CHAPTER 5. OVER-DRIVE METHOD FOR FAST FOCUSING LIQUID CRYSTAL LENS
5.4 E XPERIMENTS AND O PTIMIZATION OF O VER -D RIVING V OLTAGES
Not only the switching time but also the values of OD voltage affected the result.
Optimization for OD voltages was analyzed by applying different OD voltages to the electrodes. Typically, the focusing time was improved when the OD voltage was increased, as Figure 5-11 shows. The dash line shown in the result indicates directly applying the 45Vrms without OD method which yielded 25sec focusing time, as illustrated in Figure 5-2. As we applied OD method and increased the OD voltage to exceed 45Vrms, the focusing time started to be improved. Of course, the improvement cannot be obtained if the OD voltage is lower than that of general operation. In this experiment, the improvement was saturated as the over-drive voltage was increased to exceed 65Vrms, which means the extremely high
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Figure 5-11 The result of relation between focusing time and OD voltages. Improvement was saturated as the voltage was larger than 65Vrms.
In this experiment, GD-LC Lens was examined, as structure shown in Figure 5-12(a).
The focusing time was dramatically reduced down to 0.2sec by operating 15Vrms OD voltage.
Figure 5-12 (b) shows the relation of OD voltages to focusing time. This result also showed OD method is necessary for different structures if the fast focusing was required. A comparison of focusing response time, over-drive voltage, and stable driving voltage are listed in Table 6. Two main LC lens structures mentioned above driven by Over-drive Method with 2mm aperture size and 60um LC cell gap were discussed. The LC model was Merck nematic LC (E7). Although the external structure had slower focusing time due to insertion of high K glass, by Over-drive Method, the focusing time was effectively improved from 23.5sec to 7.1 sec. For GD-LC lens, the focusing time was significantly reduced down to 0.2sec with only 15Vrms over-drive voltage.
Table 6 The results and comparisons of two main structures of LC lenses driven by OD method. In the internal structure part, focusing response time was reduced to 0.2sec by 15Vrms over-drive voltage.
External Electrodes GD‐LC Lens
Over‐drive Voltage OD 65Vrms 15Vrms
Stable driving Voltage 45Vrms 5Vrms
Initial focusing time 23.5sec 12.5sec
Focusing time with OD method 7.1sec 0.2sec
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(a)
(b)
Figure 5-12 (a)The structure of GD LC lens with multi-electrically control, and (b) the relation of response time to over-drive voltages.
5.5 Conclusion
In summary, we proposed Over-drive (OD) method for fast focusing LC lenses. By initial OD driving, fast and instantaneous initial focusing was generated. A switching operation following the OD voltage at the time of the initial focusing can significantly improve the focusing time. By optimizing the OD voltages and the timing for the switching, the focusing response time of LC lens of two structures, external electrodes and GDLC lens, was improved from 25.3sec to 7.1sec and 12.5sec to 0.2sec with each optimized OD voltage and stable driving. This result showed Over-drive Method for LC lenses was effective and feasible, which also indicated the significance for fast focusing applications.
Chapter 6.
AF Imaging with Spherical GD-LC Lens
6.1 Introduction
In previous chapters, we proposed two kinds of structures, MeDLC lens and GD-LC lens to exhibit the different features. In MeDLC lens, the multi-electrode can finely control the lens profile to yield a wild range of focusing, although the structure is complicate for achieving spherical structure. On the other hand, GD-LC lens employed a high resistant layer as the control layer connected with the center and border electrode, and used the electrodes to initial generate the voltage difference. This new structure benefits us to significantly improve the driving voltage and focusing time while we combining OD method for LC lenses, as discussed in Chapter 5. Furthermore, the simpler structure of GD-LC lens was easy for us to construct a spherical GD-LC lens (sGD-LC lens), because there were only two control electrodes, at the center and border. For imaging applications, spherical lens is suggested as imaging and aberration theory were developed for hundred years.
In this chapter, fabrication of sGD-LC lens and its imaging system will be discussed. AF performed with GD-LC lens was also examined its optical performance, driving voltage and focusing time. It was noteworthy that GD-LC lens can exhibit an instantaneous focusing with lower driving voltage and provided superior image quality.
