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Verhoeff's circles |
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Verhoeff's circles Two black concentric circles designed for use with the duochrome test and as a target for the cross-cylinder method. The thickness and overall diameter of the inner ring are equivalent to a 6/6 (20/20) Snellen letter while the thickness and overall diameter of the outer ring are equivalent to a 6/15 (20/50) Snellen letter. Syn. Verhoeff's rings. See Snellen chart; cross-cylinder test for astigmatism; duochrome test. How to thank TFD for its existence? Tell a friend about us, add a link to this page, add the site to iGoogle, or visit webmaster's page for free fun content. |
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