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Abstract
The compound eyes of natural insects endowed with the merits of a wide field of view (FOV), high sensitivity, and detection of moving targets, have aroused extensive concern. In this work, a large-scale artificial compound eye is fabricated by a high-efficiency and low-cost strategy that involves the combination of the thermal reflow method and pressure deformation. About 30,000 ommatidia are evenly distributed on the surface of a hemisphere with an ultralow surface roughness and a large numerical aperture (NA) of 0.66. Moreover, the FOV of the artificial compound eye investigated is about 120°. The collaboration of the compound eye and CMOS sensor makes the ommatidia capturing multiple images of human organs enabled. This micro-based imaging system has considerable potential in integrated pinhole cameras, medical endoscopes, and drone navigation.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In nature, the compound eyes of arthropods are distinguished from mammalian eyes, which endows exceptional merits such as a large field of view (FOV) compared to planar microlens array(MLA), near-infinite depth of field, high sensitivity, and fast motion detection [1–11]. With the rapid development of optical microelectromechanical systems (MEMS) and standard complementary metal oxide semiconductor (CMOS) fabrication technologies [12–15], the fabrication techniques of compound eyes have experienced changes from waveguide self-writing initially to advanced femtosecond laser method through thermal reflow and self-assembly [4,16–26]. In recent years, there are also some innovative approaches springing up such as inkjet printing, inductively coupled plasma etching, etc [3–5,27,28]. A well-functioning compound eye has a wide range of applications including automatic spatial navigation, medical endoscope, digital camera which can realize automatic scanning and data acquisition system [29–33].
Recently, researchers have conducted more in-depth studies on the latest developments in artificial compound eyes. Cao et al. successfully fabricated dynamically tunable compound eyes [4]. A microfluidic device was used to be injected different volumes of water into it to change the focal length from 3.3 mm to infinity and FOV up to 180° of the compound eye. It is difficult to combine this device with a CMOS camera to fabricated a miniature imaging system and some reflection and refraction of light were introduced between water and polydimethylsiloxane (PDMS) two media, which will affect the imaging quality. Deng et al. created MLA on a curved polymethylmethacrylate (PMMA) spherical shell surface using a femtosecond-laser assisted with wet etching process and thermal embossing [2]. Although the femtosecond laser can etch structures within tens of microns, it requires expensive equipment and the shape and contour of the grooves require precisely control and adjustment of laser parameters and other factors. Moreover, it needs to scan point by point, which is time-consuming and laborious. The PMMA spherical shell surface is easily damaged and not suitable for use under extreme circumstances. He et al. produced the sandwich structural compound eye through thermal reflow method, which successfully integrated with a CMOS camera for imaging [34]. However, the process was complicated with multiple preparation steps required resulting in low efficiency and the size of the compound eye was relatively large compared with compound eyes in nature.
In this study, we fabricate a large-scale artificial compound eye by a high-efficiency and low-cost strategy that involves the combination of thermal reflow method and pressure deformation. The combination of the master mold and the auxiliary mold does not require repeated thermal reflow and pressing steps, thereby improving the production efficiency. Systematic characterizations of the three-dimensional (3D) morphology of the structural membrane obtained by multiple replications are investigated, which manifest high-quality MLA with good size and shape uniformly. The total number of ommatidia reaches 30000 with an average diameter of 40 µm, which is similar to natural compound eyes and smaller about one order of magnitude than that of previously reported eyes fabricated by thermal reflow method. Furthermore, the fabricated compound eyes have an ultralow surface roughness and a large numerical aperture (NA). The excellent optical performance was also verified, including a large FOV, optical focusing imaging and resolution detection. More importantly, the artificial compound eye integrated with a commercial CMOS camera form a miniature imaging system, which can clearly observe the human’s organs.
2. Experimental methods
The fabrication process is illustrated as shown in Fig. 1. A positive photoresist (AZ4620 PR) was spin-coated on a silicon substrate as shown in Fig. 1(a). After exposure and development, a micropillar array was fabricated through lithography and then it was heated at 160°C for 10 minutes to obtain a spherical crown structure under the effect of surface tension. The PDMS mixture was spin-coated on the photoresist array and then cured at 80°C for 1 hour to obtain PDMS concave MLA.
