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<sup>-1</sup>. 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
In this work, we introduce an optical technique to measure sound. The technique involves pointing a coherent pulsed
laser beam on the surface of the measurement site and capturing the time-varying speckle patterns using a CCD camera.
Sound manifests itself as vibrations on the surface which induce a periodic translation of the speckle pattern over time.
Using a parallel speckle detection scheme, the dynamics of the time-varying speckle patterns can be captured and
processed to produce spectral information of the sound. One potential clinical application is to measure pathological
sounds from the brain as a screening test. We performed experiments to demonstrate the principle of the detection
scheme using head phantoms. The results show that the detection scheme can measure the spectra of single frequency
sounds between 100 and 2000 Hz. The detection scheme worked equally well in both a flat geometry and an anatomical
head geometry. However, the current detection scheme is too slow for use in living biological tissues which has a
decorrelation time of a few milliseconds. Further improvements have been suggested.