2343 24.4349 24.1004
44.4517 45.0346 44.0648
65.1 66.29413 64.3134
20 40 60
6.51183 6.55116 6.47916
24.2304 24.5696 24.005
44.4582 45.3194 43.8887
65.0997 66.7478 64.0177
Visible
Mid-infrared
(a)
(a)
(b) Fig. 5. MTF diagrams showing the athermalization of visible light (a) at 40 °C and (b) at 60 °C.
shift of the system is <3 mm, thereby eliminating problems by properly choosing an athermalized compensating lens with a fine-tuned position. The results of athermalization of visible and infrared light at maximum focal shift (i.e., long focal lengths) are presented because of space limitations. Figs. 5 and 6 show the extreme temperatures that indicate no significant changes in the image quality following athermalization of the system. The transfer function of visible light is at 80 lp/mm, which is >0.5; that of mid-infrared light falls slightly at 20 lp/mm, which is >0.45 and still meets the requirements of system design. 6. Zoom curve fitting The relative position between zoom group and compensation group in the zoom process is obtained using the multiple configurations in ZEMAX. The zoom curve of the system is fitted
(b) Fig. 6. MTF diagrams showing the athermalization of mid-infrared light (a) at 40 °C and (b) at 60 °C.
via MATLAB (Fig. 7), in which the displacement of the zoom group is set to be linear. Fitting is conducted according to the relative position of the displacement between compensation and zoom group in the compensation group curve that is smooth and has no fold point, thereby meeting the design requirements of the zoom curve.
7. Conclusion The dual-band shared-aperture and co-zoom optical system is designed to realize synchronized and continuous zoom of visible and mid-infrared bands in the same optical path. The system zoom ratio is 10 with a total volume of less than 290 mm 45 mm. In the entire zoom process, the max difference in focal length of visible light and mid-infrared is less than 0.02 mm. The transfer
46
G. Ming et al. / Infrared Physics & Technology 64 (2014) 40–46
on the target. The system can be widely used in detection and observation areas. Acknowledgments The Project is being supported by Natural Science Basic Research Plan in Shaanxi Province of China (Program No. 2012JM8008). The programme is financed by Shaanxi Key Laboratory of Photoelectric Measurement and Instrument Technology. References
Fig. 7. (a) Zoom and (b) compensation group curves.
function value is >0.5 for visible light at 80 lp/mm spatial frequency, and the transfer function value is >0.45 for mid-infrared light at 20 lp/mm. The image quality meets the design requirements after realizing athermalization of the optical system at 40 to 60 °C. Compared with an ordinary dual-band zoom system, this system possesses the advantages of being small and a lightweight, having high timeliness and rapid response, as well as realizing synchronous observation, tracking, and measurement
[1] Nicholas Allan Thompson, Optical design of common aperture common focal plane, multispectral optics for military applications, Opt. Eng. 52 (6) (2013) 061308. [2] Allen Mann, Developments and trends in infrared zoom lenses from 2000 to 2010, Opt. Eng. 52 (1) (2013) 013001. [3] Jay N. Vizgaitis, Optical concepts for dual band infrared continuous zoom lenses, Proc. SPIE 7652 (2010) 76522E. [4] Vladimir Petrushevsky, David Tsur, EO/IR tactical aerial reconnaissance photography system, Proc. SPIE 8360 (2012) 836003. [5] Jeong-Yeol Han, Sergey Marchuk, Hooshik Kim, Imaging EO/IR optical system for long range oblique photography, Proc. SPIE 8020 (2012) 802009. [6] Mo Li, Weiguo Sun, Hongxu Chen, Counter-countermeasure identification based on multi-element dual band infrared detector, Opt. Eng. 52 (21) (2013) 5201–5206. [7] Yitzhak Nevo, Dual-band optics, Opt. Eng. 52 (5) (2013) 053002. [8] Jay N. Vizgaitis, Arthur Hastings, Dual band infrared picture-in-picture systems, Opt. Eng. 52 (6) (2013) 061306. [9] Antonín Mikš, Jirí Novák, Three-component double conjugate zoom lens system from tunable focus lenses, Appl. Opt. 52 (4) (2013) 862–865. [10] John Hartke, Eustace L. Dereniak, Snapshot dual-band visible hyperspectral imaging spectrometer, Opt. Eng. 46 (1) (2007) 013201. [11] C. Florea, J. Sanghera, I. Aggarwal, Broadband beam steering using chalcogenide-based Risley prisms, Opt. Eng. 50 (3) (2011). 