Talk:Laser Doppler velocimetry

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Admittedly, I may have this all backwards...

But, as described, and as corroborated by the description of this style of 'laser doppler flowmetry' expounded here (http://vtchl.illinois.edu/facilities/experimental/ldv), it is not actually the doppler effect by which flow (or, rather, the velocity of a scatterer in a flow) is measured. To do so, it would be necessary to perform some optical heterodyning, as an avalanche photodetector would be unable to detect the frequency of an incoming photon (or the frequency shift thereof).

As I understand it, the crossed-beams 'laser doppler flowmetry' works by setting up a region of structured illumination in a measurement volume having known properties. Specifically, you set up a set of equally spaced planes (interference fringes between the beams of light) of illumination in the volume. As particles flow through the volume, they will scatter light when they are in the fringes, and they will not scatter light when they are not in the fringes. As a result, detecting the light scattered from the volume could allow you to determine the velocity of a scatterer through the flow by determining a frequency at which the scatterer scatters light toward your detector. The velocity equals the distance between the fringes divided by the transit time between the fringes (measured by the frequency of pulses of reflected light, or the time difference between subsequent such pulses).

In this way, you are measuring the speed of an object based on a frequency of a signal emitted by the object; however, since the object is not reflecting a signal whose frequency is related to a velocity-dependent change in frequency of a signal reflected or emitted by the object, I don't see how the 'Doppler effect' is actually at work here.

Of course, this measurement technique is obviously commonly referred to as laser doppler flowmetry. However, I think the description is unclear (and possibly erroneous); further, since one seeing 'laser doppler' would likely think that the optical frequency (i.e., wavelength) of light illuminating scatterers is changed and measured by this method, I think the description should further point out the distinction. Also, a graphical illustration is warranted (description of directions perpendicular to the beams, etc., are very ambiguous).

I proposed to change the application of laser Doppler velocimetry for both sections: vibration & acoustics as well as the mouse section. Both description refers to laser Doppler vibrometer, and quite clearly what is measured by LDV is the velocity component along the optical axis. For the case of the mouse application, the surface velocity is derived from this velocity component. My understanding for laser Doppler velocimetry, two beams must be superimposed on the surface of the objects. For both applications, neither is the case. — Preceding unsigned comment added by Kusg11 (talk • contribs) 09:42, 27 November 2016 (UTC)[reply]