USING DYNAMIC LOW-COHERENCE INTERFEROMETRY TO IMAGE BROWNIAN-MOTION WITHIN HIGHLY SCATTERING MEDIA

Dynamic low-coherence interferometry was used to measure Brownian moti on of submicrometer particles within highly scattering media. Strong r ejection of multiply scattered light was obtained by combination of a coherence gate with a confocal microscope, thus allowing particle char acterization methods generally reserved for optically dilute materials to be applied to optically concentrated suspensions. The Brownian dif fusion coefficient of highly scattering media was determined with an a ccuracy better than 5%. show that spatial variations in the Brownian d iffusion coefficient can he imaged with an axial resolution determined by the coherence length of the light source (similar to 30 mu m). The experiments also show broadening of the power spectrum as a function of depth into the sample, most likely as a result of detecting multipl y scattered light. (C) 1998 Optical Society of America.