Holographic Motility Contrast Imaging of Live Tissues - Biomedical Optical Phase Microscopy and Nanoscopy - page 204

CHAPTER 11
Holographic Motility Contrast Imaging of
Live Tissues
David D. Nolte, Ran An, Kwan Jeong, John Turek
Dept. of Physics and Dept. of Basic Medical Sciences, Purdue University,
West Lafayette IN 47907
Editor: Natan T. Shaked
11.1 Introduction and Review
Phase sensitivity in optical imaging opens a window on the many subtle features that
characterize living systems, displaying them with a sharp contrast that goes beyond what only
scattered intensities provide. The phase of light transmitted or reflected from biological
specimens uses the wavelength as the ruler against which everything is measured, achieving
sensitivities of a small fraction of that scale. That is why phase-contrast microscopy, which is
a form of imaging interferometry, has such power in biological applications [1] .
High-resolution phase-contrast imaging requires the average phase to vary slowly across a
lateral distance of a wavelength or at least to have regions of slowly varying phase that may
have sharp boundaries. Sharp boundaries and phase heterogeneities diffract light, which
produce diffraction patterns in the image that obscure structure. When the spatial scale of
the heterogeneities gets too small, and if the phase modulation gets too large, the diffraction
patterns overlap and generate speckle—the random intensity patterns that appear as noise in
coherent or phase-sensitive imaging systems.
Speckle has played a persistent and controversial role in the history of imaging science. On the
one hand, it is a noise source, always present in any system that uses partially coherent light.
From the very early days of the laser, it was already viewed as a source of background that
limited the sensitivities of interferometry and holography [2] . On the other hand, and also from
early on, speckle was recognized as a highly interesting and complex phenomenon that
carried information about the optical system and targets in its statistical fluctuations [3,4] .
A very early success of speckle statistics, even prior to the invention of the laser, was the