A theoretical model is presented and experimentally validated that allows the prediction of the effect of speckles on the depth sensitivity of laser Doppler perfusion imaging. It is shown that the influence of speckles on depth sensitivity is large. In particular the sensitivity to particle motion in superficial layers is strongly beam diameter dependent: decreasing the beam diameter on the tissue surface increases the sensitivity to superficial motion to a much stronger extent than sensitivity to motion at a larger depth. This can be explained through the effect of beam diameter changes on the fractional coherence areas generated by photons with different penetration depths in the tissue.
Rajan, V., Varghese, B., van Leeuwen, T., & Steenbergen, W. (2007). Effect of speckles on the depth sensitivity of laser Doppler perfusion imaging. Optics express, 15(17), 10911-10919. https://doi.org/10.1364/OE.15.010911