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principal point

noun, Optics.
the point where a principal plane intersects the axis.
Origin of principal point
1695-1705 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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principal-point in Medicine

principal point n.
One of the two points in an optical system where the axis is cut by the two principal planes. Lines drawn from these to corresponding points on the object and the image will be parallel.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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