Hexagonal Liquid Crystal Micro-Lens Array with Fast-Response Time for Enhancing Depth of Light Field Microscopy

Transcription

1 Hexagonal Liquid Crystal Micro-Lens Array with Fast-Response Time for Enhancing Depth of Light Field Microscopy Chih-Kai Deng 1, Hsiu-An Lin 1, Po-Yuan Hsieh 2, Yi-Pai Huang 2, Cheng-Huang Kuo Institute of Lighting and Energy Photonics, NCTU, Tainan, 71150, Taiwan Department of Photonics, NCTU, Hsinchu, 30010, Taiwan is the distance from refocusing plane to MLA. To Abstract obtain the continuous and reasonable ERR at different depth, we propose using the hexagonal liquid crystal micro-lens array (HLC-MLA) in the light field microscope as shown in Figure 1. The Instead of fixed lens array, here we propose a fast response hexagonal liquid crystal micro-lens array (LC- MLA) for 3D light field microscopy. The liquid crystal lens can change its focal length electrically. With this property, we can extend the working range from 50 um to 350 um in light field microscopy. 1. Introduction Light field microscope can capture the light field information and reconstruct the 3D image without mechanical movement 1,2,3. This 3D image is captured without mechanical movement, so it can save lots of observation time. However, depth of field (DoF) of 3D light field microscope is usually narrow because the effective resolution decreases rapidly. But due to the trade-off between the working range and effective resolution in the light field system, the working range of light field microscope is not enough. To fix this problem, Perwaß and Wietzke proposed utilizing a multi-focus microlens array (MLA) with three different focal length type lenses to extend the working range 4. Furthermore, our group has produced a hexagonal liquid crystal micro- lens array in SID2016. It is able to tune the focal length electrically 5,6,7. In this paper, we propose a HLC-MLA with 10um cell gap which has faster response time for depth enhanced 3D light field microscope. 2. Depth Enhanced Light Field Microscope The advantage of light field microscope is that it can record 3D information by the microlens array 1,2,3. However, the issue is that the effective resolution of reconstruction image will drop rapidly once the measuring object is too far away from micro-lens array (MLA). The effective resolution ratio (ERR) formula is as equation 1 4 shown below. ERR = 1 P, v 1 (1) B 1 In this formula, B represents the distance between photo sensor and MLA, p is pixel size, D is lens pitch, and f is focal length of MLA. The magnification v is equal to a divide by B, where a ERR of boundary point a1 - in the DoF1 is supposed to be equal to the ERR of boundary point a2 + in the DoF2. We would choose the most appropriate light field image for each depth. Therefore, the ERR of rendering image could maintain, and the total working range in 3D light field microscope would extend. Figure 1. Principle scheme of extending working range of 3D light field microscope implemented with HLC- MLA. 3. Hexagonal Liquid Crystal Lens Array 3.1 High Resistance Layer Liquid crystal (LC) lens is a tunable lens which has been wildly used in different optical applications. The electrically tunable focallength without any mechanical movement makes LC lens lighter and compacter than conventional glass lens. Due to the large optical anisotropy of this material, if we apply voltage on surrounding electrodes, the fringing field from electrodes will modulate the distribution of orientation of liquid crystal molecules and induce lens-like phase retardation in LC cell. When an incident plane wave passes through a lens-like phase difference of LC directors, the wave will be converged or diverged depending on the type of voltage applied to the electrodes. According the applied voltage power, the focal length of LC lens will be changed continuously without any shape chaining or mechanical movement. To further improve the optical performance of LC lens, high resistance layer (Hi-R layer) 6 is applied to be the internal continuous-

