The underwater machine I saw used a single photomultiplier tube as the
detector, only one pixel!  I suspect the operation was different than
the laser velocimetry you describe.
        At the time I was working on the next instrument over, a diffractometer
which measured silt particle size.

Sherpa Doug

> -----Original Message-----
> From: Brandon Fosdick [mailto:bfoz@GLUE.UMD.EDU]
> Sent: Monday, August 06, 2001 10:56 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]: Amateur radar?
>
>
> Douglas Butler wrote:
> > This might be a little out of the realm of amateur weather
> radar, but
> > there is a technique I know of to measure the speed of silt
> particles in
> > seawater using a laser.
>
> Laser Velocimetry. Its used in wind tunnels too, except with dust in
> moving air. There are two problems with it though, 1. tracking lots of
> particles takes an enourmous amount of computing power, not to mention
> the video capture requirements; 2. Near particles can obscure far
> particles. Normally you don't think of a dust particle being
> big enough
> to obscure anything, but since they're bright they can take up several
> pixels, which will obscure other particles, or at least make the
> boundary ambiguous. And then when the two particles seperate
> again, you
> have to figure out which one is which. Tracking millions of water
> droplets would be a big task.
>
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