Systems and methods are provided for detecting the depth, flow rate, and other properties of regions of blood flow in biological tissue by illuminating the biological tissue with beams of coherent light and detecting responsively emitted light. This includes detecting lights emitted from the tissue having a plurality of respective different exposure times. The relationship between the intensity of the received light and the exposure times is determined and used to determine the depth, flow velocity, or other properties of regions of flow within the biological tissue. This can include determining a spatial and/or temporal contrast of the received light intensity. Determining the depth of a region of flow can include comparing determined properties of light received from the tissue at two different polarizations. Determining the depth of a region of flow can include comparing determined properties of light received from the tissue at two different locations.
