Information about Laser Doppler Velocimetry
Laser Doppler velocimetry (LDV, also known as laser Doppler anemometry, or LDA) is a technique for measuring the direction and speed of fluids like air and water. In its simplest form, LDV crosses two beams of collimated, monochromatic, and coherent laser light in the flow of the fluid being measured. The two beams are usually obtained by splitting a single beam, thus ensuring coherency between the two. The two beams are made to intersect at their waists (the focal point of a laser beam), where they interfere and generate a set of straight fringes. The sensor is then aligned to the flow such that the fringes are perpendicular to the flow direction. As particles pass through the fringes, they reflect light (only from the regions of constructive interference) into a photodetector (typically an avalanche photodiode), and since the fringe spacing 
is known (from calibration), the velocity can be calculated to be
where
is the frequency of the signal received at the detector. A diagram of the method is in the second page of this brochure.
Another form of flow sensor that is typically referred to as laser Doppler velocimeter has a completely different approach akin to an interferometer. A beam of monochromatic laser light is sent into the flow, and particles will reflect light with a Doppler shift corresponding to their velocities. The shift can be measured by interfering the reflected beam with the original beam, which will form beats according to the frequency difference. This is the type of sensor depicted here.
LDV may be unreliable near non-specular solid surfaces, where stray reflections corrupt the signal. NASA is working on a variation that uses two pairs of beams so that polarization can be used to improve noise rejection. (citation?)
LDV is chosen over other forms of fluid velocity measurement such as Pitot tube measurements because the measurement equipment can be outside of the flow being measured and therefore have no effect on the flow.
In principle a monochromatic laser beam is directed at the skin surface. Light that is reflected off stationary tissue undergoes no shift whilst light that is reflected off cells with velocity (like red blood cells) undergoes Doppler shift. The degree of Doppler shift is proportional to the velocity of the cell into which it collided. This light is randomly reflected back out of the tissue and onto a photodetector which calculates the average velocity of cells within the tissue.
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is known (from calibration), the velocity can be calculated to be
where
is the frequency of the signal received at the detector. A diagram of the method is in the second page of this brochure.
Another form of flow sensor that is typically referred to as laser Doppler velocimeter has a completely different approach akin to an interferometer. A beam of monochromatic laser light is sent into the flow, and particles will reflect light with a Doppler shift corresponding to their velocities. The shift can be measured by interfering the reflected beam with the original beam, which will form beats according to the frequency difference. This is the type of sensor depicted here.
LDV may be unreliable near non-specular solid surfaces, where stray reflections corrupt the signal. NASA is working on a variation that uses two pairs of beams so that polarization can be used to improve noise rejection. (citation?)
LDV is chosen over other forms of fluid velocity measurement such as Pitot tube measurements because the measurement equipment can be outside of the flow being measured and therefore have no effect on the flow.
Medical Applications
LDV is used in clinical research as a mechanism to partially quantify blood flow in human tissues such as skin. However, within the clinical environment, LDV is referred to as laser Doppler flowmetry (LDF.) It has gained popularity because it is simple to use, painless and non-invasive.In principle a monochromatic laser beam is directed at the skin surface. Light that is reflected off stationary tissue undergoes no shift whilst light that is reflected off cells with velocity (like red blood cells) undergoes Doppler shift. The degree of Doppler shift is proportional to the velocity of the cell into which it collided. This light is randomly reflected back out of the tissue and onto a photodetector which calculates the average velocity of cells within the tissue.
External links
- GRC Optical Instrumentation Technology Branch (NASA) (this link is broken)
See also
- Particle tracking velocimetry
- Particle image velocimetry
- Hot-wire anemometry
- Molecular tagging velocimetry
FLUID (Fast Light User Interface Designer) is a graphical editor that is used to produce FLTK source code. FLUID edits and saves its state in text .fl files, which can be edited in a text editor for finer control over display and behavior.
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Water is a common chemical substance that is essential to all known forms of life.[1] In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor.
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laser is a mechanical device that produces coherent radiation. The term "laser" is an acronym: Light Amplification by Stimulated Emission of Radiation.
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Avalanche photodiodes (APDs) are photodetectors that can be regarded as the semiconductor analog to photomultipliers. By applying a high reverse bias voltage (typically 100-200 V in silicon), APDs show an internal current gain effect (around 100) due to impact ionization (avalanche
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Doppler effect, named after Christian Doppler, is the change in frequency and wavelength of a wave as perceived by an observer moving relative to the source of the waves. For waves that propagate in a wave medium, such as sound waves, the velocity of the observer and of the source
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National Aeronautics and Space Administration
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Motto: For the Benefit of All[1]
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Formed 29 July 1958
Headquarters Washington D.C.
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NASA logo
Motto: For the Benefit of All[1]
NASA seal
Agency overview
Formed 29 July 1958
Headquarters Washington D.C.
Annual Budget $16.
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polarization (Brit., polarisation) is the property of electromagnetic waves, such as light, that describes the direction of the transverse electric field. More generally, the polarization of a transverse wave describes the direction of oscillation in the plane
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A Pitot (IPA: [pito]) tube is a pressure measuring instrument used to measure fluid flow velocity, and more specifically, used to determine the airspeed of an aircraft.
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For other uses, see .
Particle tracking velocimetry (PTV) is one of velocimetry methods, i.e a technique to measure velocity of particles. The name suggests that the particles are tracked, and not only recorded as an image as it is suggested in another..... Click the link for more information.
Particle image velocimetry (PIV) is an optical method used to measure velocities and related properties in fluids. The fluid is seeded with particles which, for the purposes of PIV, are generally assumed to faithfully follow the flow dynamics.
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Molecular Tagging Velocimetry (MTV) is a specific form of velocimetry. In its simplest form, a single "write" laser beam is shot once through the sample space. Along its path an optically induced chemical process is initiated, resulting in the creation of a new chemical species or
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