Multi-Spectra Artificial Compound Eyes, Design, Fabrication and Applications

Transcription

1 Multi-Spectra Artificial Compound Eyes, Design, Fabrication and Applications Yupei Yao, and Ruxu Du Dept. of Mechanical and Automation Engineering, The Chinese University of Hong Kong, Hong Kong SAR, China Abstract - This paper introduces a Multi-Spectra Artificial Compound Eyes (MSACE) imaging system. It is made in two parts: First, multi-spectra filters are made by depositing pigment based photo-resist material on glass substrate. Second, a micro lens array (the artificial compound eye) is made by means of photolithography and thermal reflow. The resulting MSACE has a number of distinct features. First, it has an accurate lens profile. Second, it can catch different spectral information. Third, it is rather inexpensive to make. It is expected that the MSACE system will find many potential applications in the near future, such as endoscopic diagnosis and currency counterfeit checking. Keywords: Artificial Compound Eyes, micro lens array, thermal reflow, multi-spectra imaging 1 Introduction It is well known that animals have different vision systems than human. First, many insects have compound eyes. It has very large view angle and is particularly effectively to see moving objects [1]. Second, animals have different viewing spectra. For example, bee s vision focuses on the color of the stamen and pistil of the flowers in ultraviolet [2], while snake sees infrared [3]. Inspired by these nature wonders, researchers around the world are avidly trying to develop multi-lens multi-spectrum imaging systems to see what human cannot see in naked eyes. For instance, Duparre and his team developed two apposition Artificial Compound Eyes (ACE) systems, one has planar structure [4] and the other has spherical structure [5]. The two systems have large field of view but their resolutions are low. Tanida and et al developed a planar ACE system with image reconstruction software [6]. Its reconstructed color image has higher resolution [7, 8]. Subsequently, they developed several applications, such as fingerprint capturing, multispectral imaging and color imaging [9]. Also, Lin and Tian [10] developed a 16 channel integrated narrow band-pass filter using etching technique, which can decompose an ordinary image into a number of images with specific spectral bands. Our team has been working on ACE for several years [11 ~ 13]. In particularly, we developed a simple method to make ACE using the thermal reflow. The objective of this paper is to integrate the ACE with multi-spectrum filters to make a Multi-Spectrum Artificial Compound Eyes (MSACE) system. The rest of the paper is divided into three sections. Section 2 presents the fabrication procedure. Section 3 gives the testing results. Finally, Section 4 contains conclusions and future work. 2 Fabrication It is known that the ACE can be made by a number of different methods. One is to use the traditional molding technology [14, 15]. First, a mold is made using ultraprecision lathe with diamond cutter. Then, using the mold and a precision injection molding machine, ACE can be made with materials such as PMMA. Figure 1 shows a sample ACE designed by us and made by Hong Kong Polytechnic University. This method is capable of making precision ACE in large quantities. Though, the mold is expensive and it requires additional steps to make MSACE. Figure 1: sample ACE made by conventional molding method The other method is to use MEMS fabrication techniques, which was adopted in our early research [11 ~ 13]. It has a number of advantages, such as low fabrication cost, flexible, and most importantly, being capable of making MSACE. Therefore, this method is used in this paper. The MEMS fabrication method consists of two steps. First, a multi-spectrum filter is fabricated, and then, the ACE structure (a micro-lens array) on top of the multi-spectrum filter is fabricated. The details of the fabrication processes are presented below. Figure 2 shows the fabrication process for making the multi-spectrum filter. It uses the pigment-dispersed method that is generally used in liquid crystal display fabrication [16]. The process consists of a number of repetitive steps: Step 1: Clean the glass substrate using acetone and dehydrate the substrate in the oven;

