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<p>This study presents the development of a microlens array (MLA) for floating image applications, addressing challenges in both optical design and mold manufacturing. Passive optical floating image systems that operate without external power sources employ various approaches, such as retro-reflectors and dihedral corner reflector arrays. Among these, the MLA offers the distinct advantage of higher image brightness. However, research on floating image devices employing MLA remains limited. This is likely due to constraints on resolution and viewing angle in the MLA method, as well as significant tool wear encountered during mold machining. To overcome these challenges, the authors conducted a study on optical design and mold manufacturing for MLAs. In the optical design phase, a custom macro program in CODE V was used to optimize the MLA geometry for simultaneous improvement of resolution and viewing angle. Floating images formed by the optimized MLA were subsequently simulated using LightTools. The resolution of floating images was evaluated by spot diagrams, and the illuminance of the simulated floating images observed at various angles was also quantitatively assessed. In the mold manufacturing phase, cutting experiments were conducted on a 6.5-inch diagonal electroless Ni-P workpiece using an ultraprecision 5-axis machine tool ROBONANO UiA manufactured by FANUC Corporation. Two types of single-crystal diamond tools—a normal tool and a newly developed wear-resistant tool—were applied. After the experiments, the form error and surface roughness of the molds were evaluated using a laser-probe 3D measurement system (NH-3SP manufactured by Mitaka Kohki Co., Ltd). To summarize the results, the optical design achieved a viewing angle of 45° and an RMS spot diagram radius of 0.41&#xa0;mm, effectively suppressing ghost rays. In addition, the developed MLA produced a brighter primary image than a conventional MLA, maintaining 78–90% of the front-view illuminance when observed from an angle of 17.5°, and 11–24% even when the observation angle was increased to 22.5°. In the mold cutting experiments, the wear-resistant tool maintained stable form accuracy and surface roughness Ra below 6&#xa0;nm even after machining 68,000 microlenses. However, the initial form error varied from 0.42&#xa0;μm to 1.03&#xa0;μm. A contributing factor to this variation was the mismatch between the nose radius of the tool and the design value of the lens curvature radius.</p>

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Development of optical design and mold cutting technologies for microlens arrays for floating images

  • 期刊论文

  • 发表日期：

    2026年1月29日

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  • Hiroshi Saito,
  • Tsuneyuki Kobayashi,
  • Yugo Sasaki,
  • Satomu Kanazawa,
  • Takako Matsumura,
  • Kazumi Sawamura,
  • Hirotsugu Yamamoto

摘要

This study presents the development of a microlens array (MLA) for floating image applications, addressing challenges in both optical design and mold manufacturing. Passive optical floating image systems that operate without external power sources employ various approaches, such as retro-reflectors and dihedral corner reflector arrays. Among these, the MLA offers the distinct advantage of higher image brightness. However, research on floating image devices employing MLA remains limited. This is likely due to constraints on resolution and viewing angle in the MLA method, as well as significant tool wear encountered during mold machining. To overcome these challenges, the authors conducted a study on optical design and mold manufacturing for MLAs. In the optical design phase, a custom macro program in CODE V was used to optimize the MLA geometry for simultaneous improvement of resolution and viewing angle. Floating images formed by the optimized MLA were subsequently simulated using LightTools. The resolution of floating images was evaluated by spot diagrams, and the illuminance of the simulated floating images observed at various angles was also quantitatively assessed. In the mold manufacturing phase, cutting experiments were conducted on a 6.5-inch diagonal electroless Ni-P workpiece using an ultraprecision 5-axis machine tool ROBONANO UiA manufactured by FANUC Corporation. Two types of single-crystal diamond tools—a normal tool and a newly developed wear-resistant tool—were applied. After the experiments, the form error and surface roughness of the molds were evaluated using a laser-probe 3D measurement system (NH-3SP manufactured by Mitaka Kohki Co., Ltd). To summarize the results, the optical design achieved a viewing angle of 45° and an RMS spot diagram radius of 0.41 mm, effectively suppressing ghost rays. In addition, the developed MLA produced a brighter primary image than a conventional MLA, maintaining 78–90% of the front-view illuminance when observed from an angle of 17.5°, and 11–24% even when the observation angle was increased to 22.5°. In the mold cutting experiments, the wear-resistant tool maintained stable form accuracy and surface roughness Ra below 6 nm even after machining 68,000 microlenses. However, the initial form error varied from 0.42 μm to 1.03 μm. A contributing factor to this variation was the mismatch between the nose radius of the tool and the design value of the lens curvature radius.