dave@onfcanim.UUCP (Dave Martindale) (03/13/89)
>>>A laser will spread 3.5 feet in 1/5 mile? Are you sure? >>>That doesn't sound like coherent light. >> >>You've got to remember that the beam is passing thru the air, and dust, dirt, >>water molecules, etc. will all diffract the beam to some extent. Thus you get >>a spreading of the beam. Diffraction will spread any light beam, even in a vacuum. If you start out with a laser beam that is somewhat smaller than 0.5 mm in diameter and absolutely parallel, it will spread several feet in 1000 feet. However, if you use an optical system (essentially a telescope in reverse) to expand the beam by a factor of N, the divergence is reduced by the same factor (if the optics are perfect). Using this, you could keep the spot down to a few inches in size at 1000 feet, with optics not much larger than a good rifle sight. >Another thing to think about - a beam a few feet wide is going to reflect >a beam a bit bigger than that, but not much. If the copper is on the side >of the road, he's going to have to "wiggle" the laser around until it hits >a perpendicular plane on your car. If, for example, he hits the hood, most >of the beam is going to be angled upwards into the sky. His best bet is to >aim somewhat low, since it isn't going to bounce off of the windshield >or any other part of the car. The grill/radiator/bumber would probably >be the best bet. You're assuming that the light has to be reflected directly back to the source as a beam. Not true - just hitting the car's paint should send light in all directions, including back to the source. A small telescope should be able to collect enough to measure. Existing radar units don't need perpendicular surfaces to work - why should optical systems? >As far as the scope - it had better be pretty damn >accurately aligned. If it's off by only 4 or 5 feet, it would be very easy >to hit a car next to you. Or what if a semi was coming up from behind you >at a pretty good pace - if he aimed at your windsheid, it would go through >the front windshield, through the passenger's compartment, through the rear >windshield, and bounce off the semi. Kind of scary. It would seem that the aiming scope needs to be aligned with the laser to within, say, one foot at 1000 feet. The sort of mechanism used in a riflescope should easily allow adjusting to that sort of accuracy.