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Fabrication and investigation of the bionic curved visual microlens array films
Optical Materials 66 (2017) 630e639
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Fabrication and investigation of the bionic curved visual microlens array films Wen-Kai Kuo a, Syuan-You Lin a, Sheng-Wei Hsu a, Hsin Her Yu b, * a b
Graduate Institute of Electro-Optical and Materials Science, National Formosa University, 64 Wunhua Road, Huwei, Yunlin, 63208, Taiwan Department of Biotechnology, National Formosa University, 64 Wunhua Road, Huwei, Yunlin, 63208, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2017 Received in revised form 23 February 2017 Accepted 14 March 2017 Available online 21 March 2017
The compound eyes of insects are smaller, lighter, and have a wider field of view and high sensitivity to moving targets. In recent years, these advantages have attracted many researchers to develop minimized optical devices. In this study, a high performance microlens was fabricated, mimicking the biological visual feature. Polystyrene (PS) microspheres were synthesized by dispersion polymerization first, and then a close-packed monolayer of PS microspheres was assembled by the LangmuireBlodgett (LB) deposition method. Following this, a 2D polydimethylsiloxane (PDMS) concave mold was fabricated by a soft lithography technique. Different aperture sizes of poly(methyl methacrylate) (PMMA) curved microlens array replicated films were prepared using traditional Chinese medicine cupping tool with a temperature-controllable hot plate, which eliminated the need for inconvenient metal modeling. The optical performance of the curved microlens films were evaluated by a system of optical microscopy (OM) and a home-made image capture charge-coupled device (CCD). The field of view (FOV) and the light intensity distribution of the curved microlens array were also investigated. We found that a broader field of view corresponded to a smaller aperture size of the curved microlens films, as the convex heights of the films are identical. The resolution of the curved microlens films was not affected by their aperture sizes, but was determined by their interommatidial angle and the diameter of the microlens. © 2017 Elsevier B.V. All rights reserved.
Keywords: Monolayer Langmuir-Blodgett (LB) Curved microlens array Field of view (FOV) Ommatidia
1. Introduction A lens is an optical element for converging or diverging light, and has been used by humans for centuries in such applications as starting fires or for the treatment of myopia. Nowadays, whether medical, military, imaging, or biotechnology, the lens has assumed an indispensable role. Lens technologies began exploring miniaturization to develop a variety of miniaturized optical products. The microlenses produced by micro-optical technology are gradually being taken more seriously. The advantages of microlenses include their small size and light weight, as well as having the capability of being arrayed, making them one of the more important optical components in micro-opto-electro-mechanical systems (MOEMS) [1]. Microlens arrays can also be extended to industrial applications such as optical reading systems, charge coupled devices (CCD), fiber optic communications systems, and liquid crystal display
* Corresponding author. E-mail addresses: [email protected] (W.-K. Kuo), [email protected] (S.-Y. Lin), [email protected] (S.-W. Hsu), [email protected] (H.H. Yu). http://dx.doi.org/10.1016/j.optmat.2017.03.020 0925-3467/© 2017 Elsevier B.V. All rights reserved.
backlights. The term “bionics” was first proposed by Steele [2] in the 1960s. It implies understanding and mimicking biological structures or functions from nature and apply them to our lives. Many researchers have paid attention to the compound eyes of insects in recent years. The compound eyes of insects are composed of thousands of identical structures of curved array ommatidia. The ommatidia are composed mainly of the cornea, crystal cone, rhabdom, photoreceptor cells, and nerve cells. Incident light passes through the cornea and lens to the rhabdom, and is then, by way of photoreceptor cells in the rhabdom, converted into signal information, which is passed to the brain to produce images. A number of ommatidia of different shapes are arranged on the surface of the eyes of insects. The number of ommatidia can range from 4000 for a housefly to 30000 for a dragonfly, and from 12000 to 17000 for a butterfly [3]. The diameters of the ommatidia of insects generally range between 5 and 50 mm. As insect eyes cannot focus to obtain a sharper image, the visual distance of insects is limited, typically to approximately 1/60 to 1/80 of that of the human eye. At the horizon of their eyes, dragonflies can generally only see 5e6 m, whereas
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houseflies can only see 4e7 cm. The orientation and distance of objects can be detected by insects, depending on the interommatidial angle of their compound eyes [4]. However, the compound eyes of insects are a complex system composed of many ommatidia. Each individual ommatidium collects incident light and produces a partial image of the objective separately; the complete image of the object being viewed is then constructed from these images [5,6]. The field of view (FOV) of the curved compound eyes of insects is wide [7]. Human eyes possess an FOV of approximately 180 , while those of dragonflies average about 360 , which is far more advantageous than the simple eyes of most mammals. With the progress of technology and innovation, the quality of electronic digital imaging products has gradually become more sophisticated gradually to keep up with people's demand. Miniaturization of products, wide-angle lenses, and high-resolution images are the current development tendencies in mobile phones, cameras, monitors and even detection systems. Traditional optical elements are not able to satisfy the requirements of current scientific and technological research, resulting in a focus on microoptical technologies. Owing to the characteristics of the compound eyes of insects, such as smaller size, lighter weight, and a wider visual field, they have inspired the development of a bionic curved microlens array. At present, the main methods for preparing the microlens array are sol-gel [8e12], laser etching [13e15], soft lithography [16e18], and press molding. However, the equipment used in these fabrication procedures are not only expensive but also complicated. With our previous work [19], a close-packed 2D PS crystals arrays was arranged as both self-assembly dip-drawing and magnetic stirring methods were used simultaneously. Biomimetic convex compound eye-replicating films of different hemispherical heights could be obtained directly by a glass ball thermo-pressing method. However, a perfect curved microlens array replicating film is difficult to obtain since a uniform surface temperature of the glass ball is not easily controlled. In this paper, we report a facile method to fabricate the bionic curved microlens array replicating films with the same height but different aperture sizes by a self-assembled Langmuir-Blodgett (LB) deposition method and soft lithography, and shaped the film by a traditional Chinese medicine cupping tool equipped with a temperature-controllable hot plate. The surface morphologies of these films were explored by optical microscopy (OM), fieldemission scanning electron microscopy (FESEM), and atomic force microscopy (AFM). Furthermore, the field of view (FOV) and the beam intensity distribution based on these films were also evaluated. 