033001 0091–3286. [12] Antonín Miks, Method of zoom lens design, Appl. Opt. 47 (32) (2008) 1678– 1682. [13] R. Kingslake, R.B. Johnson, Lens Design Fundamentals, second ed., Academic Press, Burlington, 2010. Ch. 5. [14] Lain A. Neil, Optical design dependence on technology development, Opt. Eng. 50 (12) (2011) 121706. [15] Jim Schwiegerling, Carmen Paleta-Toxqui, Minimal movement zoom lens, Appl. Opt. 48 (10) (2009) 1932–1935. [16] Tao Chun kan, Zoom Lens Design, National Defence Industry Press, Beijing, 1988, pp. 38–41. [17] Chih-Wei Kuo, Jr.-Ming Miao, Chang-Hsien Tai, Mid-wave infrared optical zooming design and kinoform degrading evaluation methods, Appl. Opt. 50 (18) (2011) 3043–3049. [18] K. Fuerschbach, J.P. Rolland, K.P. Thompson, A new family of optical systems employing-polynomial surfaces, Opt. Express 19 (22) (2011) 21919–21928. [19] Yu-Hung Lin, Yen-Liang Liu, Guo-Dung J. Su, Optical zoom module based on two deformable mirrors for mobile device applications, Appl. Opt. 51 (11) (2012) 1804–1810. [20] Ho-Soon Yang, Hagyong Kihm, Il Kweon Moon, Three-shell-based lens barrel for the effective athermalization of an IR optical system, Appl. Opt. 50 (33) (2011) 6206–6213.
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Design of dual-band shared-aperture Co-zoom optical system Gao Ming, Chen Yang ⇑, Liu Jun, Lv Hong School of Optoelectronic Engineering, Xi’an Technological University, Xi’an 710021, China
h i g h l i g h t s Optical system, fusing visible/mid-IR light in the same light path, is designed. A method is obtained to compensate zoom ratio difference between visible and mid-IR. Volume of dual-band continuous zoom optical system is reduced. This optical system can conduct synchronous observation, tracking and measurement with dual-band.
a r t i c l e
i n f o
Article history: Received 15 August 2013 Available online 15 February 2014 Keywords: Dual band Shared aperture Co-zoom Zoom ratio difference compensation Athermalization
a b s t r a c t An optical system that features visible plus mid-infrared light, shared aperture, synchronous and continuous zoom is designed with a 10 zoom ratio. Analysis is performed to differentiate visible plus midinfrared light focal length and zoom ratio during zooming, and the change law of this difference. Upon combination with two-group and three-group zoom theories and upon derivation of the conditions for compensating zoom ratio difference, a method has been obtained to directly compensate this difference. The focal length and the zoom ratio for visible/mid-infrared light at any zoom location are similar with this method, thereby conducting synchronous observation, tracking, and measurement on the target. Design results have shown that the system is small, has a fast response, has an excellent in overall image quality, and is athermal for temperatures between 40 °C and 60 °C. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction A dual-band continuous zoom optical system incorporates the characteristics of a dual-band system (e.g., round-the-clock detection and accurate and comprehensive access to target information) with the advantages of a continuous zoom system (e.g., wide detection range, rapid search, and continuous observation on target). This system has been gained research interest worldwide [1,2]. Vizgaitis [3] has designed an 11.7 military MWIR/LWIR continuous zoom system in 2010 that adopts the reflective-refractive mixed light path and the infrared dichroic focal plane array in concurrent imaging at medium/long-wave infrared band. In his article, Petrushevsky and Tsur [4] has analyzed a Goodrich DB-110 aerial camera having mutually independent visible light and infrared optical systems, which realizes round-the-clock detection and accurate target observation. Han et al. [5] have respectively folded both visible light and infrared dual-band systems via multiple reflections to reduce the system volumes, which is the main problem for these systems. An optical system that fully adopts the ⇑ Corresponding author. Tel.: +86 15129398622. E-mail address: [email protected] (G. Ming). http://dx.doi.org/10.1016/j.infrared.2014.01.014 1350-4495/Ó 2014 Elsevier B.V. All rights reserved.