Fig. 1. (a) The spherical crown photoresist cylinders array. (b) The preparation of PDMS concave MLA and fabrication of convex PMMA MLA.(c) PMMA MLA was pressed into a spherical shell as the master mold. (d) The formation of auxiliary mold and finally curved PDMS MLA was obtained.
Next, PMMA solid powder was dissolved in organic solvents composed of toluene and chloroform in the weight ratio of 1:1 to obtain 30 wt% PMMA solution. Then it was poured on the PDMS membrane and then cured at room temperature for 24 hours as shown in Fig. 1(b). The next step was the fabrication of curved PMMA spherical shell mold. Figure 1(c) displayed a steel ball with a radius of curvature (ROC) of 9.5 mm heated to 105°C higher than the glass transition temperature of PMMA (95°C) to press the planar-convex PMMA MLA for 10 s and not release it until cooling down to room temperature. In Fig. 1(d), a glass slide attached to PMMA spherical shell was placed upside down in a petri dish without a lid filled with PDMS. then the whole device was put at room temperature and cured after 24 hours. The structure of convex ommatidia was replicated into PDMS and a curved concave MLA was obtained after peeling off the glass slide carefully. The PMMA spherical shell was master mold and the curved concave MLA was auxiliary mold, which both can be utilized repeatedly many times. Before padding PDMS into the auxiliary mold, a hydrophobic and buffer layer of Parylene-C was deposited on it by chemical vapor deposition (CVD). Good uniformity and conformality of the deposited films have been demonstrated in Fig. S1 (Supplementary document). Finally, a curved compound eye was completed after the PDMS mixture curing.
PMMA is a kind of hard plexiglass and its rigidity is relatively large which can remain primary profile at room temperature. Although the prepared PMMA spherical shell can also be used for imaging which has also been introduced in previous literatures [2,21,35,36], the shape of the thin spherical shell is more prone to be damaged due to the lack of support at the bottom and it is not suitable for application in high temperature environment (higher than 95°C) because PMMA is easy to be deformed and not appropriately integrated with planar CMOS cameras. However, the PMMA spherical shell can be used as a replication mold which can make mass production of curved PDMS artificial compound eyes repeatedly enable. PDMS is a flexible and transparent organic material with biocompatibility and chemical stability. It has good cold and heat resistance and can be used for a long time at the range from −50°C to 200°C.
3. Results and discussions
3.1. Characterization of morphologies of the compound eye
The melted convex photoresist was characterized by a laser scanning confocal microscope (LSCM 800) shown in Fig. 2(a). We can see that the morphologies were in good size and uniform shape. The cross-section profile was extracted from the blue line in the LSCM image. The diameter and height of the convex MLAs were homogeneous, about 40 µm and 13 µm respectively. The planar PDMS MLA was prepared through the replication process. The concave PDMS MLA was also characterized in Fig. 2(b). The diameter and height were almost consistent with the morphology of melted photoresist. A convex PMMA MLA with a thickness of about 110 µm was replicated from PDMS MLA and its surface profile was also characterized by the LSM 800 as shown in Fig. 2(c). The uniformity of diameter and height were almost the same as photoresist and concave PDMS saw from the cross-section image in Fig. 2(b), which means the microlens’ structures were completely replicated.
Fig. 2. (a) LSM images of the photoresist spherical crown as well as the cross-section profile. (b) The inverse structure and the cross-section profile of the concave PDMS MLA. (c) LSM 800 images of the planar PMMA convex MLAs and the cross-section profile.
The appearance of ommatidia on curved compound eye was characterized by SEM at the focused top parts and front-left side as shown in Fig. 3(a) and (b), respectively. Figure 3(c) displayed the contour focused at the right rear side of the fabricated PDMS compound eye. The ommatidia were distributed on the top uniformly and the edges were not severely damaged or deformed, which indicated that the PDMS compound eye prepared by this method was of high quality. Each of them can bulge to form a separate convex lens, which was the prerequisite foundation for each ommatidium of the artificial compound eye to achieve imaging.
Fig. 3. SEM images of the structural portions focused at (a) the top and (b) front-left of the artificial compound eye. (c) The SEM image displayed the contour focused at the right rear.
