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Speeded-Up Focus Control of Electrically Tunable Lens by Sparse Optimization

Electrically tunable lenses (ETL), also known as liquid lenses, can be focused at various distances by changing the electric signal applied on the lens. ETLs require no mechanical structures, and therefore, provide a more compact and inexpensive focus control than conventional computerized translation stages. They have been exploited in a wide range of imaging and display systems and enabled novel applications for the last several years. However, the optical fluid in the ETL is rippled after the actuation, which physically limits the response time and significantly hampers the applicability range. To alleviate this problem, we apply a sparse optimization framework that optimizes the temporal pattern of the electrical signal input to the ETL. In verification experiments, the proposed method accelerated the convergence of the focal length to the target patterns. In particular, it converged the optical power to the target at twice the speed of the simply determined input signal, and increased the quality of the captured image during multi-focal imaging.


Focus control results in multi-focal imaging. (a) Experimental setup. (b) Input electric currents. (c) Photodiode outputs representing the resulting optical powers and the target pattern, respectively (blue: na\"{i}ve, red: proposed, black: target). V_{d100}, V_{d200}, and V_{d1500} represent the photodiode output values when the ETL focused at the targets, respectively. (d) Images captured per period in the naive method (left) and their PSNR values (right), computed by comparing the captured images with those focused at the targets in (a). (e) As for (d), but for the proposed method. The green, purple, and orange plots correspond to the near, middle, and far targets, respectively.

Publications

  • Daisuke Iwai*, Hidetoshi Izawa*, Kenji Kashima*, Tatsuyuki Ueda, and Kosuke Sato, "Speeded-Up Focus Control of Electrically Tunable Lens by Sparse Optimization," Scientific Reports, Vol. 9, Article No. 12365, 2019. (*equal contribution) [Open access]