Vol.2 No.4 2010

9/68

Research paper : Development of a real-time all-in-focus microscope (K. Ohba)−239−Synthesiology - English edition Vol.2 No.4 (2010) sec. that is sufficiently smooth for human observation, synchronized high-speed movement of the focal distance at 30 Hz and the image capture and processing speed of 240 frames/sec. at 30 × 8 are required (Fig. 6). In case of 240 frames of black-and-white 512 × 512 pixels, the pixel rate will be close to 100 MHz. At the same time, the focal distance of camera, the object distance, or the image distance must be moved at 30 Hz.(a) High-speed variable focus mechanismFor Prototype 1 and Prototype 2, the varifocal lens developed by Denso Corporation was used as the high-speed variable focus mechanism[8]. It is driven by a piezo element, and the focal distance changes according to the voltage applied. The structure is simple without any motor. The focal distance is changed by moving the glass diaphragm using the bimorph actuator. By changing the voltage applied to the PZT bimorph, the lens can be changed from a convex to a concave lens. It has been demonstrated that the frequency response is possible up to around 150 Hz without delay in phase. Figure 7 shows the varifocal lens, Fig. 8 shows the structure, and Fig. 9 shows the details of the lens driving mechanism. When no voltage is applied, the lens is a planar glass. The greatest characteristic of this varifocal lens is its high speed. Since the glass diaphragm is directly driven using the piezo element, high-speed movement of the focal distance is possible.(b) High-speed image capture mechanism, high-speed image processing arithmetic circuitFor the high-speed image capture mechanism and the high-speed image processing arithmetic circuit in Prototype 1 of section 3.1, we used the vision chip that included an image capture device, ADC, and a processing system. By using this vision chip, processing was concluded within the vision chip, so high-speed communication was unnecessary. Here, since the volume of image data per unit time and image processing capacity were high, we used the column parallel type vision chip, MAPP2200, from Integrated Vision Products AB of Sweden. The basic configuration was CMOS image sensor with 256 × 256 pixels, 256 ADCs, and 256 parallel processors. As mentioned in the previous chapter, using the image-processing algorithm that allows SIMD (single instruction multiple data)Term 3 processing, the image capture and processing can be done at high speed.(c) Configuration with parallel processingIn 2000, when we started the development, although the image-processing technology at an ordinary frame rate was already in practical use, the image capture and processing at a frame rate one digit higher would not be realized unless a special vision chip was developed. Of course, we were told that development of such vision chip required about a hundred million yen in cost. We visited several companies, but none gave us encouragement. In Prototype 1, we used the general-use vision chip MAPP2200 that was developed in Sweden to implement the parallel processing algorithm. Two-second processing speed was realized, but this was far from the real-time processing at our target frame rate.Fig. 9 Principle of varifocal lens function.Fig. 7 External appearance of varifocal lens.Fig. 8 Configuration of varifocal lens.Fig. 6 System configuration for all-in-focus microscope.High-speed image capture mechanismHigh-speed communicationHigh-speed variable focus mechanismHigh-speed image processing arithmetic circuitAll-in-focus imageDepth imageGlass diaphragm(t 50 µm)Glass diaphragm(φ14 mm, t 50 µm)Transparent fluid16 mmStainless steel plate(φ14 mm, t 50 µm)PZT bimorph actuatorφ 5 mmPZTGlass diaphragmTransparent fluidPZT bimorph actuatorPullPushPipeBimorph cell

※このページを正しく表示するにはFlashPlayer9以上が必要です