Originally Posted by Chaparral
What power output and wavelength did you try this with?
I have a 1200 mW 808nm lab laser that makes one helluva nice beam but I can only see the beam from several dozen feet if it's just pointing up in the air. Maybe if it's humid or foggy I can see it a little better but it's been my experience that lasers have to be hitting reflective objects or some sort wavelength specific photodiode to really generate a usable signal.
Also, properly collimated lasers even in the 1W range ain't gonna be cheap, especially if you need 20 of the darn things. Maybe some focusable 850 nm units from Aixiz that put out say, 5 to 15 mW might work. At a distance of 200 yards of so, the beam can be focused to be quite large and should be safe for even non-autogated intensifier tubes as well as human and animal eyes.
Interesting. To be honest, the only places I have tried this out have been either coastal areas with significant humidty/marine layer, or desert areas with lots of airborne dust, or a combination of both. I could see how in an area with nice clean air, the IR laser idea might not work very well. One more reason to go with a radio signal device motion detector I guess.