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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Photoacoustic Doppler effect ： ウィキペディア英語版 Photoacoustic Doppler effect The photoacoustic Doppler effect, as its name implies, is one specific kind of Doppler effect, which occurs when an intensity modulated light wave induces a photoacoustic wave on moving particles with a specific frequency. The observed frequency shift is a good indicator of the velocity of the illuminated moving particles. A potential biomedical application is measuring blood flow. Specifically, when an intensity modulated light wave is exerted on a localized medium, the resulting heat can induce an alternating and localized pressure change. This periodic pressure change generates an acoustic wave with a specific frequency. Among various factors that determine this frequency, the velocity of the heated area and thus the moving particles in this area can induce a frequency shift proportional to the relative motion. Thus, from the perspective of an observer, the observed frequency shift can be used to derive the velocity of illuminated moving particles. ==Theory== To be simple, consider a clear medium firstly. The medium contains small optical absorbers moving with velocity vector $\backslash vec$. The absorbers are irradiated by a laser with intensity modulated at frequency $f\_$. Thus, the intensity of the laser could be described by: $I=\_\; \backslash left(1+cos\; \backslash left\; (\; 2\; \backslash pi\; f\_t\; \backslash right\; )\; \backslash right)\; /2$〔H. Fang, K. Maslov, L.V. Wang. "Photoacoustic Doppler Effect from Flowing Small Light-Absorbing Particles." Physical Review Letters 99, 184501 (2007)〕 When $\backslash vec$ is zero, an acoustic wave with the same frequency $f\_$ as the light intensity wave is induced. Otherwise, there is a frequency shift in the induced acoustic wave. The magnitude of the frequency shift depends on the relative velocity $\backslash vec$, the angle $\backslash alpha$ between the velocity and the photon density wave propagation direction, and the angle $\backslash theta$ between the velocity and the ultrasonic wave propagation direction. The frequency shift is given by: $f\_=-f\_\; \backslash frac\; \backslash frac$ is the speed of light in the medium and $c\_$ is the speed of sound. The first term on the right side of the expression represents the frequency shift in the photon density wave observed by the absorber acting as a moving receiver. The second term represents the frequency shift in the photoacoustic wave due to the motion of the absorbers observed by the ultrasonic transducer.〔 In practice, since $\backslash frac\}\backslash sim\; 10^$ and $v\; \backslash ll\; c\_$, only the second term is detectable. Therefore, the above equation reduces to: $f\_=\; f\_\; \backslash frac\; cos\; \backslash theta$〔〔H. Fang, K. Maslov, L.V. Wang. "Photoacoustic Doppler flow measurement in optically scattering media." Applied Physics Letters 91 (2007) 264103〕 In this approximation, the frequency shift is not affected by the direction of the optical radiation. It is only affected by the magnitude of velocity and the angle between the velocity and the acoustic wave propagation direction.〔 This equation also holds for a scattering medium. In this case, the photon density wave becomes diffusive due to light scattering. Although the diffusive photon density wave has a slower phase velocity than the speed of light, its wavelength is still much longer than the acoustic wave.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Photoacoustic Doppler effect」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.