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optical flat[′äp·tə·kəl ′flat]
A disk of high-grade quartz glass approximately 2 centimeters thick, with a deviation in flatness usually not exceeding 0.05 micrometer all over, and a surface quality of 5 microfinish or less; used in determinations of surface contour and in comparison of lineal measurement.
A plane surface, with deviations from a plane surface generally not exceeding one-tenth of a wavelength of light, used to redirect light in a telescope or other optical instrument.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.