Straight light beam in opaque water

[hemness]

Hello!
As this is my first post - I'm Mike from Poland, currently studying product design on Academy of Fine Arts in Warsaw.
I'm working on a sculpture - light installation.
I want to achieve an effect of a regular beam of white light - light cylinder in slightly opaque water. Sort of similar effect to light diffusion in fog. I did some experiments with lenses but what I got was fuzzy edges and focused or spread beam. I suppose I would have to use some certain combination of lenses, or a light collimator. Could you help me with this?

 

[Danger]

Welcome to PF.
Could you just enclose a "string" of fluorescent light tubes in a frosted transparent waterproof sheath?

 

[hemness]

Hmm, the Idea is that the light beam becomes clearly visible when it hits the side of the tank and continues as a visible cylinder until it hits a surface.

 

[Danger]

I'm afraid that I really don't quite understand what you mean. Is there some way that you can post a sketch? I find things like this easier with a visual description.

 

[hemness]

It may not be the best explanation, but it's almost 5am here ;P

 

[Danger]

Okay, I see what you mean now... I think. The beam is to be invisible in air, and become a solidly defined cylinder in the water? I'm afraid that it can't be done the way you want because white light is incoherent. You would need a laser, which has to be one distinct colour and would be far narrower than you desire.
The only thing that immediately comes to mind is that you could use a plexiglass rod that almost exactly matches the refractive index of the water so it can't be seen from outside the tank. If it's glued to the sides of the box, you could shine a high-intensity light (such as a slide projector) onto the wall from outside right at the junction point. Ideally, the rod would then capture that light and transmit it to the other end with enough leakage that it would glow. There's no need for me to sketch up what I mean, because it would look exactly like what you've already drawn. I really have no idea as to whether or not that will work, but it might be worth some small-scale experimentation.

 

[hemness]

These solutions seem to be what I'm looking for:

http://www.thorlabs.de/newgrouppage9.cfm?objectgroup_ID=7541
http://www.autospeed.com/cms/articl...rformance-LED-Lighting-System-Part-2&A=109715
http://www.mightexsystems.com/family_info.php?cPath=&categories_id=225

They have a spreading lens in front but the beam seems to be slightly narrowing. How is that?

 

[Danger]

They have a spreading lens in front but the beam seems to be slightly narrowing. How is that?

I have no idea. Optics scares the hell out of me as a subject because the math is several times more complex than that needed to build a nuclear bomb. I have trouble balancing a chequebook. If you insist upon doing it optically rather than mechanically, I can be of no assistance to you. I really hope that you succeed, because it would be something awesome to see as a spectator and I admire any art that deliberately involves scientific principles. I'm limited to things that I can put together with a screwdriver or a welder.
Even if you get no more input here, please keep me informed of your progress. You can message me directly through PF even if you don't post more.

 

[NascentOxygen]

that the light beam becomes clearly visible when it hits the side of the tank and continues as a visible cylinder until it hits a surface.

Many LED flashlights allow you to form a broad or narrow beam, would a flashlight be bright enough, though? What diameter of beam in the water are you after? What is the path distance through the water? Through the air? Will the beam be eventually fixed in position, or do you want it to move around in the tank?