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Close-up Technology for Lensless Light-field Imaging

— Presentation at International Symposium on Imaging, Sensing, and Optical Memory 2017 —

November 29, 2017

Report from Presenter

Photo 1 Presentation scene

International Symposium on Imaging, Sensing, and Optical Memory 2017 (ISOM'17) was held at Matsue Kunibiki Messe in Shimane prefecture from October 22nd to 25th. There were 41 oral presentations and 39 poster presentations given at the symposium covering a wide variety of topics such as optical memory, imaging, and sensing. Hitachi, Ltd. gave 5 oral presentations and poster presentations and I gave a presentation on close-up technology for lensless light-field imaging as well as the demonstration of close-up imaging (Photo 1).

Lensless light-field imaging technology enables to capture an image using a film on which a concentric-circle pattern (Fresnel zone aperture, FZA) is printed instead of traditional lenses (Figure 1). The technology can make a visual sensing device thinner and lighter. Furthermore, the ability to capture light-field information is unique to this technology, which enables the focus position to be adjusted even after capturing images. Our team has been developing the close-up technology for lensless light-field imaging to make it possible to capture an image even in restricted space utilizing its size.

Fig. 1 System

Fig. 2 Issues of lensless close-up imaging

The first experiments to find out how the image looked at close range revealed that there was strong close-up noise. The noise appeared over all images captured with the distance between the FZA and the object under 3 mm (Figure 2). We supposed that the noise was derived from the uneven distribution of the intensity detected on the image sensor after focusing on the fact that the noise becomes stronger as the distribution is more uneven.

To solve the problem, we developed noise cancelling technology (fringe scan) based on differential detection which cancels the uneven distribution and extracts the signal by subtraction of sensor images. The technology uses different FZAs whose phase are mutually different by 180 degrees to capture sensor images whose intensity distribution are reversed from each other. After verification by simulation and experiments using a liquid-crystal FZA which can display different FZA patterns, it is confirmed that the technology is effective to remove close-up noise and enables close-up capturing with 2 mm distance (Figure 3). Along with the technology, fly-eye FZA has also been developed which can expand the field of view even when capturing an image at close-range (Figure 4).

Fig. 3 Cancellation of close-up noise

Fig. 4 Expansion of field of view

This time we also gave the demonstration at the poster session where we can directly discuss the technology with participants. As the result, we are honored to receive the Best Paper Award. We will continue to work on improving the image quality and developing further applications to put it to practical use.

(By SAO Mayu)

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