Generally LIDARs work by rotating a mirror that the laser bounces off of giving you a 2D plane or cone where you can detect things.  Super expensive LIDARs like the one on the Google car use a number of cones with different angles to approximate a 3D system and cost tens of thousands of dollars.  Other systems like the ones you see in the DARPA challenge might rotate the plane of the LIDAR through a full circle, but that takes a while.<p>Spinning mirrors are also somewhat delicate which can be a problem for automotive applications.