refraction form to fuse visible light/infrared in the same light path and realizes synchronized and continuous zoom is yet to be reported. The standards for optical system performance have increased with the rapid development of social demand and the increasingly complicated environmental applications [6–10]. However, most of the existing visible light/infrared dual-band zoom systems comprise two separate systems with a large system volume and complex reconnaissance equipment structure. Target search and focal length readjustment prior to another target observation are necessary because of external environment changes, such as target obscurity, smoke interference, alternation of day and night, and change in light path, resulting in a long change process and a slow system response. Furthermore, the target may be lost provided that the target being tracked and observed moves fast. This design obtains continuous zoom for dual bands (visible light/mid-infrared) in the same optical path. Both systems have the same focal length and zoom ratio at any location upon compensating the zoom difference for dual bands. Changing the optical path is unnecessary and can be observed directly upon observation with different bands. This system yields synchronous observation, tracking and measurement on the target with dual bands,
41
G. Ming et al. / Infrared Physics & Technology 64 (2014) 40–46 Table 1 Optical design specifications. Visible
Mid-infrared
Wave
0.48–0.64 lm
Wave
3.7–4.5 lm
Zoom ratio F/# Field of view Focal length
10 5 18–1.8° 6.5–65 mm
Zoom ratio F/# Field of view Focal length
10 3.5 18–1.8° 6.5–65 mm
improves the response rate of the optical system, and avoids the target loss during optical path change. 2. Indicator and structural design of optical system 2.1. Optical system design indicator An optical system is required to implement round-the-clock observation and yield excellent resolution in infrared band, thereby selecting dual bands. The total zoom ratio is designed at 10 to ensure that the system has a larger scope of observation. The overall length of this system is less than 300 mm because of small size and lightweight requirements. Table 1 lists the specific design indicators. The visible-light detector is a 1=400 CCD having a 6.5 lm 6.25 lm pixel size; the mid-infrared detector is a 1=400 uncooled focal plane array having a 25 lm 25 lm pixel size. 2.2. Optical system structure design This system is required to realize synchronous continuous zoom at dual bands. However, a traditional zoom system cannot meet structural design requirements, thereby redesigning this structure comprising five parts: public zoom group, dispersion prism, zoom difference compensation group, visible-light fixed rear lens group, and mid-infrared fixed rear lens group (Fig. 1). (1) The public zoom group comprises three units: front fixation group, zoom group, and compensation group. This portion goes through the visible and infrared light simultaneously, and realizes the synchronous zoom at dual bands.
