Re: Visibility and wavelength

[Dick Morrisson <rmorrisson@unidial.com>]

This sounds like a great idea that will become useful when the technology can
catch up.  Light travel 186,000 miles per second.  60% is around 100,000 miles
per second.  A millionth of that is about 500'.  So the strobe could keep the
illumination down to 500' of silt.  It would require a sensor that could be
phased to the strobe (lagging) very accurately and only catch the reflection
which, for say 50', would only be 1x10^-7 seconds.  These sound like pretty
difficult numbers to achieve.  Maybe a light on the end of a long holder would
do the same thing (the low tech approach) or, even better, an ROV with a light
and a camera on it!

Jonathan Shawl wrote:

> Another kind of remote viewing. Many years ago I saw a Cousteau video or
> another video about Harold Edgerton, a friend of Cousteau's and the inventor
> of the strobe flash we all have on our cameras now. In it Edgerton and
> Cousteau was working on a under water vision system that would allow much
> better visibility in silty waters. It was to work something like this. A
> very bright, very short flash of light would be generated at the sub then
> using a very high speed electronic shutter system it would capture only the
> returning light that bounced of the object you are trying to see, and not
> the back scattered light that was bouncing off all the silt between the sub
> and the bottom.  In theory at least I can see how it could work. It would be
> like having the lights mounted up close to the object you wanted to see
> while you sat off in the dark back ground. In effect you are seeing around
> or between the silt in the water as the silt would be back lit, it just
> doesn't show up as much that way. Ever been to a Omnimax theater? It is a
> perforated screen, when it is front lit during the movie you can't see the
> small holes or the sound system behind the screen,  but when they turn out
> the theater lights and back light the sound system behind the screen then
> the screen becomes almost transparent and you see all the speakers. Cool
> huh!
> A strobe flash even back then could put out flashes of light in the
> millionth of a second in duration but the tricky part then was to catch the
> right part of the returning flash of light.  They didn't have high speed
> liquid crystal shutters or very fast and sensitive CCD sensors like we have
> today. So I have always wondered if our technology has reached the point
> that we can make this work now. Is it true that the speed of light is about
> 60% slower in water? If true this would help with the timing of things. Any
> body in the group up on this subject?
> How about you John Brownlee, it sounds like this might be right up your
> alley.
>
> Jonathan Shawl
>
> Michael B Holt wrote:
>
> > On Mon, 05 Jul 1999 15:43:06 GMT hylands@ibm.net (Jon Hylands) writes:
> > >I am not aware of *any* visible wavelength that will penetrate water
> > >to any extent.
> >
> > If the water is clear, how far can we see?    In some places, the Secchi
> > disk is visible for a long way.
> >
> > I asked about the exact frequency of lights, seeking a color that would
> > permit me to illuminate "around" particulate matter.   (I wish I'd saved
> > my notes.)   As I recall, the best was a blue-green light that had
> > thallium
> > in it.
> >
> > Mike Holt
> >