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MM4-Computer Generated Hologram Metrology Techniques

Dr. Shelby Ament (AOM – Arizona Optical Metrology)

Monday, October 26, 2026 (8:00 AM – 12:00 Noon)

Computer-generated holograms (CGHs) are commonly used as null correctors for interferometric measurement of aspheric and freeform surface figure error. This tutorial begins with the principle of operation: how a flat, two-dimensional lithographic pattern diffracts light into a three-dimensional wavefront for use in an interferometric test. We then turn to the traceability and error analysis that separate a defensible measurement from a pretty fringe pattern. Traceability of the CGH is tied to that of the tools it is fabricated with and the interferometer it is used with. Uncertainty is broken down term by term (substrate error, encoding error, pattern writing error, wavelength variation, mounting distortion, and measurement noise) along with methods for reducing each.

Reframing the CGH as an optical geometry reference, we see how it can assess more than just surface figure. Combined with reflective targets like flats, spheres, and corner cubes, light from a single CGH can be used for measuring multiple degrees of freedom and tying them to wavefronts.  This is especially useful for measuring the geometrical properties of an optical element in addition to the surface form.  Additional examples will demonstrate CGH-enabled measurement of radius of curvature, wedge, decenter, and thickness error, along with the location of the optical surface relative to its datums, often in a single setup.

Learning Outcomes

Upon completion of this tutorial, attendees will be able to:

1. Explain how a CGH generates a custom wavefront, and how that wavefront nulls an aspheric or freeform surface in an interferometric test.
2. Break CGH measurement error into its uncertainty terms and identify methods for reducing each.
3. Describe how auxiliary alignment patterns and precision datum targets extend a CGH beyond surface figure to measure radius of curvature, wedge, decenter, thickness error, and surface location relative to datums.
4. Review a technical drawing to identify which callouts a CGH can address, and specify, procure, and validate a hologram for their own application.