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A new kind of dynamic dual-foveated imaging system in the infrared band is designed and optimized in this paper. Dual-foveated imaging refers to the variation in spatial resolution at the two selected fields across the image. Such variable resolution imaging system is suitable for a variety of applications including monitoring, recognition, and remote operation of unmanned aerial vehicle. In this system, a transmissive spatial light modulator (SLM) is used as an active optical element which is located near the image plane instead of pupil plane creatively in order to divide the two selected fields.
© 2015 Optical Society of America
1. Introduction
In conventional imaging systems, it is difficult for an optical designer to balance the requirement of high image quality and wide-angle FOV. Increasing the f-number of the optical system by stopping down the entrance pupil is a common method. However, small entrance pupil will cut down the resolution of the optical system and the amount of light the image plane receives. Another solution is using more optical elements to balance off-axis aberrations, multiple optical elements bring more variable parameters which make the design processing easier to reach the requirement of high-resolution of the entire FOV but at the cost of making the system more complex, heavier, bulkier and expensive. Meanwhile, the rate of transmission and processing of high-resolution digital video frames would be slow down because of the large data contained in high-resolution images across the entire FOV.
But on some occasions, such as monitoring, object recognition, and remote operation of unmanned aerial vehicle, small size and light weight is important to further achieve high-resolution of the field of interest (FOI) and fast real-time processing simultaneously. This kind of optical system are required to be compact, fast (low f-number) and cheap.
To solve the above problems, foveated imaging is proposed by researchers, which is a new kind of variable resolution imaging system with large FOV and high-resolution only in the FOI, where the aberration is well corrected [1]. But when more than one object need to be observed, dual-foveated-imaging is more suitable. On the fundamental study of foveated imaging, the dual-foveated imaging, which is inspired by the operation of some birds eyes, is proposed to improve the ability of perceiving objects [2]. Here we achieved an new optical design of a dynamical dual-foveated imaging system in the infrared band for two FOIs. In this system, a transmissive spatial light modulator (SLM) is used as an active optical element, and we placed it near the image plane instead of the pupil plane creatively in order to separate the rays of two different selected fields.
2. Foveated imaging and dual-foveated imaging
The concept of foveated imaging comes from studies of simulating human eyes. Human beings have a special variable resolution vision system, the resolution is very high in a limited area which is called fovea in the region of macula, and the resolution reduces quickly toward the peripheral FOV [1].
Foveated imaging can be achieved by combining images from two sensors in a conventional imaging system, one is creating low resolution images over the FOV and the other one high over the FOI [3,4]. However, this technique is at the hardware level which lacks real-time and fast features, these disadvantages can also be found in another way of foveated imaging at software level by applying foveation algorithms to images from a conventional imaging system [5,6]. It is now popular to achieve foveated imaging by utilizing active optical elements such like deformable mirror(DM), micro-electro-mechanical system (MEMS) [7,8] and spatial light modulator [9–11]. When taking into account the costs and complexity, transmissive liquid crystal (LC) phase SLM is the better choice. A variety of researchers are focused on the performance of SLM and discussed the details of using SLM to achieve foveted imaging [11].
Studies have indicated that some certain birds have a greater vision and stronger ability of perceive distant moving objects than humans and many other animals, which is due to the specificity of their eyes. This kind of birds have two foveae, meaning that there are two high resolution regions in their retina, a fovea is located in the temporal retina besides to the more common centrally located fovea, called shallow fovea and deep fovea respectively [12].
Dual-foveated imaging is proposed by simulating the visual system of such these birds, refers to the variation in spatial resolution at the two selected fields across the image. As for the foveated imaging system, dual-foveated imaging system is also a new kind of variable resolution imaging system with wide FOV and local high-resolution in two FOIs. It can have simpler and lighter structure compared with the conventional imaging systems under same design requirements, therefore the disadvantages (e.g. it is bulky, complex and expensive) of conventional imaging systems have been corrected. Another important benefit of these local high-resolution imaging systems is that they reduce the data needed for transmission and processing.
However, dual-foveated imaging systems have two high resolution FOIs, which means it can correct the residual aberration of two selected fields dynamically while the other fields have low resolution. Therefore, dual-foveated imaging have a wider range of applications. First of all, when it is necessary to perceive more than one moving objects or distant moving objects, dual foveated imaging can provide two high resolution FOIs, and when the speed of transmission and processing is required to be quick, dual-foveated imaging also have great potential application value.