2 distributed electrode to achieve low operating voltage and improve the focusing time simultaneously. It could generate linearly varying electrical potential from the center to the edge. The resistance layer for LC lens successfully demonstrates low operating voltage, fast response time, and high optical performance. 3.2 Measurement of HLC-MLA As Figure 2 shows, the hexagonal electrode type liquid crystal micro-lens array (HLC-MLA) with dense packing design can reduce the pixels consuming comparing with the conventional circular electrode type liquid microlens array. Besides, the light field microscope has blinded region in front/back of MLA plane as the green region shown in Figure 3. The object in the blinded region cannot be captured completely on the light filed image because the lens pitch of micro-lens array. Therefore, the objects which are too close to MLA plane would not able to be rendered. To minimize the blinded region of light field microscope, one solution is to reduce the distance between MLA and photo sensor B, which will also reduce the ERR following equation 1. Another solution is to minimize the lens pitch D with hexagonal microlens array as we chose in this paper. As the structure of HLC-MLA in Figure 4, the lens pitch of micro-lenses is 100 um, the cell gap is 10 um, and the LC material is E7. To obtain the high resistance layer HLC-MLA, we take aluminum and ITO as top and bottom electrode. The width of aluminum electrode is 10 um, and we coat Nb2O5 with 10 nm thickness on electrode pattern as high resistance layer by sputter machine. When we apply voltage on the electrode of HLC-MLA, the refractive index variation of LC molecule would induce lens-like phase retardation in LC cell. The driving signal is a square wave. We could adjust the frequency of driving signal to control the focal length of HLC- MLA. As shown in Figure 5, higher driving frequency would induce larger phase retardation difference in the LC cell, and decrease the focal length of HLC-MLA. Figure 4. Structure of high-resistance layer hexagonal liquid crystal lens array. (a) (b) Figure 2. Fringing patterns of (a) circular and (b) hexagonal electrode type LC lens array. Focal length (mm) Driving frequency (khz) Figure 5. Relation between driving freq. and focal length of HLC-MLA. (a) (b) Figure 3. The blinded region of light field microscope system of (a) circular and (b) hexagonal electrode type LC lens array. 4. Experimental Results In this section, we will discuss how HLC-MLA working in the light field microscope system. Our experimental setup is shown in Figure 6, the focal length of objective lens and main lens are 50 mm and 200 mm. We choose three kinds of photomasks which have aperture size of 100 um, 350 um and 1000 um separately, and then place them at the focal plane of

3 the objective lens. And then the image of our samples could be captured by HLC- MLA and effective photo sensor. Figure 7. Photomasks with lens aperture of 100 um, 350um and 1000um, were placed at a = -1.3 mm, -2.9 mm and -5.8 mm separately. Figure 6. Experimental setup. As shown in Figure 7, these three photomasks are at depth a = -1.3 mm, -2.9 mm and -5.8 mm separately. The focal length of HLC-MLA was adjusted from 1.2 mm to 1.6 mm and 3.5 mm to focus at different depth regions. The light field images are recorded on the photo sensor by time sequential method. Since the highest ERR of these light field images are at different depth, we could engage the working range boundary of each two light field images. Then we could render 3D images with continuous and reasonable ERR from three different depth. Comparing the rendering image at different depth, the results are shown below. First, we can see Figure 8 that the rendering image refocuses at the photomask with depth a = -1.3 mm. With HLC-MLA focal length f = 3.5 mm, ERR is We could see that the light field image is very clear. Then we let the rendering image refocus at depth a = -2.9 mm as shown in Figure 9. The photomask remains clear on the rendering image from light field image of HLC-MLA focal length f = 1.6 mm which has ERR of But the rendering image from HLC-MLA focal length f = 3.5 mm becomes blur because its ERR is which is too low at that depth. At last, Figure 10 shows the rendering images refocusing at depth a = -5.8 mm. The photomask is also blur with HLC-MLA focal length f = 3.5 mm because of its low ERR of But it would render a clearer image with HLC-MLA focal length f = 1.2 mm, which has a higher ERR. To make a brief conclusion, the image quality depends on ERR corresponding to HLC-MLA focal length when the image focuses at specific depth. (f = 3.5 mm) (f = 3.5 mm) Figure 8. Rendering image at a = -1.3 mm. (Upper) Both the right and left side is with HLC- MLA focal length f = 3.5 mm. (Lower) ERR with f = 3.5 mm. (f = 1.6 mm) (f = 3.5 mm) Figure 9. Rendering image at a = -2.9 mm. (Upper) The right side is with HLC-MLA focal length f = 3.5 mm, and the left side is with f = 1.6 mm. (Lower) ERR with f = 3.5 mm and 1.6 mm. 100 um 350um 1000um