2 Step 2: Spin-coat the pigment photoresist on the substrate (1st run: spin speed = 300 rpm, time = 20 s; 2nd run: spin speed = 600 rpm, time = 60 s, final thickness = 1 μm); Step 3: Remove the residual solvent using pre-bake (temperature = 80 ºC, time = 2 min); Step 4: Expose the photoresist with UV light under the cover of the prepared mask (exposure dose = 150 mj); Step 5: Develop in the specific KOH developer (KOH:Water = 1:49, develop time = 40 s), by which one section of the filter is formed. Step 6: Cure the photoresist completely using hard bake (temperature = 230 ºC, time = 80 min). 2nd run: spin speed = 2000 rpm, time = 60 s, 3rd run: spin speed = 3000 rpm, time = 1.2s); Step 2: re-bake temperature = C, time = 10 min); Step 3: Repeat Steps 1 and 2 so that the thickness of the photoresist reaches 20 ~ 25 μm; Step 4: Expose the photoresist with UV light under the cover of the prepared mask (exposure dose = 250 mj); Step 5: Develop (time = 80 s); Step 6: Conduct thermal reflow (temperature = 130 ºC, time = 120 s), which will give the artificial compound eyes on the multi-spectrum filters. This process gives one section of filter and is repeated for other color filters as shown in Figure 2. As shown in the figure, a total of four color sections are made including red, green, blue and red + green + blue, which is nearly black accepting only near-infrared to pass. Figure 3: the fabrication processes of the ACE using thermal reflow Figure 2: the fabrication process of the multi-spectrum filters Figure 3 shows the fabrication process for making the artificial compound eyes(ace) on the top of the multispectrum filters. The detailed steps are as follows: Step 1: Spin-coat the photoresist (Model: AZ-4620, Manufacturer: Shipley, USA) on the substrate with the multispectrum filter (1st run: spin speed = 500 rpm, time = 20 s, The aforementioned fabrication process shall be conducted in great care. In particularly, there are several important notes. First, because the exposing and developing steps are repeated for several times, any residual photoresist or moisture left on the substrate must be completed removed, as residual photoresist will cause uneven surface and moisture will cause bubbles. Second, to compensate the effect of thermal reflow, the diameter of the lens on the mask should be slightly larger than that of the design. This is because the thermal reflow will melt a part of the material away. Based on our experience, for the lens of 750 μm in diameter, a margin of 2 ~ 3 μm will have the best result.

3 Last but not the least, controlling the temperature during the thermal reflow is very important. If the temperature is too high (above 140 ºC) the mobility of the photoresist will increase causing the distortion of the shape of ACE. On the other hand, if the temperature is too low (under 120 ºC), the mobility of the photoresist will be too low to form the ACE, which means the photoresist will solidify before forming the lens contour. 3 Fabrication Results Using the aforementioned fabrication procedure, we have made a number of MSACE samples. Figure 4 shows a sample. It uses a simple design: the MSACE is made of four sections (red, blue, green and red + blue + green), and each section consists of 5 x 5 identical ACE. All ACE components are the same, 750 μm in diameter and 25 μm in height. From the figure, the MSACE patterns can be clearly seen. Figure 5: the center portion of the sample under background lighting with an amplification of 5 times Figure 4: A sample MSACE Figure 5 shows center portion of the sample with an amplification of 5 times, it is taken under the background lighting while Figure 6 is taken under the normal lighting. From figure 5, with adjusting the focus of microscope to the bottom of the lens, it is seen that the lenses have sharp boundaries, and from figure 6, the uniform reflation of normal lighting indicates the lenses have smooth surface, proving that the aforementioned method is successful. It shall be pointed out that usually about 95% of the lenses are in good shape while the other 4 ~ 5% of the lenses are in bad shape. The bad lenses are randomly distributed in different places, they may be caused by many factors, such as the cleaning of the substrate, the contact between the substrate and the baker, the temperature distribution of the baker and etc. These problems may be resolved using better equipment. Figure 6: the center portion of the sample under normal lighting with an amplification of 5 times Next, the contours of the lenses are carefully measured using a profiler (Model: Alpha-Step 500, Maker: Tencor Instruments Co., USA). It is found that the diameter of the lenses is about 760 to 7 0 μm, which is slightly larger than the cylinder before malting, while the height of the lenses is 4 ~ 6 μm smaller than the cylinder. Figure 7 compares the designed profile and the measured profile. From the figures, it is seen that the presented method is effective. Figure 7: A comparison between the designed lens contour and the actual lens contour

