Measuring the reflectance of hidden color objects with acoustically modulated laser speckle
Optics Letters (Impact Factor: 3.29). 10/2012; 37(19):4092-4. DOI: 10.1364/OL.37.004092
We demonstrate a technique to measure the reflectance of colored objects, including blue, green, and yellow, hidden behind an opaque slab based on acoustically modulated laser speckle. One colored paper at a time was placed behind a 1 cm thick opaque slab with an air gap of 5 mm. Small periodic movements (nanometer scale) at 200 Hz were induced in the colored paper. A coherent red He-Ne laser illuminated the front of the slab, producing acoustically modulated speckle patterns, which were captured by a CCD camera. The magnitude of the time-varying speckle intensity is indicative of the hidden colored paper's reflectance.
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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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ABSTRACT: The acoustically modulated laser speckle contrast technique has been employed to quantify and classify 25 colors (made up by different percentages of the two base colors cyan and magenta) hidden behind a 5 mm thick opaque layer with 0.24% transmittance. The main components included two He-Ne lasers (543 and 633 nm), a consumer grade digital camera (Nikon 1 J1), focusing optics and a loudspeaker. The camera captured the laser speckle patterns with the sound on and off, respectively, from which the speckle contrast differences were calculated and used in a nearest neighbor classification algorithm. The classification accuracy was between 55% and 88% depending on the underlying reflectance of all the colors to be classified.
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