The overall contour of the compound eye was characterized by a depth-of-field microscope (VHX-5000), whose diameter and height measured were approximately 9.3 mm and 3.44 mm respectively as illustrated in Fig. 4. It is proved that PMMA can be pressed to form a convex structure with the surface shape remaining intact. Therefore, the number of ommatidia is figured out by the equation [35]:
where Shemi is the surface area of the fabricated PDMS with the shape of a hemisphere, Smicro is the area of a single ommatidium, so N is calculated about 30000. The lens profile of each ommatidium can be approximated as a parabola and the focal length of this lens can be calculated as [37] : where d and h are the diameter and sag height of the ommatidium, respectively and n is refractive of PDMS. According to the cross-section profile numerical data, d = 40 µm, h=13 µm, and n=1.41, f is calculated to be 37.52 µm. Furthermore, the NA of ommatidium is obtained by the equation [38]: where θ and r are the aperture angle and half of the diameter, respectively. The value is about 0.66, which can reduce crosstalks and obtain a larger depth of field [28]. To obtain a large NA, the height of the ommatidium can be increased appropriately.Fig. 4. The morphology of the whole curved compound eye captured by depth-of-field microscope (VHX-5000) and the height profile of MLAs.
The roughness of the PDMS ommatidium at the area of 2 µm × 2 µm characterized was 3.25 nm by AFM as illustrated in Fig. 5. The lower the roughness of PDMS proves the higher the imaging quality of the ommatidia. Furthermore, a standard resolution test card (USAF 1951) to define the resolution of the artificial compound eye in different locations and found that Fig. 0 to 6 in the fourth column can basically be seen while the fifth column was very blurry as shown in Fig. S2. After calculation and comparison, the obtained resolution is about 28.5 lp/mm.
3.2 Optical performance
To demonstrate the imaging performance, an optical microscope imaging system composed of a white light source, object stage, objective lens (OL), and charge-coupled device (CCD) camera was established as shown in Fig. 6(a). A transparent mask printed with “SJTU” was placed between the light source and the compound eye. The imaging focal spots of the small lens were found by adjusting the distance between the OL and the compound eye, and then SJTU images were obtained by continually adjusting the distance between the mask and the compound eye.
Fig. 6. Optical microscope imaging test. (a) The schematic diagram of the principle of imaging by microscope. (b) The image “SJTU” of the compound eyes focused on the outer circle and (c) is focused on the middle region. (d) and (e) are higher magnification images of the left zone and right zone in (d) respectively. (f) A clear and magnified image of the compound eye in the middle region.
Figure 6(b) showed the focused images of “SJTU” at the outer circle while the images in the middle region were blurred. Since it was a curved compound eye, when the OL moved upward, the focus position of the light spot will gradually move to the central region as illustrated in (c). Figure 6(d) and (e) were magnified images of the left zone and the right zone where half blurred and half clear, respectively. It is noted that every ommatidium of the compound eye has the ability to form clear images owing to the high quality and uniformity of each unit, which demonstrates the compound eye has good optical performance. Figure 6(f) shows a clear and magnified image of the compound eye focused in the middle region.
3.3 FOV measurement
In addition to optical imaging performance, another important factor for evaluating the quality of compound eye is the achieved FOV. The optical test platform for measuring FOV as shown in Fig. 7(a) includes a He-Ne laser with a wavelength of 632.8 nm, beam expander, a rotating platform, and sliding stages, OL, CCD camera, and a computer screen for receiving images. The light intensity distribution was analyzed as shown in Fig. 7(b), which demonstrates that the formed light spots have good uniformity. The acceptance angle of each ommatidium is defined as the full width at half maximum (FWHM) of the angular sensitivity function (ASF), The measured ASF of the imaging units in the compound eye was fitted with a Gaussian distribution to obtain the acceptance angle of the small eye of approximately 6.5° as shown in the Fig. 7(c). The angle (ΔΦ) between ommatidia can be calculated by the equation [4]:
where r is the radius of the ommatidium, and R is the radius of the hemisphere. The calculated result is that 0.48° much smaller than 6.5°, indicating that large NA has the effect of attenuating the crosstalks between adjacent optical paths.Fig. 7. (a) Schematic diagram of the experimental optical system for measuring the FOV of the fabricated compound eye. (b) The 3D normalized intensity distribution analyzed by Matlab software. (c) The measured ASF of the imaging units in the compound eye.
The focused light spots in the center of the compound eye were captured as illustrated in Fig. 8(a). Figure 8(b) and (c) show the spot information varied with different angles by rotating the platform and the corresponding degrees were recorded as 30° and 60°, respectively. The ultimate experimental result of the FOV measured was 120°.
