As a scientific equipment company, we are here to introduce: How does a laser doppler work? Prior to our discussion, you should have known that the industry recognized the disadvantages of the older laser doppler blood flow monitoring method.

Laser doppler velocity measurement (LDV), also known as laser doppler anemometry (LDA), is an optical technique that uses doppler shifting in the laser beam to measure the speed in clear or semi-clear liquid flows, or in linear or oscillatory movements in opaque, reflecting surfaces. A standard laser doppler vibrometer is a contactless speed transmitter that works by measuring the doppler frequency shift in a laser beam that is scattered by a moving target using an interferometer. In its simplest and currently most widely used form, laser doppler velocimetry measures two beams of collimated, monochromatic, coherent laser light through the fluid flow.

Laser doppler anemometry’s working principle is based on doppler effects on scattered light coming from micron-sized tracers particles moving along with the fluid. By analyzing the doppler-equivalent frequencies of laser light scattered by the microns (intensity modulation inside a probe cross-beam volume), the local velocity of the fluid can be determined. For a point P moving with V speed across the crossing points of two laser beams, the frequencies of the two laser beams are doppler-shifted according to the formulas given earlier.

The low-power laser beam (typically, a laser diode) penetrates skin enough that it is scattered with a doppler shift by red blood cells, and returns concentrated on the detector. The depth at which measurement is made is also dependent upon the properties of the tissue, such as the structure and density of the capillary beds, pigmentation, oxygenation, etc. In normal skin, with Perimes Laser Doppler instruments, probes of standard fiber spacing (0.25 mm), and 780 nm wavelength laser, typical depths of measurement would be in the order of 0.3-0.5 mm, and small portions of the signals affected by the flow of blood in 1-mm or greater.

To measure deeper tissue perfusion, such as that of muscles or the brain, a new kind of Near-IR laser doppler instrument has been developed. After early publications on the potential for doppler effects caused by blood cells to measure blood flow, laser doppler flowmetry took several steps to develop into a clinically applicable instrument. In this review, we will discuss a relatively new technology, laser doppler imaging (otherwise called scanned laser doppler), that provides direct measurements of microcirculatory flow.