3. Optical design
For imaging systems, the feature that some birds have two foveae can be applied in the imaging systems that have two selected highly resolved FOIs and peripheral fields with uncorrected aberrations in a wide FOV [2].
The main design concept that we applied in this study is to use SLM to correct the residual aberration of two FOIs. SLM is a kind of pixelate monochrome device that is used to modulate the amplitude or phase of the incident light. For phase SLMs, they are reflective or transmissive devices used to control the optical wavefront by dynamically changing the optical path difference (OPD) across the aperture. By applying a small voltage to an individual pixel, the index of refraction in the direction of propagation is changed, then the optical retardance of that pixel is also changed.
In our study, aberration correction is accomplished by systematically altering the optical path across the wavefront for FOIs. By applying the appropriate voltage to each pixel, the optical path can be adjusted to correctly compensate the aberrated wavefront. The mathematical expression of the OPD created by applying voltage is described as Eq. (1).
where is the change in index of refraction in the direction of propagation, and z is the thickness of the liquid crystal of the SLM. In this paper, a transmissive phase SLM is used to correct the residual aberration of the two selected fields of the dual-foveated imaging system.We use a commercial optical design software (CODE V) to do the system design and optimization. The SLM was modeled as a Zernike phase surface, which is an infinitely thin phase plate and described by a Zernike polynominal. The Zernike phase surfuce was optimized at the two FOIs to minimize the wavefront error.
Theoretically, the optical design is advanced by placing the SLM near the image plane instead of pupil plane. In this way, the two selected FOIs can be divided and the error of the wavefront at any field can be corrected separately in our design, as shown in Fig. 1.
Fig. 1 Optical 3D layout for dual-foveated imaging system (f/# = 2) with a +/−10 °field-of-view (a) and the separated rays of each fields (b).
In our study, the key of achieving dual-foveated imaging is separating rays of different field angles to correct the aberration of two FOIs. Because the field angles of the system are continuous, the rays of each field are totally separated only when the angular difference between two FOIs is no less than 1.3°, which is shown in the Fig. 1. In our design, all the lenses are Germanium. The system has a focal length of 140mm with the f-number of 2, and the working wavelength is 10000nm covered the full FOV of (−10°,10°).
Because the rays of each field angle are separated, and the aberrations at each field angle can be determined in advance, either by ray trace calculations or direct measurements, the set of voltages required to control the wavefront phase of two FOIs can be stored as a look-up table. So the dynamic dual-foveated imaging can be gotten. Here, in order to describe the dynamic correction for the two FOIs in a dual-foveated imaging system, three typical states were simulated, and they are State-1, State-2 and State-3. For each state we chose two representational FOIs. In these states, only the parameters of the SLM is variable while the parameters of lens are the same and freeze among the three states. The two chosen fields of State-1 are (−5°,4°) and (−5°,-5°), the field value is different in the y direction, while the State-2 are (−5°,5°) and (5°,5°) which different in the x direction, and of State-3 are (−5°,5°)and (5°,-5°), as shown in Fig. 2.
The modulation transfer functions(MTF) plots and pupil map on the surface of SLM of two FOIs for each state are shown in Figs. 3, 4, and 5. The MTF plots are very close to the diffraction limit after the aberration correction by the SLM. The pupil map can also show that the aberration of the two FOIs has been well corrected. Therefore, the dual-foveated imaging systems have high resolution only in the two FOIs while the other fields have low resolution.
Fig. 3 MTF plots and pupil map on the surface of SLM of two FOI of dual-foveated imaging system of State-1.
Fig. 4 MTF plots and pupil map on the surface of SLM of two FOI of dual-foveated imaging system of State-2.
Fig. 5 MTF plots and pupil map on the surface of SLM of two FOI of dual-foveated imaging system of State-3.
Table 1 lists the wavefront analysis without and with correction by SLM in state-1, the RMS of the FOIs decreased by 57.7% in (−5°, 4°) and 50.8% in (−5°,-5°) to about 0.045λ compared with that in the system before correction by SLM. Strehl ratio increased to above 0.91 while it is much worse in other fields. Thus the data that is needed for transmission and processing is reduced. Another benefit is that the imaging system can be light and simple in that the FOIs could be changed dynamically without adjusting the lenses mechanically due to the pre-calculated aberration of each field.
