Localized fluid flow measurements with an He-Ne laser spectrometer

Laser Doppler velocimetry uses the frequency shift produced by the Doppler effect to measure velocity. It can be used to monitor blood flow or other tissue movement in the body. Laser speckle is a random interference effect that gives a grainy appearance to objects illuminated by laser light. If the object consists of individual moving scatterers (such as blood cells), the speckle pattern fluctuates. These fluctuations provide information about the velocity distribution of the scatterers. It can be shown that the speckle and Doppler approaches are different ways of looking at the same phenomenon. Both these techniques measure at a single point. If a map of the velocity distribution is required, some form of scanning must be introduced. This has been done for both time-varying speckle and laser Doppler. However, with the speckle technique it is also possible to devise a full-field technique that gives an instantaneous map of velocities in real time. This review article presents the theory and practice of these techniques using a tutorial approach and compares the relative merits of the scanning and full-field approaches to velocity map imaging. The article concludes with a review of reported applications of these techniques to blood perfusion mapping and imaging.

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Laser Doppler, speckle and related techniques for blood perfusion mapping and imaging

A new noninvasive technique for monitoring capillary blood flow has been developed. Based on the phenomenon
of time-varying laser speckle, it is a digital version of single-exposure speckle photography. It
provides a velocity map of the area of interest in real time without the need for scanning. The results of some initial experiments on volunteers are presented.

https://www.spiedigitallibrary.org/journals/Journal-of-Biomedical-Optics/volume-1/issue-02/0000/Laser-speckle-contrast-analysis-LASCA--a-nonscanning-full-field/10.1117/12.231359.pdf

Laser speckle contrast analysis (LASCA): a nonscanning, full-field technique for monitoring capillary blood flow

Flow visualization by means of single-exposure speckle photography

Coherent light scattered from an ensemble of moving scatterers produces a time-varying speckle pattern. The intensity fluctuations observed in a single speckle can be regarded either as a time-varying interference effect or as a Doppler beating effect. Techniques based on each of these approaches have been developed to analyze the fluctuations in an attempt to measure the velocities of the scatterers. Most of these methods measure the temporal statistics of the intensity fluctuations in a single speckle, i.e., at a single point. If a map of the velocity distribution is required, some form of scanning must be introduced. One way of avoiding the need to scan is to make use of the spatial statistics of time-integrated speckle. This is the basis of a technique, already described in the literature, called laser speckle contrast analysis (LASCA). In this article, we present a brief review of the theory linking the intensity fluctuations to the velocity and of the various techniques that have been proposed to measure them. We then describe the present configuration of our LASCA technique and describe some recent developments in our search for a real-time, noninvasive, full-field technique for visualizing capillary blood flow. © 1999 Society of Photo-Optical Instrumentation Engineers.

https://www.spiedigitallibrary.org/journals/Journal-of-Biomedical-Optics/volume-4/issue-01/0000/Capillary-blood-flow-monitoring-using-laser-speckle-contrast-analysis-LASCA/10.1117/1.429903.pdf

Capillary Blood Flow Monitoring Using Laser Speckle Contrast Analysis (LASCA)

A brief survey is given of the principles and present state of the art in laser Doppler anemometry. Both the optical and signal-processing aspects are treated in some detail. The comparative features and special problems associated with the various methods of interpreting the Doppler signals are discussed, with particular reference to the more recently developed technique of photon correlation. A wide range of practical applications, including measurements on liquids and solids, is considered.

Laser Doppler anemometry

A laser anemometer capable of operating on the signal backscattered from an unseeded supersonic airflow has been used to measure velocities over a range of Mach numbers extending to 2?46. The signal processing was carried out with a photon correlator.

E. R. Pike

Papers

Laser Doppler anemometry

Laser anemometry in an unseeded supersonic wind tunnel by means of photon correlation spectroscopy of backscattered light