Measurement of Diffraction Pattern Using Two-Dimensional Image Sensor for Obtaining Instantaneous Diameter Distribution of Acoustic Cavitation Bubbles

Abstract

To shorten the measurement time for the diameter distribution of acoustic cavitation by laser diffraction, a new measurement method for laser diffraction patterns using an image sensor was proposed. The laser diffraction pattern caused by the oscillating acoustic cavitation was measured using the charge-accumulation-type image sensor and the acousto optic modulator. The diameter distribution was calculated from the diffraction pattern by solving the least squares problem that minimizes the sum of squares of the difference between the theoretical and the measured diffraction patterns. The proposed method using the image sensor enables the measurement of diffraction patterns at 18 phases of ultrasound within 15 s, which corresponds to one-sixth of the measurement time of the present method using a scanning photodetector. The diffraction pattern at each phase was measured in 22 ms and the measured diffraction pattern corresponded to the instantaneous diffraction pattern ensemble-averaged 421 times. In addition, it was shown that the proposed method could reduce the effect of the speckle noise by the spatial averaging of the diffraction pattern. The residual of the least squares method was reduced from the previous method. As a result, it was concluded that the time required for measuring diffraction patterns could be reduced and the possibility of the instantaneous measurement of diameter distribution was suggested.