  A systematic and robust laser speckle contrast imaging (LSCI) method and
procedure is presented, covering the LSCI system calibration, static scattering
removal, and measurement noise estimation and correction to obtain a true flow
speckle contrast and the flow speed from single-exposure LSCI measurements. We
advocate to use as the speckle contrast instead of the conventional contrast K
as the former relates simply to the flow velocity and is with additive noise
alone. We demonstrate the efficacy of the proposed true flow speckle contrast
by imaging phantom flow at varying speeds, showing that (1) the proposed recipe
greatly enhances the linear sensitivity of the flow index (inverse
decorrelation time) and the linearity covers the full span of flow speeds from
0 mm/s to 40 mm/s; and (2) the true flow speed can be recovered regardless of
the overlying static scattering layers and the type of speckle statistics
(temporal or spatial). The fundamental difference between the apparent temporal
and spatial speckle contrasts is further revealed. The flow index recovered in
the spatial domain is much more susceptible to static scattering and exhibit a
shorter linearity range than that obtained in the temporal domain. The proposed
LSCI analysis framework paves the way to estimate the true flow speed in the
wide array of laser speckle contrast imaging applications.
