Reconfigurable interferometric form and mid-spatial frequency metrology for extended reality freeform optics

This work presents a reconfigurable interferometric null test for precision measurement of form and mid-spatial-frequency (MSF) surface errors in rectangular freeform mirrors using a spatial light modulator (SLM) as a reconfigurable nulling element. A custom alignment strategy, optomechanical null test assembly, and metrology process chain enable repeatable interferometric measurements of high base-curvature and stress-sensitive polymer optics. Measurements on a representative near-eye display mirror show form-error repeatability within 60 nm in peak-to-valley and 19 nm in RMS values. MSF signatures are analyzed through spatial-frequency filtering and validated against a fixed computer-generated hologram (CGH), showing agreement within a 4.6 nm noise floor. This work further highlights the limitations of fixed CGH technology in managing straylight in small, high base-curvature parts, compared with reconfigurable SLM-based null tests. The demonstrated metrology process provides a pathway toward reconfigurable, high-precision testing of rectangular freeform optics for emerging extended reality technologies.