(2) Dispersion prism [11] allows transmission of visible light and reflects mid-infrared light. (3) The zoom ratio difference compensation group is set in the reflected light path, which can compensate the difference in focal length and zoom ratio of dual bands via location movement. (4) The visible and mid-infrared light rear fixed lens groups can respectively correct image aberration and converge light in their respective detectors for imaging. 3. Initial structure design and analysis of the public zoom group 3.1. Initial structure design Based on design requirements, the zoom ratio is 10, and the zoom and compensation curves are smooth to subsequently compensate the zoom ratio difference. Thus, the mechanical positive group zoom is appropriate [12]. Zoom design theory states that the displacement relationship between zoom group and compensation group will meet formula (1) when the image plane is stable, q3 2f 0 f 0 q 3f 0 q2
dq2 ¼ dq1
2 3 1 3 1ffi q1 p1 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4 0 0 0 3
q1 4f2 f3 4f3 q1
f20 þ q1
þ
q21
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi q41 4f20 f30 q31
ð1Þ
2
2ðf20 þ q1 Þ
where q1 is displacement of zoom group along optical axis; q1 is displacement of compensation group along optical axis; f20 is focal length of zoom group; f30 is focal length of compensation group. The focal length of the zoom group is a standard value, which is f20 ¼ 1 in the initial structure design. The following results are obtained upon calculation coupled with design experience: focal length of front fixation group is f10 ¼ 6, that of the compensation group is f30 ¼ 1:15, and that of the rear fixed group is f40 ¼ 0:6. A combination is made at respective magnifications b1 = 1 and b2 = 1 in zoom compensation, and compensation groups in order to the spacing value of each group when the initial system is at the medium focal length; the, spacing value of the front fixed and zoom groups is d12 = 0.95, that of zoom and compensation groups is d23 = 1.95, and that of compensation group and rear fixed group is d34 = 0.55. The actual focal length f20 in zoom group is 42 mm following the analysis and calculation.
Fig. 1. Schematic diagram of the optical system. (1) front fixed group, (2) zoom group, (3) compensation group, (4) prism, (5) zoom ratio difference compensation group, (6) visible rear fixed group, and (7) mid-infrared rear fixed group.
Table 2 Visible, mid-infrared focal length focus and zoom ratio.
Visible Mid-infrared Focal length difference Zoom ratio difference
Short-focus (mm)
Mid short-focus (mm)
Mid long-focus (mm)
Long-focus (mm)
Zoom ratio
6.509 7.482 0.973
24.090 24.10 0.01
43.7376 38.2089 4.5287
65.0079 51.8958 13.1121
10.00 6.936 3.064
42
G. Ming et al. / Infrared Physics & Technology 64 (2014) 40–46 0
3.2. Analysis of initial structure design result The focal lengths of visible and mid-infrared light are significantly different in each group upon use of ZEMAX into data on initial structure, because the wavelength ranges are different in these light sources. Even in same lens structure, difference in refractive indices results in different focal lengths. The focal length and the spacing of visible light in each group are obtained by calculations based on zoom theory during initial structure design, but those of mid-infrared light are not obtained in this manner. Thus, the latter does not meet the system design based on zoom theory. This theory states that the zoom ratio C, the focal length f0 , and the corresponding image plane displacement D of the zoom system satisfies the following formula [13]:
C¼
b2L b3L b2 b3
f 0 ¼ fL0 C ¼ fL0
ð2Þ b2L b3L b2 b3
ð3Þ
D ¼ b23 ð1 b22 Þdq1 þ ð1 b22 Þdq2
ð4Þ
where b2L is the magnification of zoom group at long system focal lengths, b3L is the magnification of compensation group at long system focal lengths, b2 is the magnification of zoom group at current zoom location, b3 is the magnification of compensation group at
current zoom location, f is focal length of system at current zoom location, and fL0 is focal length at long system focal lengths. Table 2 lists the focal length and the zoom ratio at key points for visible and infrared light, in which a difference exists with the maximum focal length difference of 13.1121 mm is reached at long focal lengths. The maximum difference in zoom ratio is 3.064. 3.3. Principle and method of zoom difference compensation The direct method compensation of zoom ratio difference directly adds a zoom group in the zoom system to increase the total zoom ratio. This method is simple in structure and high in compensation accuracy with the scope of compensation suitable for the system at zoom ratio difference of 5 and below to meet the system design requirements. Table 2 shows that zoom ratio is lower in mid-infrared than in visible light, thereby requiring an addition of a zoom group in the mid-infrared light. The zoom form of this mid-infrared band is as a linkage form comprising three adjacent zoom groups [14–17]. The zoom ratio, the focal length, and the image plane displacement in mid-infrared satisfy Eqs. (5)–(7):
C¼
b2L b3L b4L b2 b3 b4L
f 0 ¼ fL0 C ¼ fL0
ð5Þ
b2L b3L b4L b2 b3 b4L
ð6Þ
(a)
(c)
(b)
(d)
Fig. 2. MTF diagrams of visible light (a) at 6.5 mm, (b) at 24.23 mm, (c) at 43.4 5 mm, and (d) at 65.1 mm.