2. Experimental section 2.1. Synthesis of the monodispersed polystyrene (PS) microspheres The PS microspheres were synthesized from styrene (St) using a dispersion-polymerization technique while employing Poly(Nvinylpyrrolidone) (PVP) as the stabilizer and 2,2-Azobis(2methylpropionitrile) (AIBN) as the initiator. The polymerization procedure was performed as follows: a mixture of 95 ml of absolute alcohol, 5 ml of deionized water, 50 ml of St monomer, and 5 ml of PVP was placed in a 250 ml four-necked flask equipped with a reflux condenser and a mechanical stirrer. After the mixture had been mixed homogeneously at 300 rpm for 10 min, it was heated to 70 C. Then, 0.5 ml of AIBN was added to the four-necked flask, and dispersion-polymerization was reacted under a nitrogen atmosphere for 24 h. A monodispersed PS microspheres were obtained by centrifugal sedimentation (LEGEND MACH 1.6R, SORVALL) at 1200 rpm for 10 min and repeating 3 times. A suspension of uniformly-sized PS microspheres was obtained. Finally, the PS
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microspheres were dispersed in ethanol of 150 ml. 2.2. Fabrication and characterization of a monolayer microarray template of PS microspheres In order to attach a monolayer of PS microspheres on a hydrophilic glass plate, the surface of a glass substrate was etched in 10 wt% sodium hydroxide solution for 30 min before self-assembly coating. The hydrophilic glass substrate was fixed onto a LB deposition trough (KN 2002, KSV NIMA) above the surface of the sink, as shown in Fig. 1. In order that the PS microspheres in solid phase (specific gravity, 1.04) could be suspended in the liquid phase of the LB trough, we formulated a 20 wt% glucose solution (specific gravity, 1.073) as the liquid phase and poured it into the tank. A plate of platinum was hung and submerged halfway into the liquid phase for the purpose of sensing the surface pressure. After the glass substrate was lowered to the height of 50 mm, both sides of the barrier were moved toward the position of 60 mm. The 10 wt% PS/ethanol solution was uniformly dropped on the liquid phase with a pipette. The PS microspheres were allowed to spread uniformly for 10 min on the liquid phase. A monolayer of PS microspheres was allowed to assemble on the surface of the LB trough as the drawing rate of the glass substrate was fixed at 1 mm/min. The surface pressure of the liquid surface was 4.7 mN/m when both sides of the barriers were moving. Next, the substrate was placed in a vacuum oven at 70 C for 3 h to enhance how closely the PS microspheres were packed. The surface morphology of the close-packed PS microsphere template was examined by using optical microscopy (OM) (ML-803, OPTIMA), Field-emission scanning electron microscope (JSM7401F, JOEL) and Atomic force microscope (AFM) (ICON-PT, BRUKER). 2.3. Fabrication of the polydimethylsiloxane (PDMS) concave mold The PDMS prepolymer was prepared by blending the base material and the curing agent at the ratio of 5:1, uniformly; the gas bubbles in the colloid were eliminated in a refrigerator. The PDMS prepolymer was then poured into the empty spaces of the PS microsphere template and cured in a drying oven at 50 C for 8 h. A two-dimensional PDMS concave mold was then obtained after peeling it off of the PS microsphere template, as shown in Fig. 2(a). The surface morphology of this concave PDMS mold was observed by OM and FESEM (see Fig. 2). 2.4. Fabrication of poly(methyl methacrylate) (PMMA) curved microlens array films A 10 wt% PMMA/acetic acid solution was poured onto the PDMS concave mold, which was then placed on a 40 C hot plate. A replicating flat PMMA film was subsequently peeled off from the concave mold. A curved microlens array film could then be formed by employing a traditional Chinese medicine cupping tool equipped with a temperature-controllable hot plate using the following method (see Fig. 3). A flat microlens array film was inserted between two steel plates, drilled with pores of equal size, corresponding to the aperture size of the microlenses, and placed on a hot plate heated to 70 C for 10 min. The same height but different aperture sizes of PMMA curved microlens array films could be formed by controlling the air removal from the cup tool at different period of degas times. The cupping tool was then removed from the now convex-shaped sample, which was then cooled down to 25 C. This produced PMMA curved microlens array films capable of mimicking convex compound eyes. The surface morphologies of the PMMA curved microlens array films were explored by OM,
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Fig. 1. Schematic of a close-packed monolayer microarray of PS microspheres assembled by a Langmuir-Blodgett (LB) deposition process.
Fig. 2. Schematic of the process for fabricating the flat microlens array PMMA replicated films. (a) A 2D concave PDMS mode is prepared by soft lithography technology and (b) a flat microlens array PMMA replicated film is prepared by solution casting.
Fig. 3. (a) Schematic for fabrication of different apertures of curved microlens array replicating films by a traditional Chinese medicine cupping tool with a temperature controllable hot plate, (b) photograph of the experimental device and (c) partial enlarged view of (b).
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FESEM, and AFM.
opposite side through a projection object, which was a transparent plastic sheet with the letter “a” printed on it in black. Finally, the miniaturized letter was projected onto the CCD sensor (STCMC83USB, Unice E-O Services Inc.) of the image capture system. We also used another CCD image analyzer (DWC 500, GetWave) to evaluate the light intensity distribution of the flat microlens array and curved microlens array films. In order to allow laser light to cover a microlens array film in the favored measurement range, a He-Ne laser was used as the light source with a beam expander (5) mounted in front of it. (see Fig. 6). The FOV is the solid angle that the scene subtends at the lens (see Fig. 7). The FOV can be specified precisely as HFOV VFOV (H ¼ horizontal field of view and vertical field of view, respectively). The light of the halogen lamp illuminated the convex compound eye-replicating films from different directions and distances. The FOV and visual distances of the films were determined from the light spots on the domes, which were recorded by the CCD sensor. The half width of the HFOV (w1), the half width of the VFOV (w2) and the visual distance (dv) of the microlens to the FOV were calculated using Eqs. (1) and (2).
2.5. Optical performance of the bionic curved microlens array films
HFOV ¼ 2 tan1 ðw1 =dv
Fig. 4. Schematic showing a bionic curved microlens array replicating film under an optical microscope.
