you may have seen a laser doppler system (LDV) which is based on a frequency shift. this is a point measurement system. the other method described sounds like partical image velocimetry (PIV), which allows you to map out an entire velocity field. both of these have been around for some time. you may want to take a look at dantec's site for some more info (it's a commercial system but they have explanations of the principles) http://www.dantecmt.com/lda/princip/index.html -pete On Tue, 7 Aug 2001, Douglas Butler wrote: > 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. > > > > -- > > http://www.piclist.com hint: The list server can filter out subtopics > > (like ads or off topics) for you. See http://www.piclist.com/#topics > > > > > > > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > > > -- Peter Green 111 IATL peter-w-green@uiowa.edu The Department of Mechanical Engineering office: 319.335.3694 The University of Iowa lab: 319.335.3585 Iowa City, Iowa 52242 fax: 319.335.5669 -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.