  Illumination coherence plays a major role in various imaging systems, from
microscopy, metrology, digital holography, optical coherence tomography, to
ultrasound imaging. Here, we present a systematic study on the effects of
degrees of spatiotemporal coherence of an illumination (DSTCI) on imaging
quality. An optical field with arbitrary DSTCI was decomposed into wavelets
with constituent spatiotemporal frequencies, and the effects on image quality
were quantitatively investigated. The results show the synergistic effects on
reduction of speckle noise when DSTCI is decreased. This study presents a
method to systematically control DSTCI, and the result provides an essential
reference on the effects of DSTCI on imaging quality. We believe that the
presented methods and results can be implemented in various imaging systems for
characterising and improving imaging quality.
