Re: can a holographic projector project under water?

Yes Michael, in principle it can. But do NOT try it! It is a can of worms.

Water is transparent and still water is isotropic (same in all 
diretioions). So holography will work.

However the slightest water leak will kill your electronics. Furthermore, 
even if hermetically sealed, the water is likely to be cool so to prevent 
condensation the interior air will have to be very dry.

Your lenses (all that contact the water) will have to be specially 
designed as for underwater cameras or else screened from water contact - 
as with goggles worn under water. Every solid-air or solid-water 
interface will introduce undesired reflections.

All the holographic angles, under water index 1.33, will be reduced from 
the "in air" angles.

The best you might manage is to project underwater a single hologram that 
was recorded under water. Probably not the exciting project you hoped for 
and it is not clear if you wish the observers to be under water, nor how 
the hologram will be viewed.

John