Rami Arieli: "The Laser Adventure" Section 9.1.2 page 1

9.1.2 Straight line marking, or plan of reference.

Many daily applications require a precise reference line for alignment.
Examples are:

By using a vibrating (or rotating) mirror to reflect a visible laser light, a perfect plane is defined in space. The mirror is vibrating around one axis, so the light is reflected into consecutive angles continuously, thus defining a perfect plane. Since the vibration of the mirror is at a frequency greater than the persistence of vision in the brain, the viewer see a plane of light. This plane helps aligning walls, sealing, etc. in industrial construction.