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Abstract
Imaging systems are widely used in many fields. However, there is an inherent compromise between field of view (FOV) and resolution. In this paper, we propose an optofluidic zoom system with increased FOV and less chromatic aberration, which can realize switching between large FOV and high resolution. The proposed system consists of a liquid prism, a zoom objective, an image sensor and image processing module, which can realize optical zoom and deflection. The proposed system achieves non-mechanical optical zoom from f = 40.5 mm to f = 84.0 mm. Besides, the angular resolution of zoom objective is up to 26"18 at f = 84.0 mm. The deflection range is ±10°, and the whole FOV of proposed system can reach up to 30.3°. The proposed system is compact and easy to machine. In addition, we reduce chromatic aberration produced by the liquid prism significantly. The proposed system can be used in monitor system, target tracking system, telescope system and so on.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Imaging systems are widely used in many fields such as medical science, astronomy, industry, etc. Powered by advanced technology, there is a growing preference for systems which are lightweight, compact and higher performance. As the information capacity of optical systems is limited by the image plane size and optical power, the field of view (FOV) and details obtained from optical systems are limited. There is an inherent compromise between larger FOV and higher resolution due to the space-bandwidth product theorem. Some approaches have been taken, for example, image stitching, zoom imaging and moving the camera. For image stitching, multiscale cameras [1,2] are used to image piece by piece. Unfortunately, it is too complex and expensive to get larger FOV and higher resolution through increasing the number of cameras. Zoom imaging allows switching between large FOV and high resolution, but this switching only occurs in the center of the FOV and is not possible at the edges. The edge information can be magnified by moving the camera [3]. However, it usually requires mechanical movement, which makes the structure complex and large. Another way to achieve large FOV is using a beam steering device, which can realize FOV deflection by changing the direction of the light passing through the optical system. Conventional mechanical beam steering methods usually have complex structure and high energy consumption, such as Risley-prism [4,5], Double wedge prism [6], and galvanometer, etc. Combining beam steering and zoom imaging can obtain more information while switching FOV and resolution. However, such a system implemented by traditional means is usually difficult to lighten and miniaturize.
Adaptive lens provides more possibilities for these problems, due to its promise in compact and novel flexible concepts. There are different usages of adaptive lenses, for instance tunable optical power [7–10], optical steering [11–14] and aberration correction [15]. The optical zoom imaging systems using adaptive liquid lenses can be more compact and lightweight. Varifocal lens greatly alleviates the design and manufacturing challenges in zoom imaging. In recent years, there have been many researches on the application of adaptive lenses in optical systems [16–19]. Compared to conventional beam steering methods, Adaptive lens is more lightweight, and compact, which is usually based on electrowetting [14,20–25], crystal [11,12,26], dielectric force [27,28] or electromagnetic [29]. In addition, it has been used in laser scanner [21], lidar [24,30], optical switches [31] and other fields [17,32–34]. Although the FOV can be expanded [25,35], a further challenge in imaging is the ability to correct the chromatic aberration [36] generated by the liquid prism.
In this paper, we propose an optofluidic zoom system with increased FOV and less chromatic aberration. The varifocal liquid lenses are used to achieve non-mechanical optical zoom. The focal length of the system ranges from 40.5 mm to 84.0 mm, with the angular resolution up to 26"18 (f = 84 mm). To enlarge the FOV of the system, a liquid prism is used in the system, and the deflection range is ±10°. The whole FOV of proposed system can reach up to 30.3°. In addition, we study the chromatic aberration produced by the liquid prism and apply this rule to reduce chromatic aberration.
2. System structure and theoretical analysis
Schematic illustration of the proposed system is shown in Fig. 1(a), which consists of four components: a liquid prism, a zoom objective, an image sensor and image processing module. The proposed system can realize optical zoom and deflection using liquid lenses and liquid prism. Due to chromatic aberration induced by deflection, an image processing module is used in the proposed system, which helps improve the image quality a lot. As the essential element, the liquid prism is designed and fabricated to help the system achieve lateral deflection of the field of view, as shown in Fig. 1(b). The designed liquid prism consists of two immiscible liquid materials, a spacer, two window glasses. The liquid prism can be driven by a motor.
