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— Presentation at ISOM 2015 —
November 10, 2015
ISOM 2015 (International Symposium on Optical Memory 2015) was held in Toyama, Japan during October 4-8 2015. This international conference is concerned about optical memory. The participants are mainly from Asia. This year, 117 researchers attended this conference.
ISOM wanted demonstration presentation in a poster session as a new try from the current year. This writer applied for this and made a demonstration presentation entitled "Visible-Infrared Achromatic Imaging by Wavefront Coding with Wide-Angle Automobile Camera".
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. The WFC technique can be applied to various fields, and as it can remove axial chromatic aberration owing to its EDOF effect, it is also expected to be applied to a night vision camera using under the condition of mixed light of visible light and infrared light. We applied the WFC technique to a wide-angle automobile camera and confirmed the achromatic effect of it.
The lens used is the one for automobile cameras of the focal length 0.8mm and more than 180 degrees of field angle. Fig. 1 represents the wavelength characteristics of focal length of the lens. There is about 0.04 mm of change by wavelength from 550nm to 950nm. We defined the depth of focus by an index as PSNR (Peak Signal-to-Noise Ratio), and designed the depth of focus of the WFC camera as approximately three times larger than that of a normal camera as shown in Fig. 2. We fabricated a phase mask based on this design and embedded it into the lens. On this occasion, the built-in IR cut filter of the lens was removed.
Fig. 3 shows experimentally captured images of a scene with the normal camera (a)(b) and WFC camera (c)(d). Here, (a) and (c) were illuminated by visible light, (b) and (d) were illuminated by infrared light. The both cameras were adjusted to focus on infinity for the visible light condition. Fig. 4 shows partially enlarged views of Fig. 3; the letters (a)-(d) correspond to those respectively in Fig. 3. The spoke pattern shown in Fig. 4 is at a distance of 140 mm from the lens. It was in focus in visible light for the normal system (a), but blurred in infrared light (b). In the WFC system, however, there were almost no blurring in both images in visible light (c) and infrared light (d). Moreover, the image of the WFC system had sharper edges than those of the normal system, even in visible light. This is possibly because the visible light includes infrared light components.
Thus we confirmed an achromatic effect of WFC. In Center for Technology Innovation - Information and Telecommunications, more studies are advanced for product application of this WFC technology.
(By OHTA Mitsuhiko)