6.2 Fabrication
The difference between cylindrical and spherical GD-LC lens is the pattern of electrodes, as shown in Figure 6-1. The spherical structure has a central electrode without horizontal connection line because the connection line will distort the distribution of voltages after the high resistant layer coated. This situation will result in asymmetrical optical properties which
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(a) (b)
Figure 6-1 The electrode patterns of (a) cylindrical and (b) spherical GD-LC lens.
To fabricate the electrode pattern of spherical GD-LC lens, as shown in Figure 6-1 (b), a double-layer electrode structure was chosen, as show in Figure 6-2. The electrode layers on different horizontal height were separated by an insulator layer. The material of the insulation was recommended to use SiO2, SiNx, Su8, and so on. The different materials of insulator exhibit different compactness which may cause connection of two electrode lays in the overlap area, as Figure 6-2 shows. This connection will make the structure unworkable due to no voltage difference between two electrodes after applying the driving voltage. Limited by the process, we finally chose SiNx as the insulator, although SiNx still cannot 100 percent to insulate two electrode layers. Similar to cylindrical GD-LC lens, the final structure of spherical one is shown in Figure 6-3. The lens aperture was designed as 2mm with a 60um central electrode. The LC cell gap was also chosen to 60um sandwiched by two 550um glass substrates.
Figure 6-2 The double-layer electrode structure of sGD-LC lens employing an insulator to separate the electrode layers.
Figure 6-3 The structure of GD-LC lens.
6.3 Optical Properties of sGD-LC Lens
To investigate the focusing profile of sGD-LC lens, the experimental setup was the same as cylindrical GD-LC lens (cGD-LC lens). The difference is that the exposure time of CCD was
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lens. The focusing profile driven by 3.3Vrms at 4.22 kHz is shown in Figure 6-4. The left side is the top view of the focusing which obviously shows the focusing profile was different from line focusing of cGD-LC lens. The left bottom and the right side figures are cross-section and 3-D profile of the focusing. The focusing profile cannot be symmetry due to the defects of fabrication. This issue could be suppressed by improving cleanliness of the process. For different focal length, the focusing profiles of sGD-LC lens were also modulated by driving optimized voltages and frequencies, as the cross sections shown in Figure 6-5. The profiles of focal length from 7 to 15 cm can be maintained to higher focusing ability by the voltage-frequency driven. As the focal length was out of the range, the focusing profiles started to decrease. This not only means there were defects but also higher control freedom was required, as mentioned in Chapter 3. Figure 6-6 also shows the relationship between focal length to operating voltage and frequency. The results was coherent to cGD-LC lens that the decreased operating voltage and frequency generated longer focal length. Although the driving range was different from that of cGD-LC lens due to the coated resistance of high R layer, the results still shows low operating voltages which were feasible for mobile devices.
Figure 6-4 The focusing profile of sGD-LC lens driven by 3.3Vrms at 4.22 kHz.
Figure 6-5 The focusing profiles of sGD-LC lens from focal length, 7cm~15cm.
Figure 6-6 The relationship between of focal length to operating voltage and frequency
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6.4 AF Experiment with sGD-LC lens 6.4.1 Experimental Principle
The AF by LC lens can be performed with a conventional lens-head and a LC lens. The lens-head was used to provide the most ratio of lens power and to correct aberrations.
Combining with the LC lens in front of the lens-head, the lens system can change the total lens-power by operating voltages (and frequencies) to focus the near objects, as mentioned in previous sections. The structure is shown in Figure 1-9. To simplify the analysis, parallel light, which was used to describe object light source at infinity, was input to the lens-head in both case of with and without LC lens, as shown in Figure 6-7. In the LC lens case, the light source from a near object point was converged to the parallel light for the lens-head. This was assumed the LC lens has diffraction-limit performance to converge a parallel light to an image point. The benefit of the arrangement is because the lens-head was optimized for infinity object. By Gaussion and Lens makers’ formula, the relations of object distance (S), image distance (S’), focal length of LC lens and lens-head (fLC and fC), and separation of LC lens with lens-head (d) can be represented by
1 1 1
which simplified Equation (6-1) to
1 1 1 (6-5)
and the optical power was independent to d. From Equation (6-3) to (6-5), the fLC is directly corresponding to the object distance (S) and the system lens power is equal to the sum of two lenses. In the structure of Figure 6-7, the back principle plane of the lens-head is stationary
and consistent with the system back principle plane. The image distance therefore was the same as that of non-LC-lens system.