In order to explore the optical performances of the bionic curved microlens array films, we mounted the samples on a stage and placed an object printed with a letter (or a pattern) on it in front of a light source under an OM (see Fig. 4). In addition, we designed a simple optical image capture system (see Fig. 5). A halogen lamp with an adjustable luminous flux (30000 lux) was used as the light source; a polarizer was mounted in front of it, in order to decrease the intensity of the light. Curved microlens array films, of equal height but differing aperture sizes, were fixed on the movable sample stage, and the light was collimated with a doublet lens. The mounted film was illuminated by the halogen lamp from the
Fig. 5. (a) Schematic of the 3D module and (b) photograph of the laboratory-made CCD image capture system.
VFOV ¼ 2 tan1 ðw2 =dv
(1) (2)
3. Results and discussion 3.1. Analysis of the monolayer microarray template of PS microspheres In this study, the size of the synthesized PS microspheres is about 5 mm. The OM images of the self-assembled monolayer array of PS microspheres deposited on a glass substrate by the LB fabrication are shown in Fig. 8 (a, b, c)-1, which were associated with the barrier pressures 4.5, 4.6, and 4.7 mN/m, respectively, and a constant substrate drawing rate of 1 mm/min. Fig. 8 (a)-2 and (b)-2 are
Fig. 6. (a) Schematic of the 3D module and (b) photograph of the laboratory-made light intensity distribution measuring system.
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Fig. 7. Schematic showing how the field of view values and visual distance of the bionic curved microlens array replicating films were measured.
Fig. 8. Optical microscopy images of self-assembled arrays of PS microspheres on a glass substrate, as barrier pressures of (a-1) 4.5 mN/m, (b-1) 4.6 mN/m, and (c-1) 4.7 mN/m were exerted during the LB deposition procedure. (a,b)-2 are the SEM and 45 tilted SEM images of the corresponding sample in (c-1).
Fig. 9. The surface morphologies of the PDMS concave mold and the PMMA replicating film. a, b noted the PDMS concave mold and flat PMMA microlens array replicating film respectively; 1, 2, 3 represent the images observed from the OM, SEM and 45 tilted SEM, respectively.
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evident from the surface morphologies imaged by OM and FESEM and are shown in Fig. 9(a, b, c)-1 and (a, b, c)-2, respectively. The light intensity distribution of the flat PMMA microlens array is shown in Fig. 10. The coefficient of variance (CV) of the spot size from the microlens was 2%, which was calculated using Eq. (3). As the coefficient of variation is less than 5% [20], it shows that the microlens array has a relatively high uniformity overall. Fig. 11 (a) and (b) are 3D and 2D AFM images of the flat PMMA microlens array film and (c) is its profile line in (b). The microlenses were close-packed and arranged uniformly. The white profile line in Fig. 11 (c), which runs edge to edge, shows that the average diameter (d), height (h), and radius (r) of the hemispherical ommatidia were 4.272, 1.079, and 2.136 mm, respectively.
CV ¼ ðstandard deviation of spot sizes=averaged spot sizeÞ 100% (3) Fig. 10. The light intensity distribution of the flat PMMA microlens array.
the SEM and 45 tilted SEM images, respectively, corresponding to the array imaged in Fig. 8 (c)-1. When the surface pressure of both sides of the barriers was increased to 4.5 mN/m exerted on the liquid phase, the PS microspheres came to be densely packed; however, local defects and vacancies still occurred on the monolayer of the PS microspheres as the surface pressure was increased to 4.6 mN/m (see Fig. 8 (b-1)). A compact, ordered periodic structure of the PS microspheres could be observed as the surface pressure approached 4.7 mN/m (see Fig. 8 (c-1)). On the other hand, compact, ordered, and periodically arranged PS microspheres were not formed when the surface tension exceeded 4.7 mN/m, as multilayer stacking began to occur. 3.2. Characterization of the bionic curved microlens array films The intact close-packed periodic structures of the PDMS concave mold and the replicating flat PMMA microlens array film were
The cross-section profiles of curved microlens array films of differing aperture sizes are shown in Fig. 12 (a) and (b). These indicate that a high quality artificial microlens array film could be obtained easily and directly from the flat film by the Chinese medicine cupping tool without the traditional metal modes construction and it is also better than our previous glass ball thermal pressing procedures [19]. The SEM image of a bionic curved microlens array film tilted at an angle of 45 was shown in Fig. 13 (a) with the magnified image of the corresponding film was in (b), each ommatidia remained intact and without any distortion after the vacuum-socking shaping using the cupping tool and a temperature controllable hot plate. 3.3. Surface properties of the bionic curved microlens array films The radius of curvature (rC), focal length (f), and numerical aperture (NA) of the bionic curved microlens array films were calculated using Eqs. (4)e(6) [13,21]. The angle between the optical axes of adjacent ommatidia is defined as the interommatidial angle,
Fig. 11. (a),(b) are 3D and 2D AFM images of the flat PMMA microlens array replicating film and (c) is its profile line in (b).
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Fig. 12. (a) The cross-sectional profiles of equal height, but differencing aperture size, curved microlens array replicating films and (b) the perspective view images of the corresponding films of (a).
Fig. 13. (a) SEM image of a bionic curved microlens array replicating film when tilted at an angle of 45 and (b) a magnified image of the corresponding film.