4 Figure 8 shows the transmittance of the visible light through different sections of the MSACE measured by a spectrum analyzer (Model: U-3501, Manufacturer: HITACHI). Figure 8(a) shows the transmittance of the light in red, blue, and green sections respectively. Figure 8(b) shows the transmittance of the light in the red + blue + green section, which is equivalent to infrared. From the figure, the effects of the color filters can be clearly seen. blue section. This will help to extract the hidden spectral patterns of the image. Though, the image in the red + green + blue section is too dark to be see, as the light source using in experiment provides little near infrared light. Figure 8: (a) the transmittance of the light in red, blue and green sections respectively, (b) the transmittance of the light in red + blue + green section. The fabricated MSACE can work with ordinary vision sensors. Figure 9 shows the setup with an industrial grade CCD sensor (Model: UC1000-C, Manufacturer: Acutance (BeiJing) Ltd.). The sensible band of the sensor ranges from 400 nm to 1200 nm. Figure 10: A sample imaging result of the MSACE 4 Conclusions and Future Work This paper presents a novel Multi-Spectra Artificial Compound Eyes (MSACE) system. Based on the discussions above, following conclusions can be drawn: Figure 9: The experiment setup for testing the MSACE Using the aforementioned setup, we tested the MSACE using a peacock s feather. Figure 10 shows the central part of the image. From the figure, it is seen that the image consists of four parts, corresponding to the four sections of the MSACE. Moreover, the image in the red section is rather different to the images in the green section and the blue section. For instance, in the red section, the centre pattern and first ring around it has fuzzy boundary, while in the blue and green section, the contrast between the centre pattern and the first ring is very clear; what s more, in the green section, there is a highlighted ring near the periphery of the feather pattern, while the same place is not so clear in either red or (1). The MSACE is made in two parts: First, multispectra filters are made by means of depositing color pigment based photo-resists on glass substrate. Second, a micro lens array (the artificial compound eyes) is made by means of photolithography and thermal reflow. (2) The MSACE system can effectively catch the hidden color patterns of an image. (3) The MSACE is rather inexpensive and hence, can be used in many applications, such as medical diagnosis and currency counterfeit checking. It shall be mentioned that the presented MSACE is still in its infancy. From the design point of view, for instance, the micro lens can be in different sizes and shapes, which gives the light field imaging. In addition, many more filters can be added covering more spectral bands. From the fabrication point of view, many improvements can be made, such as applying the heat source on top of the substrate for thermal reflow process other than using a hot plate under the substrate; and controlling the thermal reflow temperature and time.

5 5 Acknowledgement The authors wish to thank Dr. S. Di and Mr. J. Jin of Guangzhou Chinese Academy of Sciences Institute of the Advanced Technology for helping to setup the imaging experiments. 6 References [1] [2] [3] [4] J. Duparre, P. Dannberg, and et al, Artificial Apposition Compound Eye Fabricated by Micro-Optics Technology, Applied Optics, Vol. 43, pp , [5] J. Duparre, D. Radtek and A. Tunnermann, Spherical Artificial Compound Eye Captures Real Images, roc. of SPIE; Paper No K-1-9, [6] J. Tanida and et al, Thin Observation Module by Bound Optics (TOMBO): Concept and Experimental Verification, Applied Optics; Vol. 40, pp , [7] R. Shogenji, Y. Kitamura, K. Yamada, S. Miyatake and J. Tanida, Multispectral Imaging Using Compact Compound Optics, Optical Express, Vol. 12, pp , [8] J. Tanida, R. Shogenji, and at el, Color Imaging with an Integrated Compound Imaging system, Optical Express; Vol. 11, pp , [9] R. Shogenji, Y. Kitamura, K. Yamada, S. Miyatake and J. Tanida, Bimodal Fingerprint Capturing System Based on Compound-Eye Imaging Module, Applied Optics; Vol. 43, pp , [10] B. Lin, and T. Y. Tian, Study of Fabrication of 16 Channel Micro Integrated Filter, J. Infrared Millim. Waves, Vol. 25, No. 4, pp August, [11] S. Di and R. Du, The Controlling of Microlens Contour by Adjusting Developing Time in Thermal Reflow Method, International Symposium on Photoelectronic Detection and Imaging 2009, June 17-19, China. [12] S. Di, H. Li and R. Du, An Artificial Compound Eyes Imaging System Based on MEMS Technology, IEEE Robio 2009, Dec , 2009, Guilin, China. [13] S. Di, H. Lin and R. Du, A Simple Method for Fabrication of Artificial Compound Eye, The 6th International Conference on Micro-Manufacturing (ICOMM 2011), March 7 10, 2011, Tokyo, Japan. [14] B. K. Lee, D. S. Kim, and T. H. Kwon; Replication of Microlens Arrays by Injection Molding, Microsystem Technologies, Volume 10, Issue 6-7, pp , October [15] Z. D. Popvic, R. A. Sprague and G. A. N. Connel, Technique for Monolithic Fabrication of Microlens Arrays, Applied Optics, Vol. 27, No. 7, pp , [16] R. W. Sabnis, Color Filter Technology for Liquid Crystal Displays, Displays, Vol. 20, pp , [17] G. Themelis, J. S. Yoo, and V. Ntziachristos, Multispectral Imaging Using Multiple-Bandpass Filters, Optics Letters, Vol. 33, No. 9, [18] B. E. Bayer, Color Imaging Array, U.S. atent 3,971,065, [19] J. M. Eichenholz, N. Barnett, and et. al, Real Time Megapixel Multispectral Bioimaging, roc. of S IE, aper No. 7568, Jan [20] J. Y. Hardeberg, Multispectral Color Image Capture Using a Liquid Crystal Tunable Filter, Optics Engineering, Vol. 41, No. 10, pp , October 2002.