Blue-green light has the greatest depth of penetration in seawater. Blue-green lasers have been used to map water depths in at least 200+ feet of water. They have also been experimented with for very covert communications with subs but due to the very narrow beam width of the laser, you would have to be almost over it for it to be seen. Currently green lasers are the most expensive of the various colors available and then when you consider the power required for identification (and even a lot more for 2-way communications) if it were to be feasible, you could buy many top quality EPIRBs, UQCs, pingers, and trackers. R/Jay From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of vbra676539@aol.com This isn't new tech. HBOI was using a pair of lasers to define the precise focal length for stills and video decades ago. That kind of thing is available through the camera manufacturers, I suspect. Don't know about long range, other than that Chris Chulimanus told me that green light works best. Vance |