baron,
things you need to consider about the transmission of light through water and effectively using lasers underwater for ROV survey.
water absorption
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for short ranges most people consider this to be the main thing of interest!!. Ultimately water transmits light preferentially in the visible spectrum from around 400nm to 650nm. (about violety-blue to red colours – just before it hits infrared and ultraviolet). The actual spectrum will vary with pressure, salinity, temp, etc.
Water will actually transmit light from deeper into both UV and infrared but penetration will be shorter for the same amount of power. If you are working beneath a meter or two, video cameras should have more than sufficient gain to be able to achieve the same detection using a red or a green laser (actually you should be able to use a UV laser too.)
The best wavelengths are around blue-green, but these wavelengths actually scatter light more than red light, hence there are tradeoffs here too. Ultimately, we are all limited to commercial availability of particular wavelength lasers.
Remember, when you look at "path length" of your ROV, you need to remember the light travels from the laser to your pipe and then reflects diffusely from the pipe to your camera.
water scattering co-efficients.
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Water is a molecule shaped like H-0-H and it will vibrate (scizzors movements, bending, twisting, etc) and absorb energy in order to move
(imagine light photons as bundles of energy that can be absorbed and re-emitted by a molecule.)
When the water molecule relaxes, it kicks out that energy in the form of light (actually kicks out the energy with the same color as the laser). This is a special type of scattering "Rayleigh scattering" which gets stronger the closer you go to the UV. (Another tradeoff)
There are other emission parameters at play (Raman, fluorescence emissions, etc) (but for short range and using a cheapo video camera, I tend to consider these two as the main components). You’d be hard pushed to detect anything more using any of the technology available to ROV companies at present.
If you are working beneath 70m or at night, then you don’t really need to consider the background sunlight light levels, however you also need to consider the background light levels according to ROV halogen lighting. Remember with some green lasers, if they are weak you’ll be hunting for
green shapes on a green background!).
There are other properties of light transmission which vary according to polarisation, you can play games using polarisers to increase your detection range.
We used a 700 and a 750nm laser to do line imaging. There’s no real advantage except if you use a filter and knock out all the infrared emissions from your ROV lighting. In this way, your camera can be tuned to detect only the laser wavelength and it becomes very much more sensitive. You’re better in the red and green/blue just to keep the laser power down.
In terms of research, there are hundreds of papers written on ocean optical properties – very relevant to marine biology, military, etc. Queen Mother library – Aberdeen university has a huge section on it. Optical properties of all the major waterways are tabled in it too so you can get a reasonable idea.
I’ll try and post some refernces if I can find some good ones online. Sorry if this is a little too vague.
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