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We report a zoom microscope objective which can achieve continuous zoom change and correct the aberrations dynamically. The objective consists of three electrowetting liquid lenses and two glass lenses. The magnification is changed by applying voltages on the three electrowetting lenses. Besides, the three electrowetting liquid lenses can play a role to correct the aberrations. A digital microscope based on the proposed objective is demonstrated. We analyzed the properties of the proposed objective. In contrast to the conventional objectives, the proposed objective can be tuned from ~7.8 × to ~13.2 × continuously. For our objective, the working distance is fixed, which means no movement parts are needed to refocus or change its magnification. Moreover, the zoom objective can be dynamically optimized for a wide range of wavelength. Using such an objective, the fabrication tolerance of the optical system is larger than that of a conventional system, which can decrease the fabrication cost. The proposed zoom microscope objective cannot only take place of the conventional objective, but also has potential application in the 3D microscopy.
© 2016 Optical Society of America
1. Introduction
An objective is an essential part for a digital or optical microscope. It determines the magnification and quality of the observed image. For the conventional microscope, the magnification varies by changing the objective with another. So, the conventional microscope usually has several objectives for different magnifications such as 10 × , 20 × , 40 × , 60 × , and 100 × . It is a challenge to achieve continuous zoom change for the conventional microscopes. A zoom microscope objective is proposed using mechanical movement parts to realize zooming [1]. However, the lens system is very bulky and complex to operate. Besides, the image quality is unavoidably degenerated while zooming, since the mechanical movement cannot correct aberrations. Fortunately, with the development of the adaptive lenses such as liquid lenses and liquid crystal lenses [2–7 ], it is possible to replace the glass lenses with adaptive lenses in a zoom lens system such as cameras, which enables the zoom system to vary focal length without movement parts [8–14 ]. In microscopy, liquid lenses also find useful applications. For example, a liquid lens is used in light-sheet microscopy to refocus the target without movement parts and achieve images with high speed [15] or temporal focusing [16]. To further miniaturize the system, a theoretical design of an adaptive objective is proposed [17]. The objective uses an embedded tunable lens to get a large depth scanning range. In these systems, the liquid lenses only play a role as adaptive elements for refocusing. Therefore, it is urgent to realize an objective with more compact structure, continuously zoom ability, adaptively correcting aberration ability and without mechanical movement parts.
In this paper, we propose and experimentally demonstrate a zoom microscope objective which can achieve continuous zoom change and correct the aberrations dynamically. The proposed objective consists of three electrowetting liquid lenses and two glass lenses. The magnification is changed by applying voltages on the electrowetting lenses. In contrast to the conventional objectives, the proposed objective has continuous zooming range from ~7.8 × to ~13.2 × without mechanical movement parts. And, the device has optimizing ability, which can correct aberration in any wavelength range dynamically. Besides, using such an objective, the fabrication tolerance is larger than that of the conventional system, which can decrease the fabrication cost.
2. Zoom objective and theoretical analysis
The proposed zoom objective is shown in Fig. 1(a) . In the lens system, three electrowetting liquid lenses play a major role as the zoom part to vary the focal power, as well as correct aberrations. Two glass lenses are used for two purposes: 1) They undertake part of the focal power of the lens system, since the power of the liquid lenses is limited. 2) They are fabricated in different materials with different refractive indices and Abbe numbers, so that they can help to decrease aberrations, especially chromatic aberration.
Fig. 1 Schematic cross-sectional structure of the zoom objective. (a) Structure of the zoom objective. (b) Electrowetting liquid lens.
The liquid lens is an electrowetting-actuated lens with variable focal length, which consists of an oil and a conductive liquid, as shown in Fig. 1(b). The focal length is changed due to electrowetting effect. According to Young–Lippmann equation, the relationship of the contact angle θ and the applied voltage U can be described as follows [2]:
where ε is dielectric constant of the insulating layer, d is the thickness of the insulating layer. 𝛾1, 𝛾2 and 𝛾12 are the interfacial tensions of the dielectric insulator /oil, dielectric insulator /water and oil/water, respectively.A simplified conceptual model of the zoom objective system is shown in Fig. 2 . The variable focal length of the lens system can be achieved by changing the shape of the three electrowetting liquid lenses. The effective focal length f of the three variable liquid lenses can be expressed as
where n1 is the refractive index of the oil, while n2 is refractive index of the conductive liquid. r1, r2, r3 are the radii of the three electrowetting liquid lenses, respectively.For a lens system, one liquid lens can vary the power. However, to keep the back focal distance L fixed while zooming, at least two liquid lenses are used. In the proposed lens system, we use three liquid lenses for the following reasons: the system can vary the power with fixed back focal distance, while the system has an additional variable parameter so that we can optimize the three parameters to correct aberrations while zooming.
