4 March 2013 Seeing hidden colors with acoustically modulated laser speckle sensing
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Abstract
A technique based on acoustically modulated laser speckle has been demonstrated which can quantify and classify 25 colored papers, even when they are hidden 5 mm behind an opaque slab barrier with a thickness of 5 mm and a reduced scattering coefficient of 1.8 mm-1. A small vibration at 200 Hz was induced on the colored paper by attaching it to the central diaphragm of a loudspeaker. Two He-Ne lasers (green at 543 nm and red at 633 nm) illuminated the slab surface sequentially. Although the slab blocked most of the incoming light, a small proportion of light penetrated through, interacted with the vibrating colored paper and backscattered, causing a time-varying speckle pattern on the slab surface. A consumer grade digital camera was used to capture the speckle pattern from which the speckle contrast difference was calculated and shown to be indicative of the color of the hidden object. Using the speckle contrast difference measured at 543 nm and 633 nm, the nearest neighbor classification algorithm was employed to classify the 25 hidden colors (formed by different percentages of base colors magenta and cyan), achieving an accuracy of 72%. This work has demonstrated that the acoustically modulated laser speckle technique can increase the sensitivity of spectroscopic measurements in a deeper region, which has the potential to be translated into clinical applications such as cerebral oxygenation measurement in which a superficial layer (skull) is present.
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Terence S. Leung, Shihong Jiang, "Seeing hidden colors with acoustically modulated laser speckle sensing ", Proc. SPIE 8581, Photons Plus Ultrasound: Imaging and Sensing 2013, 85813E (4 March 2013); doi: 10.1117/12.2004620; https://doi.org/10.1117/12.2004620
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