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Measuring the reflectance of hidden color objects with acoustically modulated laser speckle

The colour of an object can be easily determined by shining light on it and measuring the reflected light intensity. However, when the object is hidden behind an opaque barrier, the reflectance measurement becomes more difficult. We have pioneered a technique to measure the reflectance of an object hidden behind an opaque barrier by vibrating the object slightly and recording the time-varying laser speckle patterns on the barrier, encoded with information about the hidden reflectance. In the future, this technique can be translated into a clinical monitoring technique of blood oxygenation in the brain hidden behind the scalp and skull.