To correct aberrations, a merit function must be constructed. Since the materials (n1, n2 ...) and distances (d1, d2...) are fixed parameters, the focal length and the aberrations are determined by the three radii (r1, r2, r3). Thus, we can construct a merit function including the focal length and the aberrations. Then an optimization algorithm such as Damped Least Squares is adopted to optimize the merit function to get the best solution for zooming and aberration-corrected target. The detailed optimization theory can be found in our previous publication [18]. In fact, nowadays we can use a commercial software Zemax to optimize the three radii (r1, r2, r3) to get aberration-corrected solution. Then we convert the radius into applied voltage based on Young–Lippmann equation. Using the converted voltage, we can correct aberrations while zooming.
3. Fabrication and simulation
To fabricate a zoom objective shown in Fig. 1, electrowetting liquid lenses and glass lenses are needed. We chose a commercial liquid lens Arctic 39N0 produced by Varioptics [19] as the tunable element. The effective aperture of the liquid lens is ~3.9 mm. The materials of the liquids are oil and conductive liquid, and the materials to fabricate the glass lenses are K9 and CaF2. The refractive index and Abbe number of the materials are shown in Table 1 . The fabricated zoom objective is shown in Fig. 3 .
Table 1. Refractive index and Abbe number of the materials we used.
Fig. 3 Fabricated zoom objective. (a) Side view of the objective. (b) Bottom view of the objective. (c) Top view of the objective.
We measured the focal length of the electrowetting liquid lens at different voltages. The result is shown in Fig. 4 . The shortest positive and negative focal length of the electrowetting liquid lens are −64 mm and 32 mm, respectively, which means the curvature radius tuning range are (-∞, −7.3 mm)∪(3.2 mm, + ∞). We also simulated the proposed lens system within the radius tuning range in Zemax-EE. The detailed parameters of the proposed objective such as f, numerical aperture (NA) and magnification are shown in Table 2 . From Table 2, the focal length of the system can be tuned from ~12 mm to ~19 mm, while the corresponding magnification is tuning from ~13.2 × to ~7.8 × . The largest and smallest NAs are ~0.163 and ~0.103, respectively. In the tuning range, the proposed objective is also a dynamic optimizing device, which can vary its focal length as well as correct the aberration in any wavelength. Figure 5 shows the MTF of the proposed lens system for wavelength λ = 486 nm. The three colorful lines represent the MTF with different field of view using object height in Zemax. The black lines (DIFF. LIMIT) represent the MTF of diffraction limited resolution. In the tuning range, the MTF almost reaches the diffraction limited resolution, which means the aberration is largely reduced. For the focal length f = 19 mm, the MTF degenerates a little. The main reason is that the three liquid lenses have to take large optical power, so their ability to correct aberration is weaken. For wavelength λ = 587 nm [Fig. 6 ] and λ = 685 nm [Fig. 7 ], the MTF is also as good as that for λ = 487 nm. This is because our objective can re-optimize the radii of the three liquid lenses to correct aberration in other wavelength. Therefore, compared with conventional objective, our objective can be used in any wavelength.
Table 2. Detailed parameters of the proposed objective.
Fig. 5 MTF of the proposed objective for λ = 486nm. (a) f = 12mm. (b) f = 14mm. (c) f = 17mm. (d) f = 19mm.
Fig. 6 MTF of the proposed objective for λ = 587nm. (a) f = 12mm. (b) f = 14mm. (c) f = 17mm. (d) f = 19mm.
Fig. 7 MTF of the proposed objective for λ = 656nm. (a) f = 12mm. (b) f = 14mm. (c) f = 17mm. (d) f = 19mm.
4. Experiments and result discussions
We fabricated a digital microscope to evaluate the optical performance of the proposed objective shown in Fig. 8 . The digital microscope consists of a zoom objective, a lens cone, a LED ring-shaped light and a CMOS digital camera. In the experiment, we evaluated the zooming ability and image quality using a resolution target shown in Fig. 9 (a) . The CMOS camera of the digital microscope was used as an image plane. The pixel size in the CMOS is 2.2 μm × 2.2 μm. The resolution is 1280 × 960. The inner and outer radii of the LED light are ~64 mm and ~94 mm, respectively. The line width of No. 23 is ~11.2 μm. We first optimized the radii of the three liquid lenses to get optimized solution for each magnification in Zemax-EE. Then the optimized radii converted to applied voltages. Then the optimized voltages were applied to the three electrowetting liquid lenses. In fact, we usually need to slightly adjust each voltage until the image is clearest. The obtained images are shown in Figs. 9(b)-9(f). The magnification for Fig. 9(b) is ~7.8 × , we can see the whole target bars and the number “23” clearly. When we changed the magnification (~8.9 × ), the target bars and the number “23” were magnified, as shown in Fig. 9(c). Further changing the magnification, the image became bigger, and smaller part of the target bars and “23” can be seen. The largest image is shown in Fig. 9 (f), the corresponding magnification is ~13.2 × . From Fig. 9(b)-9(f), we see that the image is magnified by the applied voltage, and all the captured pictures are very good, which means our proposed objective has the ability to get aberration corrected image for different magnifications. During the zooming process, the working distance is fixed. No movement parts are needed to refocus or change the magnification. We also see that the colors of the five magnified pictures are slightly different. It is because that the fields of view (FOV) are different for different magnifications and the exposures are also different, which results in color difference on the CMOS camera.