Fig. 8. (a) The focal spot images at the center of the fabricated curved MLAs were illuminated by a He-Ne laser at the wavelength of 632.8 nm. (b) and (c) are the light spots obtained by rotating different angles captured by the CCD camera.
Table 1 lists the comparison of characteristics parameters of compound eyes fabricated by the thermal reflow process in recent years. It is seen that the diameter of the ommatidium is an order of magnitude smaller than other previous works and the roughness is relatively low, and also the FOV test system is established and was measured about 120°. The high NA was calculated as 0.66, which can reduce crosstalks to some extent and obtain a larger depth of field. It can be seen that the low-cost, low-consumption and rapid-production thermal reflow method can fabricate compound eyes that are close to the size and number of dragonfly in nature.
Table 1. Comparison of curved compound eyes based on thermal reflow technology (“-” means unknown)
3.4 Micro-based imaging system
The fabricated artificial compound eyes can be integrated with a commercial CMOS camera (Sony IMX 322) to realize the imaging of each ommatidium. The maximum pixel of the camera is 2 million and the maximum resolution can reach 1920×1080, which can be used for high-definition shooting and face recognition. The transmittance of this CMOS camera is above 88% at the wavelength range from 440 nm to 620 nm. Figure 9(a) shows the micro-based imaging system composed of compound eyes, an adjustable sleeve, and an optical sensor. The outer frame size of the whole device is 38 mm × 38 mm in length and width, respectively and a detailed assembly diagram and optical layout was given in Fig. S3. The focal length between the compound eyes and camera can be adjusted by rotating the sleeve for the purpose of obtaining legible sub-images. The images of the human’s eyes, mouth, teeth, and nose were captured by the self-contained software of the micro-based imaging system called AMcap as shown in Fig. 9(b) to (d). (The dynamic process captured in videos 1 and 2 in supplementary document.) The internal pictures are a magnification of the ommatidium imaging, which can clearly distinguish organs of the human body. It is expected to be used for short-distance forensics with pinhole cameras, and form curved light field camera with 3D light field reconstruction algorithm to achieve large field of view and super depth of field for 3D target detection in future [31,45–47]. Maybe two reasons are resulting in the image blurring at the edge of the pictures. One is the unmatched focal length resulting in blurred sub-images in the surrounding. Non-uniform compound eye structures with different focal lengths can be used to improve the edge imaging quality. Another reason is the optical crosstalks between the ommatidia. When the light irradiates on an object, it may be received by several adjacent light channels at the same time causing a blurred image. The phenomenon can be reduced by making an optical path isolation layer between adjacent light channels. The small macro imaging system achieves the perfect combination of artificial bionic compound eyes and commercial cameras. The clear images at close range, which provides great potential for applications in pinhole cameras, medical endoscopes, and drone navigation.
Fig. 9. (a) The micro-based imaging system composes of compound eyes, an adjustable sleeve, and a CMOS camera. (b) The sub-images of human’s eyes, (c) nose, (d) teeth and mouth captured by the optical sensor.
4. Conclusions
In conclusion, a small imaging system has been successfully developed by integrating compound eyes with a commercial CCD camera. The curved PDMS MLA was fabricated through a combined technology of thermal reflow method and microscale replication, as well as pressure deformation process. The combination of PMMA spherical shell (master mold) and PDMS curved concave MLA (auxiliary mold) permits mass-production of high-quality MLA with good size and shape uniformly. The as-prepared PDMS MLA with ∼30000 ommatidia with diameter of 40 µm, which is similar to natural compound eyes and smaller about an order of other compound eyes fabricated by thermal reflow method. Furthermore, the fabricated compound eyes have an ultralow surface roughness of 3.25 nm and a large NA of 0.66. The collaboration of compound eyes and commercial CMOS camera endows the ommatidia with the merits of capturing multiple images of human organs, which will be promising for achieving large field of view and super depth of field through integrating 3D light field reconstruction algorithm to for targets detection in future.
Funding
Joint Foundation of Pre-research of Equipment and Ministry of Education (6141A02022637).
Acknowledgments
The authors grateful to the Center for Advanced Electronic Materials and Devices (AEMD). This work was supported by Joint Foundation of Pre-research of Equipment and Ministry of Education (6141A02022637).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that support the findings of this study are available from the corresponding authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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