We Can Figure This Out.org
Virtual Lab: The Inner Workings of CD/DVD Drives
University of Virginia
© 2003-Present, John C. Bean
However, most CD/DVD drives use an even more precise way of sensing disk movement. This involves an "astigmatic" lens: a lens that is curved much more strongly side-to-side than top-to-bottom.

Because more curvature means stronger bending, this lens bends the light more quickly in the horizontal plane than the vertical. This produces the complex beam cross-section illustrated by the white rings.

But if the disk moves downward, the light rays are already "pre-bent" a little as they enter the astigmatic lens. So that lens focuses even more quickly, moving the pattern closer. And the opposite happens when the disk moves upward.
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