Fig. 9 Captured images using a resolution target. (a) Resolution target. (b) Zoom 7.8 × . (c) Zoom 8.9 × . (d) Zoom 10.1 × . (e) Zoom 11.7 × . (f) Zoom 13.2 × .
We also compare the optical performance of our objective with that of a commercial objective (10 × ), as shown in Fig. 10 . The same microscope [Fig. 8] is used for the two objectives. The two objectives were used to observe the pixels of a cell phone. The observed pixels are from the screen of Samsung cell phone G3608. The results are shown in Figs. 10 (c) and 10(d). Comparing Fig. 10(c) with Fig. 10(d), we see that both the two objectives can obtain image with high quality. If we compare the two pictures carefully, we may find that the pixel array obtained by our lens [Fig. 10 (c)] is clearer than that of the conventional objectives [Fig. 10 (d)]. It is mainly because that our objective is a dynamic optimizing device. Due to optimizing ability, the lens can obtain the image as good as that in simulation. However, for the conventional objectives, fabrication error is unavoidable, which may decrease the image quality. Besides, due to dynamic optimizing ability, the fabrication tolerance of the optical system is larger than that of the conventional system, which can decrease the fabrication cost. We also can find color difference between the two pictures [Fig. 10(c) and Fig. 10(d)]. It may result from the following reasons. The two objectives have different ability to correct chromatic aberration. Therefore, optical performances are different. Meanwhile, since the NAs of the two objectives are different, the exposures are also different. During white balance process of CMOS camera, the phenomenon of the different colors may become obvious.
Fig. 10 Comparison between the proposed objective and the conventional objective. (a) Proposed objective. (b) Conventional objective. (c) Pixels imaged by proposed objective. (d) Pixels imaged by the conventional objective.
Compared with the conventional objective with zoom ratio 10 × , the NA of our objective is smaller (~0.13). It mainly results from the aperture of electrowetting liquid lens. For Arctic 39N0, the aperture is ~3.9 mm, which limits the entrance pupil diameter. With the development of liquid lens technology, the aperture can be increased. Therefore, we believe the issue can be solved in our next objective system.
In theory, the proposed objective can get any magnification. However, for our fabricated objective, the tuning range is still limited (from ~12 mm to ~19 mm). The main reason is that the power of the electrowetting liquid lens is relatively small compared with the glass lenses. To get larger tuning range, several methods are available. If we can get an electrowetting liquid lens with larger power, the tuning range will increase. Besides, we can employ more electrowetting lenses in our lens system. For example, we can use four or more electrowetting liquid lenses as the elements. In this case, the tuning range will increase, and the ability to correct aberration will increase too. But the system will become bulky and more complex to operate simultaneously, which also decreases the FOV and increase the cost. Therefore, a trade-off should be taken between tuning range, system length, FOV and cost. In our future work, we will devote to increase the tuning range of the focal length with less electrowetting liquid lenses as possible.
5. Conclusion
In this paper we propose and experimentally demonstrate a zoom microscope objective using electrowetting lenses. The proposed zoom objective consists of three electrowetting liquid lenses and two glass lenses. The three electrowetting liquid lenses have variable optical power so that the proposed objective has continuous zoom change. Compared with the conventional objective, our proposed objective has advantages: 1) Our lens has continuous zooming range (~7.8 × to ~13.2 × ), which provides the users with more magnification choice. 2) Our lens can get aberration-corrected images while zooming and be used in any wavelengths because of its dynamic optimizing ability. 3) The fabrication tolerance zone of the system is larger than that of the conventional system, which can decrease the fabrication cost. 4) No movement part is needed to refocus or change the magnification. The proposed zoom microscope objective cannot only take place of the conventional objective, but also has potential application in the 3D microscopy.
Acknowledgments
This work is supported by the “973” Program under Grant No. 2013CB328802, the NSFC under Grant Nos. 61225022, 61320106015 and 61505127.
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