Df (or the angular resolution of the bionic curved microlens). It was calculated using Eq. (7) [13,22]: rc ¼ ðh2 þr2 Þ=2h;
(4)
f ¼ rc =ðnPMMA 1Þ;
(5)
NA ¼ ðd=2f Þ;
(6)
Df ¼ d=R;
(7)
to improve the image resolution of the bionic microlens-based visual system by increasing the diameter of the microlens to more than 10 mm. 3.4. Analysis of field of view of the bionic curved microlens array
where r is half the diameter of the ommatidia (d/2), h is the height of the ommatidia, and nPMMA is the refraction index of PMMA (approximately 1.491). R is the curvature of the curved microlens array film corresponding to different aperture sizes (5 mm, 8 mm, and 10 mm) of the curved microlens array films (approximately 3.625 mm, 8.5 mm, and 13 mm, respectively). According to Eq. (7), the interommatidial angle Df of different aperture sizes (5 mm, 8 mm, and 10 mm) of the curved microlens array films were 0.07, 0.03 , and 0.02 , respectively, which is lower than that of the members of the dragonfly family. For instance, Df of the species Anax junius, Aeshna grandis, and Odonata sympetrum are 0.24 , 0.8 , and 0.4 , respectively [4]. Our results show that the optical characteristics of the fabricated convex microlens array films were comparable to those found in nature. Clearly, the challenge ahead is
A relationship between the height and aperture size of the curved bionic curved microlens array films is illustrated schematically in Fig. 14(a). The variation in aperture size of the curved array films resulted in a change in the number of ommatidia being illuminated. This means that the CCD sensor received signals from differing numbers of light spots, owing to the differences in the FOV (the yellow zone of Fig. 14 (a)). The visual distance (dv) of these films was found to be 23 cm after measuring in the FOV testing system for every aperture size, since the heights of the three curved microlens array films we prepared are identical. FOV were shrunk gradually from 76.3 75.4 to 74.5 73.2 and to 73.3 72.4 as the aperture sizes (D) of curved microlens array films were increased from 5 mm to 8 mm and 10 mm, respectively (see Fig. 14 (b) and (c)). 3.5. Imaging performances of the bionic curved microlens array films The imaging performance of the bionic curved microlens array films with different aperture sizes were evaluated using an OM (see
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Fig. 14. (a) Schematic of the relationship between the height and aperture size of the curved microlens array replicating films, FOV measuring results exhibited in (b) 3D view and (c) the HFOV and VFOV diagrams.
Fig. 15. The imaging performance of the curved microlens array replicating films; a, b, c label the aperture sizes of the films at 5, 8, 10 mm, respectively; 1, 2, 3 represent the images obtained from the OM, laboratory-made image capture system and PSF measuring, respectively.
Fig. 15 (a, b, c)-1) and a laboratory-made CCD image capture system (see Fig. 15 (a, b, c)-2). The image quality was evaluated on the basis of the standard deviation (SD) of all the pixels. The calculated SD
results corresponding to Fig. 15 (a, b, c)-1) and Fig. 15 (a, b, c)-2) were 27.32, 29.46, 26.39 and 25.77, 28.05, 24.41, respectively. The SD, and hence, the quality of the images associated with the three
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Fig. 16. The light intensity distribution of different aperture sizes of the curved microlens array replicating films. a, b, c labelled the aperture sizes of the curved films are 5, 8, 10 mm, respectively; 1, 2, 3 represent the 2D, 3D, their horizontal profiles diagrams, respectively.
films were similar, this indicates that the aperture sizes of the films were not the predominating factor determining the optical performance. The main factor that determines the resolution of a compound eye is the interommatidial angle between the optical axes of two adjacent ommatidia. In other words, the resolution of a bionic curved microlens array film increases with a decrease in the interommatidial angle of two adjacent microlenses. In addition, Fig. 15 (a, b, c)-3 shows the measured point spread functions (PSF) of the corresponding films. The airy discs of the curved microlens array films with different aperture sizes had diameters of approximately 3.1, 2.6, and 2.5 mm, respectively. This indicates that the resolving powers of these three cases were very similar. It should be noted that a few speckles were observed in the airy patterns, as shown in Fig. 15 (a, b, c)-3; these were attributable to the scattering of contaminant particles incorporated during the replication procedures. This can be explained by the fact that the fabricated microlenses were not perfectly circular. On the other hand, the
spacing between the airy discs of the microlenses increased from 3.8 to 4.0 and 4.4 mm as the aperture of the films was increased from 5 mm to 8 mm and 10 mm, respectively. 2D, 3D, and horizontal profiles of the light intensity distributions for each aperture size of curved microlens array film are shown in Fig. 16(a, b, c)-1,2,3, respectively. The isochart of Fig. 16(a, b, c)-1 is useful for determining how much area a light fixture can cover. From Fig. 16(a, b, c)-1, we found that the light intensity distribution peaked at the center area, but decreased gradually moving outwards on the 2D diagram. Basically, the light intensity distribution shape is nearly symmetrical. Smaller aperture size of curved microlens array film, smaller the light intensity distribution area. Moreover, from the 3D and horizontal profiles in Fig. 16(a, b, c)-2 and 3, respectively, we found that the light intensity distribution decreased rapidly in the smaller aperture size case (5 mm). The defocused phenomena usually was seen to occur as the light passed through the curved microlens array film at a distance far from the
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center point, especially in the smaller aperture size case. Therefore, an obvious sharp peak could be observed in Fig. 16(a)-2 and 3, respectively. Many noises were occurred on the light intensity distribution curves, since the light interfered with the microlens of films. The resolutions of the curved microlens array films in this study could be further improved by increasing the diameter of ommatidia. However, the diffraction-limited resolution is inversely related to the diameter of ommatidia [23]. To improve the resolution of the curved microlens array film, it is necessary to increase the film's curvature. In addition, we could use Poly 2-hydroxyethyl methacrylate (pHEMA), silicone methacrylate (SMA) or modified silk fibroin to replace the PMMA for the microlens array film to increase biocompatibility with the human body. Furthermore, different imaging amplification effects can be obtained if we rotate two identical curved microlens arrays in different directions and patterns [24]. This bioangles, according to the principle of Moire logically inspired mimetic visual feature can be extended to applications such as medical diagnostic systems, light-field photography devices, and endoscopy, amongst others.
[2] [3] [4] [5] [6] [7] [8]
[9]
[10]
[11]
[12]
4. Conclusions [13]
In this study, monodispersed microsized polystyrene spheres were synthesized by dispersion polymerization. A close-packed monolayer array of PS microspheres was assembled by the Langmuir-Blodgett (LB) deposition technology, with the surface pressure exerted by the moving barriers fixed at 4.7 mN/m and the drawing rate of the glass substrate kept at 1 mm/min. Different aperture sizes, but the same height of curved microlens array replicating films could be prepared facilely by a traditional Chinese medicine cupping device with a hot plate without metal modeling. The curved microlens film with the smallest aperture size exhibited the broadest field of view, as the heights of all of the films were equal. In addition, the resolution of the curved microlens films was not affected by their aperture sizes, but was determined by the interommatidial angle and the diameter of the microlens.
