CHAPTER 6. AF IMAGING WITH SPHERICAL GD-LC LENS
6.4 AF E XPERIMENT WITH S GD-LC LENS
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 image quality still cannot compete with conventional lens-head.
Table 7 The comparisons of single tunable lenses.
Table 8 The comparison of the AF solutions after the improvements of our group for LC lens.
7.2 Future work
7.2.1 Optical design for LC lens
In current status, we significantly improve the issues of LC lenses, especially the slow focusing time and high driving voltage. In the optical quality part, however, although the
concept of multi-control can be used to optimize the focusing profiles of each focal length, the spot size still cannot be competed with that of conventional lens-heads, as well as its aberration values also cannot be corrected by this single-lens system. For camera industry, diffraction-limit performance is usually required even for the use in mobile phones. 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.
Finally, the simulation tools for lens design, such as CodeV and Zemax, and LC cells, such as 2Dimos and ExpertLCD, should be linked to simulate the imaging result of LC lens and to analysis items such as aberrations, MTF, imaging result, and tolerance. Optimization is also importance for the lens modifying. The relation of the link is shown in Figure 7-5.
Figure 7-5 The relation of lens design tool for LC lens design
7.2.2 Polarizer free LC lens
In the application of lens with LCs, polarized light is necessary to distinct the ordinary-rays and extraordinary-rays. However, this property results in a half of incoming light losing due to the usage of polarizer. To make the application of LC lens more feasible, polarizer free LC lens is required. The general method for polarizer free LC lens is to use two crossed LC lenses with perpendicular rubbing direction. Each of the lens is used to deal with one polarization.
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Therefore, our group would use DIP to compute the imaging result without employing polarizer. In such a LC lens without polarizer, it can be used for the hyper distance shooting without losing gathering light. For AF usage, the lens system firstly takes an original image before AF which includes information from ordinary-rays and extraordinary-rays, and takes another one after AF ready. The difference of these two images is that the extraordinary-rays in the second image were focused. By DIP, the information coming from ordinary-rays can be eliminated, and according to the original luminance to restore the AF image in the second image with proper intensity, as shown in Figure 7-6.
Figure 7-6 The concept of DIP for polarizer free LC lens.
7.2.3 Optical Zoom
As discussed in section 4.4, GD-LC lens can be easily performed in convex and concave mode. This feature is benefit for the application in LC optical zoom lens because zoom lens usually requires a convex and a concave lens group as the variator and compensator.
Therefore, we add another LC lens in front of the LC AF system, as shown in Figure 7-7. The structure consist of two LC lenses To achieve the concept of zoom lens, Equation (7-1) and Equation (7-2) can be used to obtain the optical power of the zoom system and fix the imaging plane on position of image sensor.
⇒ (7-1)
1
(7-2) and are the lens power of two LC lens, and other parameters are the spacing and fix optical power. As we utilize and as the variator and compensator respectively. The lens power of the system was simulated, as Figure 7-8 shows. In the simulation result, although lens power of the system, K, can be varied, but the lens power of LC lenses were limited within 0.03 mm . Therefore, the zoom ratio, as shown in Equation (7-3)
≡ (7-3)
is only around 1.5X. The next step is to demonstrate the feasibility of the LC zoom lens concept and to improve the zoom ratio.
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Figure 7-8 The variation of lens power, K, changed by LC lens power, K1 and K2
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