Visibility and wavelength

[John Brownlee <jonnie@chronic.lpl.arizona.edu>]

	I often hear everyone talk about visibility as a major factor in
U/W ops, and I commiserate. There is nothing worse than going on a dive and
not being able to see your own feet clearly. Often times when I have a
similar problem with an absorbtion and scattering medium at work, I see if
I can use a different wavelength regime where the medium is not such a
problem.
	For example, some spacecraft people decided they needed to see what
was going on beneath a soupy suspended cloud layer on Titan, a moon of
Saturn. Turns out you have to go far infra-red in a narrow band, but sure
enough there is a window of minimum absorbtion and simple scattering for
the obscuring media. We got the right filters queued up on Hubble's IR
camera and wham-o, "visible" surface. Now we know we're not putting down on
a featureless billiard ball, and that there is surface heterogeneity.
	Similar spectra must exist for various common obscurations
suspended/dissolved in water; it might even be worth looking at a
tunable-wavelength imager to maximize penetration for a given set of
conditions. Not all poor-visibility water is created
spectroscopically equal!
	In particular, I am thinking of the sub-to-millimeter detector
systems which have been recently commercialized following declassification.
Is anyone familiar with non VNIR imaging systems being applied underwater?
Sub-millimeter in particular might be interesting because it is outside the
common stretching abosrbtions of water. I saw a vision system of this sort
used in cutting through zero-zero conditions for aircraft and instead
providing 2 mile visibility! Imaging sonar is great, but an alternate
wavelength EM system would be more intuitive to the human eye, and also has
greater resolution for close-up work.
	Any thoughts? Am I out to lunch and picking up the tab here?

							John

John Brownlee
Lunar and Planetary Lab
University of Arizona
jonnie @ lpl . arizona . edu