[14]
[15]
[16]
[17]
[18]
[19]
Notes [20]
The authors declare no competing financial interest. [21]
Acknowledgements The authors gratefully acknowledge financial support from the Ministry of Science and Technology of Taiwan (MOST 104-2221-E150-038 & 104-2221-E-150-064-MY3). References [1] K.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Fabrication and investigation of the bionic curved visual microlens array films Wen-Kai Kuo a, Syuan-You Lin a, Sheng-Wei Hsu a, Hsin Her Yu b, * a b
Graduate Institute of Electro-Optical and Materials Science, National Formosa University, 64 Wunhua Road, Huwei, Yunlin, 63208, Taiwan Department of Biotechnology, National Formosa University, 64 Wunhua Road, Huwei, Yunlin, 63208, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2017 Received in revised form 23 February 2017 Accepted 14 March 2017 Available online 21 March 2017
The compound eyes of insects are smaller, lighter, and have a wider field of view and high sensitivity to moving targets. In recent years, these advantages have attracted many researchers to develop minimized optical devices. In this study, a high performance microlens was fabricated, mimicking the biological visual feature. Polystyrene (PS) microspheres were synthesized by dispersion polymerization first, and then a close-packed monolayer of PS microspheres was assembled by the LangmuireBlodgett (LB) deposition method. Following this, a 2D polydimethylsiloxane (PDMS) concave mold was fabricated by a soft lithography technique. Different aperture sizes of poly(methyl methacrylate) (PMMA) curved microlens array replicated films were prepared using traditional Chinese medicine cupping tool with a temperature-controllable hot plate, which eliminated the need for inconvenient metal modeling. The optical performance of the curved microlens films were evaluated by a system of optical microscopy (OM) and a home-made image capture charge-coupled device (CCD). The field of view (FOV) and the light intensity distribution of the curved microlens array were also investigated. We found that a broader field of view corresponded to a smaller aperture size of the curved microlens films, as the convex heights of the films are identical. The resolution of the curved microlens films was not affected by their aperture sizes, but was determined by their interommatidial angle and the diameter of the microlens. © 2017 Elsevier B.V. All rights reserved.
Keywords: Monolayer Langmuir-Blodgett (LB) Curved microlens array Field of view (FOV) Ommatidia
1. Introduction A lens is an optical element for converging or diverging light, and has been used by humans for centuries in such applications as starting fires or for the treatment of myopia. Nowadays, whether medical, military, imaging, or biotechnology, the lens has assumed an indispensable role. Lens technologies began exploring miniaturization to develop a variety of miniaturized optical products. The microlenses produced by micro-optical technology are gradually being taken more seriously. The advantages of microlenses include their small size and light weight, as well as having the capability of being arrayed, making them one of the more important optical components in micro-opto-electro-mechanical systems (MOEMS) [1]. Microlens arrays can also be extended to industrial applications such as optical reading systems, charge coupled devices (CCD), fiber optic communications systems, and liquid crystal display
* Corresponding author. E-mail addresses: [email protected] (W.-K. Kuo), [email protected] (S.-Y. Lin), [email protected] (S.-W. Hsu), [email protected] (H.H. Yu). http://dx.doi.org/10.1016/j.optmat.2017.03.020 0925-3467/© 2017 Elsevier B.V. All rights reserved.
backlights. The term “bionics” was first proposed by Steele [2] in the 1960s. It implies understanding and mimicking biological structures or functions from nature and apply them to our lives. Many researchers have paid attention to the compound eyes of insects in recent years. The compound eyes of insects are composed of thousands of identical structures of curved array ommatidia. The ommatidia are composed mainly of the cornea, crystal cone, rhabdom, photoreceptor cells, and nerve cells. Incident light passes through the cornea and lens to the rhabdom, and is then, by way of photoreceptor cells in the rhabdom, converted into signal information, which is passed to the brain to produce images. A number of ommatidia of different shapes are arranged on the surface of the eyes of insects. The number of ommatidia can range from 4000 for a housefly to 30000 for a dragonfly, and from 12000 to 17000 for a butterfly [3]. The diameters of the ommatidia of insects generally range between 5 and 50 mm. As insect eyes cannot focus to obtain a sharper image, the visual distance of insects is limited, typically to approximately 1/60 to 1/80 of that of the human eye. At the horizon of their eyes, dragonflies can generally only see 5e6 m, whereas
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houseflies can only see 4e7 cm. The orientation and distance of objects can be detected by insects, depending on the interommatidial angle of their compound eyes [4]. However, the compound eyes of insects are a complex system composed of many ommatidia. Each individual ommatidium collects incident light and produces a partial image of the objective separately; the complete image of the object being viewed is then constructed from these images [5,6]. The field of view (FOV) of the curved compound eyes of insects is wide [7]. Human eyes possess an FOV of approximately 180 , while those of dragonflies average about 360 , which is far more advantageous than the simple eyes of most mammals. With the progress of technology and innovation, the quality of electronic digital imaging products has gradually become more sophisticated gradually to keep up with people's demand. Miniaturization of products, wide-angle lenses, and high-resolution images are the current development tendencies in mobile phones, cameras, monitors and even detection systems. Traditional optical elements are not able to satisfy the requirements of current scientific and technological research, resulting in a focus on microoptical technologies. Owing to the characteristics of the compound eyes of insects, such as smaller size, lighter weight, and a wider visual field, they have inspired the development of a bionic curved microlens array. At present, the main methods for preparing the microlens array are sol-gel [8e12], laser etching [13e15], soft lithography [16e18], and press molding. However, the equipment used in these fabrication procedures are not only expensive but also complicated. With our previous work [19], a close-packed 2D PS crystals arrays was arranged as both self-assembly dip-drawing and magnetic stirring methods were used simultaneously. Biomimetic convex compound eye-replicating films of different hemispherical heights could be obtained directly by a glass ball thermo-pressing method. However, a perfect curved microlens array replicating film is difficult to obtain since a uniform surface temperature of the glass ball is not easily controlled. In this paper, we report a facile method to fabricate the bionic curved microlens array replicating films with the same height but different aperture sizes by a self-assembled Langmuir-Blodgett (LB) deposition method and soft lithography, and shaped the film by a traditional Chinese medicine cupping tool equipped with a temperature-controllable hot plate. The surface morphologies of these films were explored by optical microscopy (OM), fieldemission scanning electron microscopy (FESEM), and atomic force microscopy (AFM). Furthermore, the field of view (FOV) and the beam intensity distribution based on these films were also evaluated. 