Fig. 1. Schematic cross-sectional structure of the proposed system. (a) Structure of the proposed system. (b) Structure of the liquid prism. (c) Image process.
The relationship between beam steering angle and inclination angle of spacer can be obtained as follow:
Due to large deflection angle, the proposed system suffers from large chromatic aberration. Thus, an image processing module is proposed to ease the problem. The image processing module is preset in the proposed system. Thus, when the proposed system deflects the FOV, the image processing module is used to correct the chromatic aberration. The flow chart of imaging processing is shown in Fig. 1(c).
The deflected images are separated into three channels with RGB. Then we make different displacements of the pixel matrix of the R and G channels to compensate for the chromatic aberration produced by the prism. The image displacement function can be described as:
3. Design and fabrication
We designed and simulated the proposed zoom objective and liquid prism in Zemax. The 2D layout is shown in Figs. 2(a)–2(c). The zoom objective is composed of three liquid lenses and a doublet. The liquid lens is Optotune EL-10-30 [37], and optical power tuning range is −1.5m−1 ∼ + 6m−1. The liquid lens consists of an offset lens, adaptive lens material and a container glass. Their refractive index and Abbe number are shown in Table 1.
Fig. 2. Design of the zoom objective. (a) f = 40 mm. (b) f = 60 mm. (c) f = 80 mm. (d) Simulation of optical power
Table 1. Refractive index and Abbe number of designed materials
Limited by the variable optical power range of the liquid lens, the focal length of the proposed system varies from 40 mm to 80 mm as shown in Fig. 2(d). The full FOV of the system is 10.28° at f = 40 mm. The MTF (Modulation Transfer Function) curves of different focal lengths are shown in Fig. 3. When the largest image resolution is 20 lp/mm, the MTF at different magnifications are all beyond 0.3. Besides, the image resolution can reach up to 54 lp/mm at f = 65 mm. Therefore, we can conclude there are great imaging quality at 40-80 mm.
Fig. 3. MTF of zoom telescope objective at different focal length. (a) f = 40 mm. (b) f = 55 mm. (c) f = 65 mm. (d) f = 80 mm.
The window glass is Boron-Silicate Glass, and the spacer is Polymeric Methyl Methacrylate (PMMA). Liquid 1 is the water solution, liquid 2 is Iota Silicone Oil (705) [38]. And the densities of the two liquids are matched. The refractive index and Abbe number are shown in Table 2.
Table 2. Refractive index and Abbe number of liquid prism materials
The chromatic aberration of the system with different deflection angles is simulated in Zemax. The relationship of chromatic aberration and deflection angle at certain wavelength is shown in Fig. 4. Chromatic aberration varies with deflection angle as follow:
where $\lambda $ is wavelength, $\theta $ is deflection angle of liquid prism, f is the focal length of the zoom objective, $\varDelta L$ means chromatic aberration on the sensor. From this curve, we can get that the lager the angle of light steering, the more severe the chromatic aberration. Therefore, we employ this rule to reduce the chromatic aberration.Fig. 4. Correspondence between chromatic aberration and deflection angle. (a) λ=0.486um. (b) λ=0.656um.
4. Experiment and discussion
The structure of the proposed system is shown in Fig. 5. All the fabricated elements are shown in Figs. 5(b) and (c). The liquid prism consists of two immiscible liquid materials, a spacer, two window glasses, a cavity and rotating shaft. The effective size of the liquid prism is ∼20 mm × ∼19.9 mm. Three liquid lenses and a doublet are used in the zoom system. The size of the doublet is Ф18 mm × 19.4 mm.
To evaluate the optical performance of the zoom objective, we built an experimental device, as shown in Fig. 6(a). The resolution target is added in front of a collimator with the focal length of 1000 mm (Ф=40 mm). The light source illuminates the resolution target, which is imaged on the 1/2.5"CMOS sensor (MV-CE050-30UC) through the collimator and the proposed zoom objective.
