No abstract

There are several sources of error in interferometry to consider when testing surfaces in a non-null configuration. A model of the interferometer is typically used to calibrate these errors, but the model differs from the actual interferometer due to the alignment and tolerance of individual components. Reverse raytrace calibration using a model that differs from the real system corrects some errors but introduces others. Reverse optimization using measurements from known test configurations or configuration changes can produce a model that better reflects the real system. This paper addresses the tolerances required to obtain calibration precision from reverse ray tracing. The sources of error can be separated in a way that allows the amount of correction to be compared to the generated errors from misalignment. These errors can be expressed in a generic way that can be applied to any arbitrary interferometer architecture or test surface shape. The simulation results of a standard interferometer with standard tolerances shows that errors corrected by reverse ray tracing can be on the same order as the errors generated by reverse ray tracing an incorrect model. The efficacy of the calibration method resides in correction of other errors such as distortion and ray intercept coordinate error. These corrections are much larger than misalignment errors for surfaces with large departures. This method can be used to determine the level of interferometer component alignment required to accurately measure large departure surfaces with reverse ray tracing.

Abstract. Contact lens performance depends on a number of lens properties. Many metrology systems have been developed to measure different aspects of a contact lens, but none test the surface figure in reflection to subwavelength accuracy. Interferometric surface metrology of immersed contact lenses is complicated by the close proximity of the surfaces, low surface reflectivity, and instability of the lens. An interferometer to address these issues was developed and is described here. The accuracy of the system is verified by comparison of glass reference sample measurements against a calibrated commercial interferometer. The described interferometer can accurately reconstruct large surface departures from spherical with reverse raytracing. The system is shown to have residual errors better than 0.05% of the measured surface departure for high slope regions. Measurements made near null are accurate to λ∕20. Spherical, toric, and bifocal soft contact lenses have been measured by this system and show characteristics of contact lenses not seen in transmission testing. The measurements were used to simulate a transmission map that matches an actual transmission test of the contact lens to λ∕18.