A plane within a material which experiences only normal stresses i.e. no shear stresses. In three dimensions there are three principal stress planes which are mutually perpendicular. For a given state of stress these planes are unique. The stresses on these planes are called the major, intermediate and minor principal stresses, in order of decreasing stress magnitude.
(n) A plane that is seen in its true size and shape in two of the six principal views and as an edge in the other four. The three principal planes are frontal, profile, and horizontal. For example, the frontal plane is seen in its true size and shape in the front and rear views and as an edge in the other four views.
A mathematical construct useful in analyzing complex optical assemblies. The principal planes are constructed at the effective focal length distance away from the lens assembly focus and serve to represent the mathematical position of the lens assembly assuming it could be constructed as a single thin lens.
(Also called sun's meridian or sun's vertical.) In clear-sky optics, the vertical plane defined by sun, zenith, and observer. The clear daytime sky's radiance, polarization, and chromaticity patterns are largely symmetric about the principal plane.