Fig. 6. Imaging quality of the zoom telescope objective and optical power at different focal length. (a)Experimental device for measuring imaging quality. (b) Focal length is 40.54 mm. (c) Focal length is 84 mm. (d) Normalized relative intensity of angular resolution at 34"98 and focal length is 40.5 mm. (e) Normalized relative intensity of angular resolution at 26"18 and focal length is 84.0 mm. (f) Optical power curves, ‘S’ means simulated curve, ‘E’ means experimental curve.
The focal length of the proposed system can be calculated through the resolution target. It is made according to JB/T9328-1999 standard [39]. When the focal length is 40.5 mm, the angular resolution in Fig. 6(b) is 34"98. When the focal length is 84.0 mm, the angular resolution in Fig. 6(c) is 26"18. Besides, the angular resolution at 34"98 and 26"18 can be normalized relative intensity as shown in Figs. 6(d) and (e), respectively. From the test results we can conclude that the proposed system can achieve the angle resolution better than 26"18.
The optical power change of liquid lens at different focal lengths of the system is simulated, then tested by the experimental device, as shown in Fig. 6(f). Lens 1 is used as the main adjustment element of zoom, and lens 2 and 3 compensate the position of the image plane. It can be seen that the general trend of measured optical power is consistent with is simulation. However, the measured optical power of some lenses is different from that of simulation, which mainly results from the optical power tuning range of liquid lens and fabrication error. The experiment results show that the focal length of the system can vary from 40.5 mm to 84.0 mm.
In order to test the deflection ability of the liquid prism, we set up an experiment, as shown in Fig. 7(a), where a laser beam (MBL-III-473-20 mW) is used as the incident beam. d (56 mm) is the displacement of the light spot, h is the distance between the screen and the liquid prism, and α is the steering angle which can be calculated [$\alpha = arctan ({d\textrm{ / }h} )$]. This experiment shows that the liquid prism can realize the deflection of light range from −10.09° to 10.09°.
The proposed system can achieve optical zoom and deflection, which means the proposed system can magnify any detail in the FOV, especially the marginal area. Figure 8 shows the imaging quality of the proposed system. Figure 8(a) shows the captured image in large FOV. The focal length is ∼46.16 mm. When the focal length is changed to ∼70.64 mm, the detail in the center FOV is magnified as shown in Fig. 8(b). However, the marginal target “rabbit” cannot be seen. Therefore, the liquid prism is used to deflect the FOV, so that the whole “rabbit” can be seen in the center FOV as shown in Fig. 8(c). However, due to deflection the chromatic aberration can be observed. For our system, in this case, the image processing module is used to correct the chromatic aberration, and the results are shown in Fig. 8(d). Similarly, we deflect the FOV, so that the car on the right is in the center FOV, then correct the chromatic aberration as shown in Fig. 8(f).
Fig. 8. Captured images using the proposed zoom system. (a)f = 46.16 mm, without deflection. (b)f = 70.64 mm, without deflection. (c) Captured image, f = 70.64 mm, α=- 3.22°. (d)Processed image, f = 70.64 mm, α=- 3.22°. (e) Captured image, f = 70.64 mm, α= 3.68°. (f) Processed image, f = 70.64 mm, α = 3.68°.
It can be seen from these images, the imaging quality of the rabbit's eyes and ears has been significantly improved, and the large chromatic aberration spot on the car window has also reduced. However, there are still some residual rainbow bands, which are due to the acquisition of pictures under white light conditions. In subsequent work, we will improve the correction of chromatic aberration by separating more color channels. And try to change the structure of the prism to achieve two-dimensional deflection and chromatic aberration correction.
5. Conclusion
To conclude, we demonstrate an optofluidic zoom system with increased FOV and less chromatic aberration. The proposed system consists of a liquid prism, a zoom objective, an image sensor and image processing module, which can realize optical zoom and deflection. The proposed system achieves non-mechanical optical zoom from f = 40.5 mm to f = 84.0 mm. Besides, the angular resolution of zoom objective is up to 26"18 at f = 84.0 mm. The deflection range is ±10°, and the whole FOV of proposed system can reach up to 30.3°. The proposed system is compact and easy to machine. In addition, we reduce chromatic aberration produced by the liquid prism significantly. The proposed system can be used in monitor system, target tracking system, telescope system and so on.
Funding
National Natural Science Foundation of China (61927809, 61975139).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data available from the authors on request.
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