Henry Quach

Mechanical Engineer

Infinite deflectometry enabling 2π-steradian measurement range

Abstract: We present a novel deflectometry implementation termed Infinite Deflectometry. The technique provides a full aperture surface reconstruction sag map of freeform surfaces, including previously challenging to measure optics such as highly convex surfaces. The method relies on the creation of a virtual source enclosure around the tested optic, which creates a virtual 2π-steradian measurement range. To demonstrate the performance, a fast f/1.26 convex optical surface was measured with a commercial interferometer and with the Infinite Deflectometry system. After removing Zernike terms 1 through 37, the metrology tests resulted in absolute RMS surface values of 18.48 nm and 16.26 nm, respectively. Additionally, a freeform Alvarez lens was measured with the new technique and measured 22.34 𝜇m of surface sag RMS after piston, tip/tilt, and defocus had been removed. The result deviated by 488 nm RMS from a profilometer measurement while standard interferometry failed to measure the Alvarez lens due to its non-nulled wavefront dynamic range limitation.

The figure above shows the progression of virtual screens which create a "tipi"-like source area that encloses the optic. Without any ray path obscurations, the configuration effectively removes dynamic range limitations typically due to source area size and planar geometry.

The second figure presents a top-down view of the system with a slightly-tilted observer's perspective. A non-normal aerial view is chosen to more clearly show the progressive overlap of clocked virtual screens.

Journal: Optics Express
Key Words: Metrology, Deflectometry, Freeform Optics, Dynamic Range
Full Citation: Logan R. Graves, Henry Quach, Heejoo Choi, and Dae Wook Kim, "Infinite deflectometry enabling 2π-steradian measurement range," Opt. Express 27, 7602-7615 (2019).

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