In comparison to traditional planar optical devices, compound-eye structured optical elements can reduce the number of components and their volume when being applied to wide-field imaging and sensing systems. However, the fabrication process for microstructures on a curvilinear surface has many difficulties since traditional fabrication techniques are planar. In this paper, we present a cost-effective method to fabricate microlenses on a spherical surface. Microlenses, of which the fill factor was about 78%, were formed using the thermal reflow technique, followed by multiple replication processes to transfer the microlenses from the planar substrate onto a spherical surface. We produced a curved mold with concave microlenses, and it allowed this method to be replicable. A polydimethylsiloxane elastomer was employed as the material for both the microlenses and the mold. The radius of curvature of the spherical surface was approximately 6.1 mm. The variation of the microlenses was analyzed, demonstrating high uniformity. The imaging performance of the microlenses is also presented. The curved microlens arrays were combined with image sensors, and sub-images of objects at various distances are shown. The experimental results demonstrate a high potential for curved microlens arrays being applied to compact mobile camera lenses.
Artificial compound-eye structure has been studied recently due to its great applications of wide field-of-view imaging and backlight modules. However, fabrication process for microstructures on curvilinear surface has a lot of difficulties since traditional fabrication techniques are planar. In this paper, a simple and low-cost method to fabricate microlenses on spherical surface was demonstrated. Microlenses with high fill factor were formed by thermal reflow technique, followed by multiple replication processes to transfer the microlenses from planar substrate onto spherical surface. During the process, we made a curved mold with concave microlenses, which allowed this method to be duplicable easily. Polydimethylsiloxane (PDMS) elastomer was employed as the material of both microlenses and mold due to its flexibility and transparency for visible light. To prevent microlenses from being damaged during the release procedure, surface treatment using trichloro(1H,1H,2H,2H-perfluorooctyl)silane was applied before every replication process. Several PDMS domes covered with hexagonal or square microlenses on the surface were fabricated successfully. The diameter of each microlens was about 200 μm and the pitch of array was 220 μm. The radius of curvature of the spherical surface was about 6.1 mm. The uniformity of microlenses was analyzed through the intensity distribution of focused spots. Imaging performance of microlenses was shown. The curved microlens arrays were combined with image sensor, and clear images of objects at different distance are shown. The experimental results showed a high potential for curved microlens arrays being applied to compact mobile camera lens.
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