An Etymological Dictionary of Astronomy and Astrophysics
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The ratio of the peak intensity of the central core of the image of a point source to the corresponding intensity contained in an image obtained in the absence of aberration. An optically perfect system has a Strehl ratio of unity. The ratio may be computed by integrating the area under a → modulation transfer function (MTF) curve and dividing it by the integral over the diffraction-limited MTF.

In honor of Karl Strehl (1864-1940), the German physicist and mathematician who introduced the concept; → ratio.