4 (f = 1.2 mm) (f = 3.5 mm) Figure 10. Rendering image at a = -5.8 mm. (Upper) The right side is with HLC-MLA focal length f = 3.5 mm, and the left side is with f = 1.2 mm. (Lower) ERR with f = 3.5 mm and 1.2 mm. There is also another important issue of LC lens, which is the response time. We fabricate the Hi-R HLC- MLA with 10 um LC cell gap in order to reduce the response time. The fringe pattern variation is shown in Figure 11. There is a method that could reduce the response time more. The over-drive method 8 for LC lenses has been proposed, it is investigated which utilizes marginal electrodes to drive over high voltage to pre- generate a lens-shape phase retardation. By switching the overdrive voltage to optimized driving voltages for a particular focal length at an optimized switching time, the focusing ability can be maintained. At the same time, the response time is significantly reduced to about 80 ms as shown in Figure 12. Figure 11. Hi-R HLC-MLA (with lens pitch 100 um, Nb2O5 layer 10 nm, LC cell gap 10um), fringe pattern change in the response time. Figure 12. Hi-R HLC-MLA (with lens pitch 100 um, Nb2O5 layer 10 nm, LC cell gap 10um), fringe pattern change in the response time (with over-drive method). 5. Conclusions In order to extend the total working range of 3D light field microscope, here we propose a hexagonal liquid crystal micro-lens array (HLC- MLA) instead of fixed MLA. Our main purpose is rendering 3D images that have reasonable ERR in large depth region, so we adjust focal length of HLC-MLA to record light field images. HLC-MLA has the merit that it could lower the consuming of pixel and provides light field microscope a wider working range without mechanical scanning. The total working range could be extended from 50 um to 350 um by fusing light field images recorded with three different focal length of HLC- MLA. Furthermore, we use over-drive method for the 10 um LC cell gap HLC- MLA to successfully reach the fast response time of 80 ms. Referenc e 1. T. Georgiev, C. Intwala, Light-field Camera Design for Integral View Photography, Tech. rep., Adobe Systems, Inc, (2006). 2. M. Levo, R. Ng, A. Adams, M. Footer, M. Horowitz, Light Field Microscopy, Proceedings of ACM siggraph, Vol. 25 issue 3, Pages , (2006). 3. A. Lumsdaine, T. Georgiev, Full Resolution Light- field Rendering, Tech. rep., Adobe Systems, Inc, (2008). 4. C. Perwaß, L. Wietzke, Single Lens 3Dcamera with Extended Depth-of-field, Proceedings of the SPIE, Vol. 8291, (2012). 5. P. Y. Hsieh, C. Y. Chu, Y. Hsuan, Y. P. Huang, Depth Enhancement of Light Field Microscope with Hexagonal Liquid Crystal

5 Lens Array, in Society for Information Display Symp., San Francisco, USA, (2016). 6. H. Kwon, Y. Kizu, Y. Kizaki, M. Ito, M. Kobayashi, R. Ueno, K. Suzuki and H. Funaki, A Gradient Index Liquid Crystal Microlens Array for Light-field Camera Applications, IEEE Photonics Technology Letters, Vol. 27, No. 8, (2015). 7. Y. C. Chang, T. H. Jen, C. H. Ting, Y. P. Huang, High-resistance Liquid-crystal Lens Array for Rotatable 2D/3D Autostereoscopic Display, Optics Express, Vol. 22, Issue 3, (2014). 8. L. Yao. Liao, P. Y. Shieh, Y. P. Huang, Marginal Electrodes with Over-drive Method for Fast Response Liquid Crystal Lens Applications, SID Symposium Digest of Technical Papers, Vol. 41, No. 1, Blackwell Publishing Ltd, (2010).