2. Experimental section 2.1. Synthesis of the monodispersed polystyrene (PS) microspheres The PS microspheres were synthesized from styrene (St) using a dispersion-polymerization technique while employing Poly(Nvinylpyrrolidone) (PVP) as the stabilizer and 2,2-Azobis(2methylpropionitrile) (AIBN) as the initiator. The polymerization procedure was performed as follows: a mixture of 95 ml of absolute alcohol, 5 ml of deionized water, 50 ml of St monomer, and 5 ml of PVP was placed in a 250 ml four-necked flask equipped with a reflux condenser and a mechanical stirrer. After the mixture had been mixed homogeneously at 300 rpm for 10 min, it was heated to 70 C. Then, 0.5 ml of AIBN was added to the four-necked flask, and dispersion-polymerization was reacted under a nitrogen atmosphere for 24 h. A monodispersed PS microspheres were obtained by centrifugal sedimentation (LEGEND MACH 1.6R, SORVALL) at 1200 rpm for 10 min and repeating 3 times. A suspension of uniformly-sized PS microspheres was obtained. Finally, the PS
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microspheres were dispersed in ethanol of 150 ml. 2.2. Fabrication and characterization of a monolayer microarray template of PS microspheres In order to attach a monolayer of PS microspheres on a hydrophilic glass plate, the surface of a glass substrate was etched in 10 wt% sodium hydroxide solution for 30 min before self-assembly coating. The hydrophilic glass substrate was fixed onto a LB deposition trough (KN 2002, KSV NIMA) above the surface of the sink, as shown in Fig. 1. In order that the PS microspheres in solid phase (specific gravity, 1.04) could be suspended in the liquid phase of the LB trough, we formulated a 20 wt% glucose solution (specific gravity, 1.073) as the liquid phase and poured it into the tank. A plate of platinum was hung and submerged halfway into the liquid phase for the purpose of sensing the surface pressure. After the glass substrate was lowered to the height of 50 mm, both sides of the barrier were moved toward the position of 60 mm. The 10 wt% PS/ethanol solution was uniformly dropped on the liquid phase with a pipette. The PS microspheres were allowed to spread uniformly for 10 min on the liquid phase. A monolayer of PS microspheres was allowed to assemble on the surface of the LB trough as the drawing rate of the glass substrate was fixed at 1 mm/min. The surface pressure of the liquid surface was 4.7 mN/m when both sides of the barriers were moving. Next, the substrate was placed in a vacuum oven at 70 C for 3 h to enhance how closely the PS microspheres were packed. The surface morphology of the close-packed PS microsphere template was examined by using optical microscopy (OM) (ML-803, OPTIMA), Field-emission scanning electron microscope (JSM7401F, JOEL) and Atomic force microscope (AFM) (ICON-PT, BRUKER). 2.3. Fabrication of the polydimethylsiloxane (PDMS) concave mold The PDMS prepolymer was prepared by blending the base material and the curing agent at the ratio of 5:1, uniformly; the gas bubbles in the colloid were eliminated in a refrigerator. The PDMS prepolymer was then poured into the empty spaces of the PS microsphere template and cured in a drying oven at 50 C for 8 h. A two-dimensional PDMS concave mold was then obtained after peeling it off of the PS microsphere template, as shown in Fig. 2(a). The surface morphology of this concave PDMS mold was observed by OM and FESEM (see Fig. 2). 2.4. Fabrication of poly(methyl methacrylate) (PMMA) curved microlens array films A 10 wt% PMMA/acetic acid solution was poured onto the PDMS concave mold, which was then placed on a 40 C hot plate. A replicating flat PMMA film was subsequently peeled off from the concave mold. A curved microlens array film could then be formed by employing a traditional Chinese medicine cupping tool equipped with a temperature-controllable hot plate using the following method (see Fig. 3). A flat microlens array film was inserted between two steel plates, drilled with pores of equal size, corresponding to the aperture size of the microlenses, and placed on a hot plate heated to 70 C for 10 min. The same height but different aperture sizes of PMMA curved microlens array films could be formed by controlling the air removal from the cup tool at different period of degas times. The cupping tool was then removed from the now convex-shaped sample, which was then cooled down to 25 C. This produced PMMA curved microlens array films capable of mimicking convex compound eyes. The surface morphologies of the PMMA curved microlens array films were explored by OM,
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Fig. 1. Schematic of a close-packed monolayer microarray of PS microspheres assembled by a Langmuir-Blodgett (LB) deposition process.
Fig. 2. Schematic of the process for fabricating the flat microlens array PMMA replicated films. (a) A 2D concave PDMS mode is prepared by soft lithography technology and (b) a flat microlens array PMMA replicated film is prepared by solution casting.
Fig. 3. (a) Schematic for fabrication of different apertures of curved microlens array replicating films by a traditional Chinese medicine cupping tool with a temperature controllable hot plate, (b) photograph of the experimental device and (c) partial enlarged view of (b).
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FESEM, and AFM.
opposite side through a projection object, which was a transparent plastic sheet with the letter “a” printed on it in black. Finally, the miniaturized letter was projected onto the CCD sensor (STCMC83USB, Unice E-O Services Inc.) of the image capture system. We also used another CCD image analyzer (DWC 500, GetWave) to evaluate the light intensity distribution of the flat microlens array and curved microlens array films. In order to allow laser light to cover a microlens array film in the favored measurement range, a He-Ne laser was used as the light source with a beam expander (5) mounted in front of it. (see Fig. 6). The FOV is the solid angle that the scene subtends at the lens (see Fig. 7). The FOV can be specified precisely as HFOV VFOV (H ¼ horizontal field of view and vertical field of view, respectively). The light of the halogen lamp illuminated the convex compound eye-replicating films from different directions and distances. The FOV and visual distances of the films were determined from the light spots on the domes, which were recorded by the CCD sensor. The half width of the HFOV (w1), the half width of the VFOV (w2) and the visual distance (dv) of the microlens to the FOV were calculated using Eqs. (1) and (2).
2.5. Optical performance of the bionic curved microlens array films
HFOV ¼ 2 tan1 ðw1 =dv
Fig. 4. Schematic showing a bionic curved microlens array replicating film under an optical microscope.