Figure 6-7 The schematic of lens-system with and without LC lens.
In the experimental setup, sGD-LC lens and a 5-mega-pixel lens-head were prepared as shown in Figure 6-8 (a). An FPGA board was used to deliver the captured images to PC and monitor, as shown in Figure 6-8 (b). The test objects including I3A/ISO Resolution Test Chart-1X (NT56-074, Edmund) and toys were setup at different distances. The lens-head originally was tuned to focus on infinity, and utilized the sGD-LC lens to focus objects at finite distance. A control signal used to perform OD method for sGD-LC lens was connected.
The signal consisted of tree channels which were zero state, OD state and stable state, and the channels were selected by another control board. The width of the OD state can be modified by coding of the board which can significantly affect the focusing time. The function and equipment are shown in Figure 6-9.
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(a)
(b)
Figure 6-8 (a)The setup of lens-system and (b) the experimental arrangement.
(a)
(b)
Figure 6-9 (a)The function of control signal and (b) the equipmet.
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6.4.2 Results
Resolution Test Chart for different focal length
The first test object was I3A/ISO Resolution Test Chart-1X (NT56-074, Edmund) which was used to test the resolution of sGD-LC lens combined with the lens-head. The test chart was placed at the distance corresponding to the focal length of sGD-LC lens and captured by the system, as shown in Figure 6-10. The captured results of the magnified part indicated in Figure 6-10 are shown in Figure 6-11. The test resolution was chosen from 6 to 20 line-pairs/mm. From the results, the low resolution range can be easily distinguished. Even in the range from 18 to 20 line-pairs/mm, some of the focal length also can provide acceptable contrast. The ghost image appears on the black-white edge was due to the misalignment of LC directors to the direction of polarizer. Therefore, there was always a minimum ghost image due to the ordinary rays. This issue can be suppressed by DIP approach, and the relative works is under the research.
Figure 6-10 The test chart placed in front of sGD-LC lens at the distance corresponding to the focal length.
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AF test
To perform AF with sGD-LC lens, four toys were placed in front of the lens with different distances, as shown in Figure 6-12. Each of the toys was separated to out of its depth of field.
By different operation for each focal length, the toys can be individually focused, as Figure 6-13 shows. Through the result of Figure 6-13, the objects were obviously focused by operating corresponding driving voltage and frequency. As the operation changed, the toys out of the depth of field started to be blurred. The OD method used to increase the focusing time was also perform for focusing from the toys at the 90cm to the 6cm. The OD voltage was 10Vrms with 500m sec OD state and then switched stable state for focusing on the 6cm toy.
The focusing time was around 1sec.
Figure 6-12 The AF experiment and the toys placed at different distances.
Figure 6-13 (Video) The AF result of four toys at different distances, and OD result focusing from the farest toy to the nearest one.
MTF measurement
Figure 6-14 shows MTF of sGD-LC lens focusing at 10cm measured by ImageMaster® HR, Trioptics [70]. From the result, we can find this lens can provide 90%
contrast at 10lp/deg and larger than 50% for 25lp/deg. This result is similar to that of liquid lens reported by Philips Research Eindhoven in 2004 [71] which has around 70% at 25lp/deg.
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is required to higher than 30% at 280lp/mm for center (0.0 field), not to mention the requirement for corner (0.8 field) which is higher than 30% at 140lp/mm. Moreover, MTF of the LC lens in the different field was also measured, as shown in Figure 6-15. As the result shows, as the field of view increased, the contrast was decreased rapidly. At the area of large field, there was almost no signal be measured due to the undetectable focusing signal. This shows not only the image quality for the MTF at central area has to be improved, but also the large field, which requires more compensation for the image quality at the border. This is a major challenge for LC lenses to improve its image quality if we really considered its applications in consumer products.
To improve this issue, the optical design of conventional lens for LC lens can be take into consideration. In the first step, the focusing performance of LC lens should be optimized, and then utilize solid lens to correct the image quality, image aberrations, and enhance MTF.
The role of the solid lens is to obtain a balance solution for focusing at infinity and close objects.
Figure 6-14 MTF of sGD-LC lens measured by ImageMaster® HR, Trioptics.
0
MTF vs Frequency Tan. 1; EFL= 1 Sag. 1; EFL= 1
Figure 6-15 Global MTF of the LC lens with field of views from 0 to 30 degree.
