> Two laser transmitters set to converge at a spot at the far end of your
> expected range.
>
> Use a CCD to accurately measure the distance between the dots.  This is
> linearly proportional to the distance.

there is actually no need for two lasers and two dots if the camera is
mounted a fixed distance from the laser simple trigonometry and quite
accurate if the distance between laser and camera is large enough compared
to the measured distance

The position of the dot is easy to determine
laser on, store image, laser off store image, subtract images , difference >
threshold .... this is the spot

Peter