Solutions for environmentally robust interferometric optical testing

Optical interferometry is widely used for surface metrology, and phase-shifting interferometry (PSI) remains the "gold standard" for measuring surface form and texture. In the past, the environmental sensitivity of PSI relegated its use to dedicated metrology labs with well-controlled environments. As metrology requirements have expanded beyond the lab, advances in metrology techniques now enable high-precision optical testing in places previously considered inappropriate for interferometric techniques. A class of environmentally-robust methods preserve the gold-standard accuracy of PSI by measuring and accounting for rigid-body motions from vibration and large-scale airflow during the PSI acquisition. These methods, including model-based PSI, use a physical model of the interference to measure cavity rigid body motion and accommodate a wide variety of measurement geometries, surface shapes, surface departure, reflectivities, and environmental conditions without changing the user's measurement process. For more extreme environments, spatial carrier methods analogous to off-axis holography extract the surface phase fast enough to "freeze" vibration and air turbulence in a single camera frame. The price paid for single-frame acquisition is a loss of lateral resolution as well as a modest increase in sensitivity to optical aberrations, requiring calibration for compensation. This paper will describe current techniques and methods, as well as provide examples and results for practical measurement scenarios in the manufacture of high-precision optical components and systems.