6.5 Conclusion
In this chapter, we fabricated spherical GD-LC lens (sGD-LC lens) and investigated its optical properties. The AF system with sGD-LC lens was also setup. The sGD-LC lens was placed in front of a 5-mega-pixel lens-head which was connected to a FPGA board to capture the output images. OD method was also coded by a control board to demonstrate the fast focusing. As the result shows, the objects within different depth of field can be focused individually by different operating. The focusing time was also achieved in 1sec.
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Chapter 7.
Conclusion and Future Work
7.1 Conclusion
Liquid Crystal Lens (LC lens) exhibits the ultimate features, such as its focal length is electrical tunable without any mechanical movement or surface shape changing and its tiny volume is suitable for employing in mobile devices. However, the major issues, inferior optical quality, slow focusing time, and high driving voltage, result in the unpractical and unfeasible applications of LC lenses. To overcome these issues, we have reported tree main topics, MeDLC Lens, Over-driving Method, and GD-LC lens. The relations of the topics for each issue are shown in Figure 7-1. Finally, combine the features of the proposed topic, all the issues were suppressed.
Figure 7-1 The relations of the topics for each issue 7.1.1 Multi-electrode Driven LC Lens (MeDLC Lens)
The first one is Multi-electrode Driven LC Lens (MeDLC Lens) which utilized multi-electrode to finely control the orientations of LC molecules for each focal length to have superior focusing profiles. In this stage, we changed the structures of LC lenses and utilized multi-electrode rather than the single control freedom of conventional structures, as Figure 7-2 shows.
Figure 7-2 The change of LC lens structure from conventional single control to multi-control freedom.
In the experiments, we demonstrated the high control freedom of MeDLC lens can yield similar focusing profile at each focal length and minimize the spot size of these focusing. To obtain superior image quality, the multi-control was necessary to optimize the focusing.
7.1.2 Over-driving (OD) Method for LC lenses
The second topic is Over-driving (OD) Method for LC lenses which utilized an initial OD voltage to increase the deformation of LC molecules and switch the OD voltage according to the focusing profile to an optimized driving to significantly reduce the focusing time, as Figure 7-3 illustrates. Different from the OD technology wildly employed in Liquid Crystal Display (LCD) industry, OD method for LC lens was investigated according to transition of the focusing profile. In this stage, we also demonstrated the timing for switching the driving voltage was optimized at the initial focusing peak by the OD voltage.
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(a)
(b)
Figure 7-3 (a) The operation of OD method and (b) the focusing transition by OD method.
7.1.3 Gradient Driven Liquid Crystal Lens (GD-LC lens)
The most important topic, the third one: Gradient Driven Liquid Crystal Lens (GD-LC lens), was proposed to intrinsically solve the issue of high driving voltage. Compare to previous papers or the newly reports, we demonstrated a dramatic improvement for reducing the driving voltage from tens voltages or even higher than hundred voltages down to less than 5 Vrms for focal length from 6cm to infinity. The structure of GD-LC lens utilized a resistant
layer connected by tree electrodes as the control electrode. The first one benefit of this structure is that the applied electric field can be conserved inside the LC cell to be effectively employed the without leaking out. The second one is that the resistance layer controlled by these tree electrodes can initially produce gradient voltage distribution for yielding lens profile. Furthermore, the structure also can be controlled by operating voltage and frequency to achieve the concept of multi-control. In our experiment, the focusing profiles of each focal length can be optimized by this dual-control which has benefited the image quality in our auto-focusing (AF) experiments. The most important breakthrough was that the focusing time has been improved from the typical larger than 10 sec or over 1 min down to less than 1sec (~600ms) for focusing from infinity to 6cm closed object, as Figure 7-4 illustrates.
Figure 7-4 The dramatic improvement in driving voltage and focusing time with GD-LC lens combined with the volt. & freq. dual-control and OD method.
To compare with results of the world’s leading groups, as shown in Table 7, not only the driving voltage was reduced to a reasonable range that the normal IC could drive but also the
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other solutions for AF mentioned in section 1.4.1 after the improvement of our group. As mentioned, the issues of driving and focusing time have almost been solved, although the
other solutions for AF mentioned in section 1.4.1 after the improvement of our group. As mentioned, the issues of driving and focusing time have almost been solved, although the