What do a.m. and p.m. stand for?

principal plane

noun, Optics.
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. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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Examples from the web for principal plane
  • The height is measured from the floor to the principal plane of the ceiling.
  • The shortest distance from a clear pixel to a cloud in the principal plane is defined as cloud-free distance for the clear pixel.

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