sand grains. Fluctuations in the interference patterns will be observed if the scat-
tering particles are moving, which will cause intensity variations at the photode-
tector. These intensity variations result in a blurring of the speckle pattern.
Note that the formation of the laser speckle pattern is a random process, so the
speckles need to be described statistically. The goal of laser speckle imaging is to
determine the relations between the statistical characteristics of the detected
intensityfluctuations and the time-varying and structural properties of the probed
tissue. Because the temporal and spatial statistics of this speckle pattern yield
information about the motion of the scattering particles, speckle-imaging techniques
have been used for procedures such as monitoring bloodflow in capillaries, ana-
lyzing blood samples in vitro, and imaging of blood coagulation. Two-dimensional
maps of bloodflow can be obtained by examining the degree of speckle pattern
blurring that results from bloodflow. In those regions where the bloodflow is
faster, the intensityfluctuations of the speckle pattern are more rapid, which results
in a more blurred speckle pattern. The exposure time when using a CCD camera for
these measurements is typically 1–10 ms.
Whereas speckle patterns can be used advantageously for measurement proce-
dures such as bloodflow, in other cases speckles can lead to an unwanted blurring
of an image. For example, because OCT is a coherent imaging modality, speckles
can corrupt its images by giving them a grainy or irregularly spotted appearance.
On the other hand, speckle is fundamental to OCT image formation. Thus, attempts
to suppress or eliminate speckles results in a decrease in the OCT image resolution.
A discussion of the effects of speckle on OCT is beyond the scope of this chapter,
but can be found in the literature [ 29 – 33 ].
An important parameter for quantifying the blurring of the speckles is the
speckle contrast. This parameter is designated by K and can be expressed as a
function of the exposure time T of the camera and thespeckle correlation timeτc,
which is inversely proportional to the local velocity of the scattering particles. The
speckle contrast is defined as the ratio of the standard deviationσof the speckle
intensity to the mean intensity Ihiand in terms of the ratio T/τcis given by
Fig. 10.12Granular light
patterns arise when light
reflects from a diffuse or
random scattering medium
308 10 Optical Imaging Procedures