  Speckle contrast imaging is an established technique to obtain relative blood
maps over wide field of views. Currently, its most accurate implementation
relies on the acquisition of raw speckle images at different exposure times but
requires modulation of a laser pulse in duration and intensity and precise
synchronization with camera. This complex instrumentation has limited the use
of multiple exposure speckle imaging. We evaluate here a simplified approach
based on synthetic exposure images created from the sum of successive frames
acquired with a 1 ms exposure time. Both methods have been applied to evaluate
controlled flows in micro-channels. The contribution of noises to the speckle
contrast have been quantified and compared. Dark, readout and shot noise
contributions to the total contrast remains constant for modulated exposure,
while all these contributions decrease with increasing exposure time for
synthetic exposure. The relative contribution of noises to speckle contrast
depends on the level of illumination, the exposure time and the flows that are
imaged. Guidelines for accurate flow measurements in the synthetic exposure
acquisition scheme are provided. The synthetic exposure method is simple to
implement and should facilitate the translation of multiple exposure speckle
imaging to clinical setups .
