principal plane

noun

, Optics.

1. a plane that is perpendicular to the axis of a lens, mirror, or other optical system and at which rays diverging from a focal point are deviated parallel to the axis or at which rays parallel to the axis are deviated to converge to a focal point.

Example Sentences

In our consideration of flexure it has so far been supposed that the bending takes place in a principal plane.

From Project Gutenberg