In order to explore the optical performances of the bionic curved microlens array films, we mounted the samples on a stage and placed an object printed with a letter (or a pattern) on it in front of a light source under an OM (see Fig. 4). In addition, we designed a simple optical image capture system (see Fig. 5). A halogen lamp with an adjustable luminous flux (30000 lux) was used as the light source; a polarizer was mounted in front of it, in order to decrease the intensity of the light. Curved microlens array films, of equal height but differing aperture sizes, were fixed on the movable sample stage, and the light was collimated with a doublet lens. The mounted film was illuminated by the halogen lamp from the
Fig. 5. (a) Schematic of the 3D module and (b) photograph of the laboratory-made CCD image capture system.
VFOV ¼ 2 tan1 ðw2 =dv
(1) (2)
3. Results and discussion 3.1. Analysis of the monolayer microarray template of PS microspheres In this study, the size of the synthesized PS microspheres is about 5 mm. The OM images of the self-assembled monolayer array of PS microspheres deposited on a glass substrate by the LB fabrication are shown in Fig. 8 (a, b, c)-1, which were associated with the barrier pressures 4.5, 4.6, and 4.7 mN/m, respectively, and a constant substrate drawing rate of 1 mm/min. Fig. 8 (a)-2 and (b)-2 are
Fig. 6. (a) Schematic of the 3D module and (b) photograph of the laboratory-made light intensity distribution measuring system.
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Fig. 7. Schematic showing how the field of view values and visual distance of the bionic curved microlens array replicating films were measured.
Fig. 8. Optical microscopy images of self-assembled arrays of PS microspheres on a glass substrate, as barrier pressures of (a-1) 4.5 mN/m, (b-1) 4.6 mN/m, and (c-1) 4.7 mN/m were exerted during the LB deposition procedure. (a,b)-2 are the SEM and 45 tilted SEM images of the corresponding sample in (c-1).
Fig. 9. The surface morphologies of the PDMS concave mold and the PMMA replicating film. a, b noted the PDMS concave mold and flat PMMA microlens array replicating film respectively; 1, 2, 3 represent the images observed from the OM, SEM and 45 tilted SEM, respectively.
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evident from the surface morphologies imaged by OM and FESEM and are shown in Fig. 9(a, b, c)-1 and (a, b, c)-2, respectively. The light intensity distribution of the flat PMMA microlens array is shown in Fig. 10. The coefficient of variance (CV) of the spot size from the microlens was 2%, which was calculated using Eq. (3). As the coefficient of variation is less than 5% [20], it shows that the microlens array has a relatively high uniformity overall. Fig. 11 (a) and (b) are 3D and 2D AFM images of the flat PMMA microlens array film and (c) is its profile line in (b). The microlenses were close-packed and arranged uniformly. The white profile line in Fig. 11 (c), which runs edge to edge, shows that the average diameter (d), height (h), and radius (r) of the hemispherical ommatidia were 4.272, 1.079, and 2.136 mm, respectively.
CV ¼ ðstandard deviation of spot sizes=averaged spot sizeÞ 100% (3) Fig. 10. The light intensity distribution of the flat PMMA microlens array.
the SEM and 45 tilted SEM images, respectively, corresponding to the array imaged in Fig. 8 (c)-1. When the surface pressure of both sides of the barriers was increased to 4.5 mN/m exerted on the liquid phase, the PS microspheres came to be densely packed; however, local defects and vacancies still occurred on the monolayer of the PS microspheres as the surface pressure was increased to 4.6 mN/m (see Fig. 8 (b-1)). A compact, ordered periodic structure of the PS microspheres could be observed as the surface pressure approached 4.7 mN/m (see Fig. 8 (c-1)). On the other hand, compact, ordered, and periodically arranged PS microspheres were not formed when the surface tension exceeded 4.7 mN/m, as multilayer stacking began to occur. 3.2. Characterization of the bionic curved microlens array films The intact close-packed periodic structures of the PDMS concave mold and the replicating flat PMMA microlens array film were
The cross-section profiles of curved microlens array films of differing aperture sizes are shown in Fig. 12 (a) and (b). These indicate that a high quality artificial microlens array film could be obtained easily and directly from the flat film by the Chinese medicine cupping tool without the traditional metal modes construction and it is also better than our previous glass ball thermal pressing procedures [19]. The SEM image of a bionic curved microlens array film tilted at an angle of 45 was shown in Fig. 13 (a) with the magnified image of the corresponding film was in (b), each ommatidia remained intact and without any distortion after the vacuum-socking shaping using the cupping tool and a temperature controllable hot plate. 3.3. Surface properties of the bionic curved microlens array films The radius of curvature (rC), focal length (f), and numerical aperture (NA) of the bionic curved microlens array films were calculated using Eqs. (4)e(6) [13,21]. The angle between the optical axes of adjacent ommatidia is defined as the interommatidial angle,
Fig. 11. (a),(b) are 3D and 2D AFM images of the flat PMMA microlens array replicating film and (c) is its profile line in (b).
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Fig. 12. (a) The cross-sectional profiles of equal height, but differencing aperture size, curved microlens array replicating films and (b) the perspective view images of the corresponding films of (a).
Fig. 13. (a) SEM image of a bionic curved microlens array replicating film when tilted at an angle of 45 and (b) a magnified image of the corresponding film.