43
G. Ming et al. / Infrared Physics & Technology 64 (2014) 40–46
D ¼ b24 b23 ð1 b22 Þdq1 þ b24 ð1 b22 Þdq2 þ ð1 b24 Þq3
ð7Þ
where b4L is the magnification of zoom ratio difference compensation group at long system focal lengths, b4 is the magnification of zoom ratio difference compensation group at current system focal length of system, and q3 is the displacement of zoom ratio difference compensation group. The following equations should be met to satisfy the requirement that focal length and zoom ratio of visible light and infrared in any zoom location are similar and that the image plane displacement is compensated in the mid-infrared band based on Eqs. (2), (5), (7):
C¼
b2L b3L b4L b2 b3 b4L
ð8Þ
DI ¼ b24I b23I ð1 b22I Þdq1I þ b24I ð1 b22I Þdq2I þ ð1 b24I Þq3I
ð9Þ
where the parameters with subscripts V and I represent visible and infrared light. Eq. (8) indicates that the zoom ratios of visible and mid-infrared light are similar, and Eq. (9) indicates that mid-infrared band meets the requirement for image plane stability. 4. Optical system design result and image quality evaluation 4.1. Optical system design The requirement of independent aberration correction in each group shall be met to reduce the burden of aberration correction
of rear fixed group and simplify the system structure. Because of large color difference in visible light plus mid-infrared bands as well limited choice of materials, the analysis and design shall focus on public zoom group. The material and the system structure meet the requirement, the materials used for front fixation group are CAF2 and IRG11, and the structure form is dual separation from calculations based on three-level aberration theory and achromatic equation. The materials used for zoom group are ALN and KCL, both with excellent optical performances, in which the lenses are designed into a thick meniscus shape and the two lenses are combined in the manner of dual separation. The materials used for zoom group are meniscus lenses CAF2 and ALN. The final structure of the system comprised 12 lenses and a group of dispersion prisms, with a total length of 281.432 mm in visible light, and a size of 254.379 mm 42.323 mm in mid-infrared band. One diffraction plane and four aspheric planes are added in the system to improve the image quality [18,19].
4.2. Optical system image quality evaluation Fig. 2 shows the visible light transfer function diagram. The transfer function value at each focal length is >0.5 for visible light at 80 lp/mm, and the imaging quality is at or near the diffraction limit. Fig. 3 shows the mid-infrared transfer function diagram. The transfer function value at each mid-infrared focal length is >0.45 at 20 lp/mm that indicates a good image quality of the system.
(a)
(c)
(b)
(d)
Fig. 3. MTF diagrams of mid-infrared light (a) at 6.5 mm, (b) at 24.23 mm, (c) at 43.45 mm, and (d) at 65.1 mm.
44
G. Ming et al. / Infrared Physics & Technology 64 (2014) 40–46
Table 3 Visible/mid-infrared focal length and zoom ratio.
