title: Large-aperture, equal-path interferometer for precision measurements of flat transparent surfaces.
abstract: The measurement of flat optical components often presents difficulties because the presence of parallel surfaces generates multiple reflections that confuse conventional laser-based interferometers. These same parts have increasingly demanding surface finish tolerances as technologies improve over time, further complicating the metrology task. Here we describe an interferometric optical system for high-accuracy noncontact evaluation of the form and texture of precision flat surfaces based on an equal-optical-path geometry that uses extended, broadband illumination to reduce the influence of speckle noise, multiple reflections, and coherent artifacts by a factor of 10 when compared to laser-based systems. Combined with a low-distortion, fixed-focus imaging system and 4-Mpixel camera, the 100 mm aperture instrument offers surface height resolutions of 0.1 nm over lateral spatial frequencies extending from 0.01 to 10 cycles/mm. The instrument is vibration resistant for production-line testing of flat optics such as glass hard disks for the data-storage industry and flat-panel-display substrates.
authors.forename: Leslie L
authors.initials: LL
authors.lastname: Deck
authors.forename: Peter J
authors.initials: PJ
authors.lastname: de Groot
authors.forename: James A
authors.initials: JA
authors.lastname: Soobitsky