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— Presentation at ISOM'14 —
December 1, 2014
ISOM'14 (International Symposium on Optical Memory) was held in Hsinchu, Taiwan during October 20-23 2014. This international conference is concerned about optical memory. The participants are mainly from Asia. This year, 173 researchers attended this conference.
Representatives of Yokohama Research Laboratory were 4 members. This writer made a presentation entitled "Rotationally Symmetric Wavefront Coding for Extended Depth of Focus with Annular Phase Mask."
Recently, approaches to extended depth of focus (EDOF) for imaging systems have been actively studied. Especially Wavefront Coding (WFC) is notable as a simple set up technology. A phase mask inserted in an optical system blurs an image almost independently of defocus. The uniform blurs of the image is removed by digital image processing to generate an image of wide depth of focus. Fig. 1 (1) shows an example of WFC with conventional phase mask. This system uses a cubic phase mask (CPM). It needs a square aperture , therefore it losses light power. And its asymmetric shape causes several disadvantages such as remarkable artifacts in images, necessity of rotational adjustment in assembling, and lateral image displacement. To overcome these disadvantages, several rotationally symmetric phase masks were proposed.
About realizing WFC with rotationally symmetric phase mask, we examined the design of a phase mask composed of plural annuluses as shown in Fig. 1 (2), intending a phase mask with simpler form than former proposes for manufacturability.
We intended to make realize a symmetric phase mask composed of plural annuluses as shown in Fig. 1 (2). As interferences of lights from each annulus occur, making uniform blurs was difficult. But we discovered that the interferences can be removed by setting phase steps between annuluses. As a result we could design an annular phase mask (APM) which has same EDOF effect as CPM. Fig. 2 shows the APM we fabricated.
Images in Fig. 3 show EDOF effect. In normal system, a QR code of 100㎜ is on focus, but a finger of 60㎜ and a spork pattern of 1.1m are both blurred. On the other hand in WFC system with APM and CPM, there is no blur. Besides, there are fewer artifacts around a finger image in APM than CPM.
Fig. 4 shows a result of an experiment of sensor tilt tolerance. We took photos of a test pattern with 3 degrees of sensor tilt. In normal system, the outer pattern is blurred. But in APM and CPM there is no blur. Furthermore fewer artifacts are observed in APM than CPM.
Thus we designed a new type of a phase mask which overcomes several disadvantages of CPM, and confirmed its effect. In Yokohama Research Laboratory, we will continue the research of WFC technique toward applying to productions.
(By OHTA Mitsuhiko)