conoid of Sturm

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Sturm, conoid of 

The bundle of rays formed by an astigmatic optical system consisting of a primary focal line (called Sturm's line), a circle of least confusion and a secondary focal line (Sturm's line) perpendicular to the first. See astigmatism; circle of least confusion.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann