prin'cipal plane'

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.

Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.

principal partsprincipal point

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