Laser Doppler imaging (LDI), also called laser Doppler perfusion imaging (LDPI), is a non-contact optical technique for mapping signals related to microvascular blood perfusion in tissue. It illuminates tissue with coherent laser light and analyzes temporal fluctuations in the backscattered light caused by scattering from moving red blood cells; these fluctuations contain Doppler-broadened frequency components. LDI typically reports a perfusion-related index proportional to the amount of moving blood cells and their velocities within the sampled volume, rather than a direct absolute volumetric flow rate.