Laser Doppler measures the total local microcirculatory blood perfusion including the perfusion in capillaries (nutritive flow), arterioles, venules and shunting vessels. The technique is based on the emission of a scanning beam of laser light.
When laser light penetrates the tissue under study it is scattered and partly absorbed. Some of the scattered light returns to the tissue surface, where it is registered by a photo detector inside the instrument. This signal is then processed to extract information about the microcirculatory blood flow. According to the Doppler principle, light particles which hit moving blood cells undergo a change in wavelength/frequency (a Doppler shift), while light particles which encounter static structures return unchanged. The perfusion can be calculated since the magnitude and frequency distribution of the doppler shifted light are directly related to the number and velocity of blood cells but unrelated to their direction of movement.