Visible Mid-infrared Focal length difference
Short-focus (mm)
Mid short-focus (mm)
Mid long-focus (mm)
Long-focus (mm)
Zoom ratio
6.50093 6.50272 0.0179
24.2303 24.2344 0.0041
43.4517 43.4514 0.0003
65.1 65.099 0.0001
10.0 10.0
4.3. Zoom difference compensation result The focal length in the zoom ratio difference compensation group is 19.7 mm after calculation and optimization. Table 3 gives the compensation for the zoom ratio difference at each focal length, which shows that the maximum difference in focal length value at each focal length location for visible and mid-infrared is 0.0179 mm and the total zoom ratios for them are similar. The displacement of zoom group q1is taken as the x-coordinate and the focal length of visible and mid-infrared light as the y-coordinate. Two focal length curves were fitted using MATLAB, and Fig. 4(a) shows the results in the same coordinate system, which fully overlapped with each other. Subtraction is performed in the fitting formula for both light sources to obtain the difference between their focal lengths as it changes with the displacement in zoom group (Fig. 4(b)). The maximum difference of <0.02 mm at
short focal lengths exists for visible and mid-infrared light. The design requirements should have a focal depth of ±0.027 mm for visible light. 5. Analysis of optical system athermalization The public zoom group adopts materials that can both get through visible and infrared light [20]; some of these materials are greatly influenced by temperature, thereby considering athermalization for visible and infrared light. Analysis is conducted for the system at a temperature ranging from 40 °C to 60 °C. Table 4 shows specific data of focal lengths for these light sources at different temperatures that are affected by temperature and causes the production of focal shifts. The change of focal shift follows the similar law of change. This shift is relatively small at short focal lengths and maximum at long focal lengths. The maximum focal
Fig. 4. (a) The focal length of visible and mid-infrared light and (b) the focal length difference between these light against the displacement in zoom group.
45
G. Ming et al. / Infrared Physics & Technology 64 (2014) 40–46
Table 4 Visible, mid-infrared focal length change with temperature. Temperature (°C)
Short-focus (mm)
Mid short-focus (mm)
Mid long-focus (mm)
Long-focus (mm)
20 40 60
6.51093 6.52472 6.50181
24.2343 24.4349 24.1004
44.4517 45.0346 44.0648
65.1 66.29413 64.3134
20 40 60
6.51183 6.55116 6.47916
24.2304 24.5696 24.005
44.4582 45.3194 43.8887
65.0997 66.7478 64.0177
Visible
Mid-infrared
(a)
(a)
(b) Fig. 5. MTF diagrams showing the athermalization of visible light (a) at 40 °C and (b) at 60 °C.
shift of the system is <3 mm, thereby eliminating problems by properly choosing an athermalized compensating lens with a fine-tuned position. The results of athermalization of visible and infrared light at maximum focal shift (i.e., long focal lengths) are presented because of space limitations. Figs. 5 and 6 show the extreme temperatures that indicate no significant changes in the image quality following athermalization of the system. The transfer function of visible light is at 80 lp/mm, which is >0.5; that of mid-infrared light falls slightly at 20 lp/mm, which is >0.45 and still meets the requirements of system design. 6. Zoom curve fitting The relative position between zoom group and compensation group in the zoom process is obtained using the multiple configurations in ZEMAX. The zoom curve of the system is fitted
(b) Fig. 6. MTF diagrams showing the athermalization of mid-infrared light (a) at 40 °C and (b) at 60 °C.
via MATLAB (Fig. 7), in which the displacement of the zoom group is set to be linear. Fitting is conducted according to the relative position of the displacement between compensation and zoom group in the compensation group curve that is smooth and has no fold point, thereby meeting the design requirements of the zoom curve.
7. Conclusion The dual-band shared-aperture and co-zoom optical system is designed to realize synchronized and continuous zoom of visible and mid-infrared bands in the same optical path. The system zoom ratio is 10 with a total volume of less than 290 mm 45 mm. In the entire zoom process, the max difference in focal length of visible light and mid-infrared is less than 0.02 mm. The transfer
46
G. Ming et al. / Infrared Physics & Technology 64 (2014) 40–46
on the target. The system can be widely used in detection and observation areas. Acknowledgments The Project is being supported by Natural Science Basic Research Plan in Shaanxi Province of China (Program No. 2012JM8008). The programme is financed by Shaanxi Key Laboratory of Photoelectric Measurement and Instrument Technology. References
Fig. 7. (a) Zoom and (b) compensation group curves.
