LDF makes use of the fact that when tissue is illuminated by a coherent, low powered laser, light is scattered by both moving and static structures within the microcirculatory beds. Photons, scattered by moving blood cells are spectrally broadened according to the Doppler effect. Maximum Doppler shifts occur when blood cells are moving in a direction parallel to the incident light beam and the detected light (scattered light) from the cells is detected in a direction opposite to its origin.

For example, a Doppler shift of about 4 kHz is obtained when laser light (of a wavelength of 830 nm) is backscattered from a particle in water moving at 1 mm/s parallel to the light beam. In general a continuous range of Doppler frequency shifts is expected. Photons scattered by static structures alone do not undergo Doppler shifting.