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"t the .esearch !aboratories of 8rown 2ngineering >ompany %later $eledyne 8rown

2ngineering), this phenomenon was used in developing the first laser Doppler flowmeter

using heterodyne signal processing;<

$he instrument was soon called the !aser Doppler ?elocimeter %!D?) and the technique

!aser Doppler ?elocimetry, also abbreviated !D? "nother application name is laserDoppler anemometry %!D") 2arly !D? applications ranged from measuring and

mapping the e3haust from rocket engines with speeds up to *@@@ mAs to determining flowin a near-surface blood artery " variety of similar instruments were developed for solid-

surface monitoring, with applications ranging from measuring product speeds in

 production lines of paper  and steel mills, to measuring vibration frequency and amplitudeof surfaces;0<

[edit] Operating principles

=n its simplest and most presently used form, !D? crosses two beams of collimated,

monochromatic, and coherentlaser light in the flow of the fluid being measured $he two beams are usually obtained by splitting a single beam, thus ensuring coherence between

the two !asers with wavelengths in the visible spectrum %9@-B1@ nm) are commonly

usedC these are typically 6e-7e, "rgon ion, or laser diode), allowing the beam path to beobserved " transmitting optics focuses the beams to intersect at their waists %the focal

 point of a laser beam), where they interfere and generate a set of straight fringes "s

 particles %either naturally occurring or induced) entrained in the fluid pass through thefringes, they reflect light that is then collected by a receiving optics and focused on a

 photodetector  %typically an avalanche photodiode)

$he reflected light fluctuates in intensity, the frequency of which is equivalent to the

Doppler shift between the incident and scattered light, and is thus proportional to thecomponent of particle velocity which lies in the plane of two laser beams =f the sensor is

aligned to the flow such that the fringes are perpendicular to the flow direction, the

electrical signal from the photodetector will then be proportional to the full particle

velocity 8y combining three devices %egC 6e-7e, "rgon ion, and laser diode) withdifferent wavelengths, all three flow velocity components can be simultaneously

measured;1<

"nother form of !D?, particularly used in early device developments, has a completelydifferent approach akin to an interferometer  $he sensor also splits the laser beam into

two partsC one %the measurement beam) is focused into the flow and the second %the

reference beam) passes outside the flow " receiving optics provides a path that intersectsthe measurement beam, forming a small volume &articles passing through this volumewill scatter light from the measurement beam with a Doppler shiftC a portion of this light

is collected by the receiving optics and transferred to the photodetector $he reference

 beam is also sent to the photodetector where optical heterodyne detection produces anelectrical signal proportional to the Doppler shift, by which the particle velocity

component perpendicular to the plane of the beams can be determined;4<

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5imilar arrangements using optical heterodyning are also used in laser Doppler sensors

for measuring the linear velocity of solids and for measuring vibrations of surfacesC the

latter sensor is usually called a laser Doppler vibrometer , also abbreviated !D?;B<

[edit] Applications

=n the decades since the !D? was first introduced, there has been a wide variety of laser

Doppler sensors developed and applied

[edit] Flow esearch

!aser Doppler velocimetry is often chosen over other forms of flow measurement 

 because the equipment can be outside of the flow being measured and therefore has no

effect on the flow 5ome typical applications include the following:

• Wind tunnel velocity e3periments for testing aerodynamics of aircraft, missiles,

cars, trucks, trains, and buildings and other structures

• ?elocity measurements in water flows %research in general hydrodynamics, ship

hull design, rotating machinery, pipe flows, channel flow, etc)

• Fuel inection and spray research where there is a need to measure velocities

inside engines or through no''les

• 2nvironmental research %combustion research, wave dynamics, coastal

engineering, tidal modeling, river hydrology, etc);< 

ne disadvantage has been that !D? sensors are range-dependentC they have to be

calibrated minutely and the distances where they measure has to precisely defined $his

distance restriction has recently been at least partially overcome with a new sensor that is

range independent;9<

[edit] !edical Applications

!aser Doppler velocimetry is used in hemodynamics research as a technique to partiallyquantify blood flowin human tissues such as skin Within the clinical environment, the

technology is often referred to as laser Doppler flowmetry %!DF) $he beam from a low-

 power laser %usually a laser diode) penetrates the skin sufficiently to be scattered with a

Doppler shift by the red blood cells and return to be concentrated on a detector $hesemeasurements are useful to monitor the effect of e3ercise, drug treatments,

environmental, or physical manipulations on targeted micro-si'ed vascular  areas;*@<

$he laser Doppler vibrometer is being used in clinical otology for the measurement of

tympanic membrane %eardrum), malleus %hammer), and prosthesis head displacement inresponse to sound inputs of @- to *@@-d8 sound-pressure level =t also has potential use

in the operating room to perform measurements of prosthesis and stapes displacement;**<

[edit] e"erences

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* # White, " D, and J D .igden, E>ontinuous #as /aser peration in the

?isibleE Proc IRE , vol 1@, p *49B: July *94+, p *49B (5 &atent +0+09

+ #ea, , and 6 G >ummingsC H!ocali'ed Fluid Flow /easurements with an 6e- 7e !aser 5pectrometer,I Appl. Phys. Letters, vol 0, /ay *940, pp *B4-*B

#Foreman, J W, Jr, 2 W #eorge, and . D !ewisC H/easurement of !ocali'ed

Flow ?elocities in #ases with a !aser Doppler Flowmeter,I Appl. Phys. Lettersvol B "ug *941, pp BB-B

0 #Watson, . >, Jr, . D !ewis, and 6 J WatsonC H=nstruments for /otion

/easurement (sing !aser Doppler 6eterodyning $echniques,I ISA Trans., vol ,no *, *949, pp +@-+

1 # Drain, ! 2C The Laser Doppler Technique, John Wiley 5ons, *9@, =587

@0B*+B4+B 

4 # Durst, F, " /elling, and J 6 WhitelawC Principles and Practice of Laser Doppler Anemometry, "cademic &ress, !ondon, *9B4, =587 @*+++1+1@@ 

B # /iller, Frederic , "ghes F ?andome, and John /c8rewsterC Laser Doppler

Vibrometer , ?D/ &ub 6ouse, +@*@, =587 4*+1*+@+ 

#Dantec Dynamics, I!aser Doppler "nemometryIChttp:AAwwwdantecdynamicscomADefaultasp3K=DL41 

9 #/oir, >hristopher =: E/iniature laser doppler velocimetry systems,E SPIEonference Proceedin!s" #ptical Systems" $%%&, vol B14

*@ #5tern, /ichael DC E!aser Doppler velocimetry in blood and multiply scattering

fluids: theory,E Appl. #pt., vol +0, no *, %*91), pp *94-*94** ##oode, .ichard !C #eoffrey 8all, 5hinsei 7ishihara, and Moshiro 7akamuraC

H!aser Doppler ?ibrometer,I American '. #tolo!y vol *B, no 4, %*994), pp *-

++

[edit] $%ternal lin&s

• !D"A!D? principle 

• !D? overview 

[edit] 'ee also

• &article tracking velocimetry 

• &article image velocimetry 

• 6ot-wire anemometry 

• /olecular tagging velocimetry 

•

&hoton Doppler velocimetry • !aser surface velocimeter  

.etrieved from Ehttp:AAenwikipediaorgAwAinde3phpKtitleL!aserNDopplerNvelocimetryoldidL01B*+4E

$rustworthy

bective

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