function value is >0.5 for visible light at 80 lp/mm spatial frequency, and the transfer function value is >0.45 for mid-infrared light at 20 lp/mm. The image quality meets the design requirements after realizing athermalization of the optical system at 40 to 60 °C. Compared with an ordinary dual-band zoom system, this system possesses the advantages of being small and a lightweight, having high timeliness and rapid response, as well as realizing synchronous observation, tracking, and measurement
[1] Nicholas Allan Thompson, Optical design of common aperture common focal plane, multispectral optics for military applications, Opt. Eng. 52 (6) (2013) 061308. [2] Allen Mann, Developments and trends in infrared zoom lenses from 2000 to 2010, Opt. Eng. 52 (1) (2013) 013001. [3] Jay N. Vizgaitis, Optical concepts for dual band infrared continuous zoom lenses, Proc. SPIE 7652 (2010) 76522E. [4] Vladimir Petrushevsky, David Tsur, EO/IR tactical aerial reconnaissance photography system, Proc. SPIE 8360 (2012) 836003. [5] Jeong-Yeol Han, Sergey Marchuk, Hooshik Kim, Imaging EO/IR optical system for long range oblique photography, Proc. SPIE 8020 (2012) 802009. [6] Mo Li, Weiguo Sun, Hongxu Chen, Counter-countermeasure identification based on multi-element dual band infrared detector, Opt. Eng. 52 (21) (2013) 5201–5206. [7] Yitzhak Nevo, Dual-band optics, Opt. Eng. 52 (5) (2013) 053002. [8] Jay N. Vizgaitis, Arthur Hastings, Dual band infrared picture-in-picture systems, Opt. Eng. 52 (6) (2013) 061306. [9] Antonín Mikš, Jirí Novák, Three-component double conjugate zoom lens system from tunable focus lenses, Appl. Opt. 52 (4) (2013) 862–865. [10] John Hartke, Eustace L. Dereniak, Snapshot dual-band visible hyperspectral imaging spectrometer, Opt. Eng. 46 (1) (2007) 013201. [11] C. Florea, J. Sanghera, I. Aggarwal, Broadband beam steering using chalcogenide-based Risley prisms, Opt. Eng. 50 (3) (2011). 033001 0091–3286. [12] Antonín Miks, Method of zoom lens design, Appl. Opt. 47 (32) (2008) 1678– 1682. [13] R. Kingslake, R.B. Johnson, Lens Design Fundamentals, second ed., Academic Press, Burlington, 2010. Ch. 5. [14] Lain A. Neil, Optical design dependence on technology development, Opt. Eng. 50 (12) (2011) 121706. [15] Jim Schwiegerling, Carmen Paleta-Toxqui, Minimal movement zoom lens, Appl. Opt. 48 (10) (2009) 1932–1935. [16] Tao Chun kan, Zoom Lens Design, National Defence Industry Press, Beijing, 1988, pp. 38–41. [17] Chih-Wei Kuo, Jr.-Ming Miao, Chang-Hsien Tai, Mid-wave infrared optical zooming design and kinoform degrading evaluation methods, Appl. Opt. 50 (18) (2011) 3043–3049. [18] K. Fuerschbach, J.P. Rolland, K.P. Thompson, A new family of optical systems employing-polynomial surfaces, Opt. Express 19 (22) (2011) 21919–21928. [19] Yu-Hung Lin, Yen-Liang Liu, Guo-Dung J. Su, Optical zoom module based on two deformable mirrors for mobile device applications, Appl. Opt. 51 (11) (2012) 1804–1810. [20] Ho-Soon Yang, Hagyong Kihm, Il Kweon Moon, Three-shell-based lens barrel for the effective athermalization of an IR optical system, Appl. Opt. 50 (33) (2011) 6206–6213.