Df (or the angular resolution of the bionic curved microlens). It was calculated using Eq. (7) [13,22]: rc ¼ ðh2 þr2 Þ=2h;
(4)
f ¼ rc =ðnPMMA 1Þ;
(5)
NA ¼ ðd=2f Þ;
(6)
Df ¼ d=R;
(7)
to improve the image resolution of the bionic microlens-based visual system by increasing the diameter of the microlens to more than 10 mm. 3.4. Analysis of field of view of the bionic curved microlens array
where r is half the diameter of the ommatidia (d/2), h is the height of the ommatidia, and nPMMA is the refraction index of PMMA (approximately 1.491). R is the curvature of the curved microlens array film corresponding to different aperture sizes (5 mm, 8 mm, and 10 mm) of the curved microlens array films (approximately 3.625 mm, 8.5 mm, and 13 mm, respectively). According to Eq. (7), the interommatidial angle Df of different aperture sizes (5 mm, 8 mm, and 10 mm) of the curved microlens array films were 0.07, 0.03 , and 0.02 , respectively, which is lower than that of the members of the dragonfly family. For instance, Df of the species Anax junius, Aeshna grandis, and Odonata sympetrum are 0.24 , 0.8 , and 0.4 , respectively [4]. Our results show that the optical characteristics of the fabricated convex microlens array films were comparable to those found in nature. Clearly, the challenge ahead is
A relationship between the height and aperture size of the curved bionic curved microlens array films is illustrated schematically in Fig. 14(a). The variation in aperture size of the curved array films resulted in a change in the number of ommatidia being illuminated. This means that the CCD sensor received signals from differing numbers of light spots, owing to the differences in the FOV (the yellow zone of Fig. 14 (a)). The visual distance (dv) of these films was found to be 23 cm after measuring in the FOV testing system for every aperture size, since the heights of the three curved microlens array films we prepared are identical. FOV were shrunk gradually from 76.3 75.4 to 74.5 73.2 and to 73.3 72.4 as the aperture sizes (D) of curved microlens array films were increased from 5 mm to 8 mm and 10 mm, respectively (see Fig. 14 (b) and (c)). 3.5. Imaging performances of the bionic curved microlens array films The imaging performance of the bionic curved microlens array films with different aperture sizes were evaluated using an OM (see
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Fig. 14. (a) Schematic of the relationship between the height and aperture size of the curved microlens array replicating films, FOV measuring results exhibited in (b) 3D view and (c) the HFOV and VFOV diagrams.
Fig. 15. The imaging performance of the curved microlens array replicating films; a, b, c label the aperture sizes of the films at 5, 8, 10 mm, respectively; 1, 2, 3 represent the images obtained from the OM, laboratory-made image capture system and PSF measuring, respectively.
Fig. 15 (a, b, c)-1) and a laboratory-made CCD image capture system (see Fig. 15 (a, b, c)-2). The image quality was evaluated on the basis of the standard deviation (SD) of all the pixels. The calculated SD
results corresponding to Fig. 15 (a, b, c)-1) and Fig. 15 (a, b, c)-2) were 27.32, 29.46, 26.39 and 25.77, 28.05, 24.41, respectively. The SD, and hence, the quality of the images associated with the three
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Fig. 16. The light intensity distribution of different aperture sizes of the curved microlens array replicating films. a, b, c labelled the aperture sizes of the curved films are 5, 8, 10 mm, respectively; 1, 2, 3 represent the 2D, 3D, their horizontal profiles diagrams, respectively.
films were similar, this indicates that the aperture sizes of the films were not the predominating factor determining the optical performance. The main factor that determines the resolution of a compound eye is the interommatidial angle between the optical axes of two adjacent ommatidia. In other words, the resolution of a bionic curved microlens array film increases with a decrease in the interommatidial angle of two adjacent microlenses. In addition, Fig. 15 (a, b, c)-3 shows the measured point spread functions (PSF) of the corresponding films. The airy discs of the curved microlens array films with different aperture sizes had diameters of approximately 3.1, 2.6, and 2.5 mm, respectively. This indicates that the resolving powers of these three cases were very similar. It should be noted that a few speckles were observed in the airy patterns, as shown in Fig. 15 (a, b, c)-3; these were attributable to the scattering of contaminant particles incorporated during the replication procedures. This can be explained by the fact that the fabricated microlenses were not perfectly circular. On the other hand, the
spacing between the airy discs of the microlenses increased from 3.8 to 4.0 and 4.4 mm as the aperture of the films was increased from 5 mm to 8 mm and 10 mm, respectively. 2D, 3D, and horizontal profiles of the light intensity distributions for each aperture size of curved microlens array film are shown in Fig. 16(a, b, c)-1,2,3, respectively. The isochart of Fig. 16(a, b, c)-1 is useful for determining how much area a light fixture can cover. From Fig. 16(a, b, c)-1, we found that the light intensity distribution peaked at the center area, but decreased gradually moving outwards on the 2D diagram. Basically, the light intensity distribution shape is nearly symmetrical. Smaller aperture size of curved microlens array film, smaller the light intensity distribution area. Moreover, from the 3D and horizontal profiles in Fig. 16(a, b, c)-2 and 3, respectively, we found that the light intensity distribution decreased rapidly in the smaller aperture size case (5 mm). The defocused phenomena usually was seen to occur as the light passed through the curved microlens array film at a distance far from the
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center point, especially in the smaller aperture size case. Therefore, an obvious sharp peak could be observed in Fig. 16(a)-2 and 3, respectively. Many noises were occurred on the light intensity distribution curves, since the light interfered with the microlens of films. The resolutions of the curved microlens array films in this study could be further improved by increasing the diameter of ommatidia. However, the diffraction-limited resolution is inversely related to the diameter of ommatidia [23]. To improve the resolution of the curved microlens array film, it is necessary to increase the film's curvature. In addition, we could use Poly 2-hydroxyethyl methacrylate (pHEMA), silicone methacrylate (SMA) or modified silk fibroin to replace the PMMA for the microlens array film to increase biocompatibility with the human body. Furthermore, different imaging amplification effects can be obtained if we rotate two identical curved microlens arrays in different directions and patterns [24]. This bioangles, according to the principle of Moire logically inspired mimetic visual feature can be extended to applications such as medical diagnostic systems, light-field photography devices, and endoscopy, amongst others.
[2] [3] [4] [5] [6] [7] [8]
[9]
[10]
[11]
[12]
4. Conclusions [13]
In this study, monodispersed microsized polystyrene spheres were synthesized by dispersion polymerization. A close-packed monolayer array of PS microspheres was assembled by the Langmuir-Blodgett (LB) deposition technology, with the surface pressure exerted by the moving barriers fixed at 4.7 mN/m and the drawing rate of the glass substrate kept at 1 mm/min. Different aperture sizes, but the same height of curved microlens array replicating films could be prepared facilely by a traditional Chinese medicine cupping device with a hot plate without metal modeling. The curved microlens film with the smallest aperture size exhibited the broadest field of view, as the heights of all of the films were equal. In addition, the resolution of the curved microlens films was not affected by their aperture sizes, but was determined by the interommatidial angle and the diameter of the microlens.
[14]
[15]
[16]
[17]
[18]
[19]
Notes [20]
The authors declare no competing financial interest. [21]
Acknowledgements The authors gratefully acknowledge financial support from the Ministry of Science and Technology of Taiwan (MOST 104-2221-E150-038 & 104-2221-E-150-064-MY3). References [1] K.
Najafi,
Micropackaging
technologies
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