Table 1. Wavefront analysis without and with correction by SLM of State-1
4. Imaging simulation and results of dual-foveated imaging system
We used the image named “USAF1951_array” included in the function “2D Image Simulation” of CODEV as our object, as shown in Fig. 6.
Figs. 7(a), 8(a), 9(a) shows the images simulated by CODEV in the above dual-foveated imaging system in each state. We had the surface of SLM matched by using MATLAB, as it is shown in the Figs. 7(c), 8(c), 9(c),where a different color means different OPD (or phase) introduced by SLM in different areas. Dual-foveated imaging is correcting the aberrations in the two FOIs without in other fields in this way. Magnifying the region of two FOIs and another region, we can see that the details are more clearer in the two FOIs than in other regions, as it is shown in the Figs. 7(b), 8(b), 9(b), the green color regions represent the FOIs and the red color means the un-FOIs.
5. Conclusion
In this paper, we introduced a transmissive SLM near the image plane to correct the residual aberration of the two FOIs. We designed and optimized a new kind of dynamic dual-foveated imaging system with a +/−10 ° FOV in the infrared band. According to the simulated images for the dual-foveated imaging, we conclude that the two FOIs among the entire FOV can be highly resolution while the other fields appear blurred by using SLM near the image plane in this application. Compared with conventional imaging systems, dual-foveated imaging system simplifies the wide-FOV optical system by using SLM which also decrease the data needed for program processing and transmission. So it is able to observe more than one objects and distant moving objects. This kind of multi-resolution imaging system can fit in many applications for detection, identification and tracking, etc.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (Grant No. 61178041) and Aeronautical Science Foundation of China (Grant No. 20150172002).
References and links
1. T. Martinez, D. Wick, and S. Restaino, “Foveated, wide field-of-view imaging system using a liquid crystal spatial light modulator,” Opt. Express 8(10), 555–560 (2001). [CrossRef] [PubMed]
2. F. Chi, C. Jun, and H.-B. Yang, “Design of dually foveated imaging optical system,” J. Acta Phys. Sin 64(3), 034201 (2015).
3. J. Yang, Miami US Patent 7973834 (2011).
4. H. Hua and S. Liu, “Dual-sensor foveated imaging system,” J. Appl. Opt. 47(3), 317–327 (2008). [CrossRef] [PubMed]
5. Y. Qin, Z. Zheng, and H. Hua, “Multi-resolution foveated laparoscope,” in Proceedings of FIO/Laser Science Conference XXVIII, OSA Technical Digest (online) (Optical Society of America), paper FTh1F.4 (2012). [CrossRef]
6. W. S. Geisler and J. S. Perry, “Real time foveated multiresolution system for low-bandwidth video communication,” Proc. SPIE 3299, 294–305 (1998).
7. B. E. Bagwell, D. V. Wick, W. D. Cowan, O. B. Spahn, W. C. Sweatt, T. Martinez, S. R. Restaino, J. R. Andrews, C. C. Wilcox, D. M. Payne, and R. Romeo, “Active zoom imaging for operationally responsive space,” Proc. SPIE 6467, 64670D (2007). [CrossRef]
8. X. Zhao and X. W. Zhao, “Broadband and wide field of view foveated imaging system in space,” J. Opt. Eng. 47(10), 1065–1074 (2008).
9. D. Wick, T. Martinez, S. Restaino, and B. Stone, “Foveated imaging demonstration,” Opt. Express 10(1), 60–65 (2002). [CrossRef] [PubMed]
10. G. Curatu and J. E. Harvey, “Lens design and system optimization for foveated imaging,” Proc. SPIE 7060, 70600P (2008).
11. G. Curatu and J. E. Harvey, “Analysis and design of wide-angle foveated optical systems based on transmissive liquid crystal spatial light modulators,” J. Opt. Eng. 8(4), 043001 (2009).
12. M. E. C. Fitzgerald, P. D. R. Gamlin, Y. Zagvazdin, and A. Reiner, “Central neural circuits for the light-mediated reflexive control of choroidal blood flow in the pigeon eye: a laser Doppler study,” J. Vis. Neurosci. 13(4), 655–669 (1996). [CrossRef] [PubMed]
