Innovators at NASA Marshall, in conjunction with the Center for Applied Optics at the University of Alabama in Huntsville, developed a system that increases the spatial resolution of interferometric measurements of optical surfaces beyond the fundamental Nyquist Limit of the interferometer’s imaging system/detector. The Sub-Pixel Spatial Resolution Interferometry (SSRI) system enables optical fabricators to accurately qualify surface features of spatial frequencies that could not be accurately resolved using standard interferometers.
Current trends in optical design are producing new types of specifications, requiring distinct spatial frequency bands and calling for larger aperture optics; however, the ability to characterize these features over the full aperture necessitates improvements. The SSRI system meets this need by increasing the spatial resolution of full aperture interferometric measurements using a technique that incorporates interlaced stitching of low-resolution measurements taken at sub-pixel lateral shifts to enhance spatial resolution.
The SSRI system increases the spatial resolution of the interferometric measurements by performing multiple measurements of an optical surface with sub-pixel shifts between a charged coupled device (CCD) detector and the optic under test between measurements. The measurements are then combined to create a single measurement with higher spatial resolution.
The measurements are combined using a stitching algorithm. This method relieves the spatial resolution constraint defined by the pixel size and spacing. It is assumed that the optical system has been designed to allow the higher spatial frequency features to pass. This new technique will enable precision optics to be accurately qualified against the specifications and allow fabricators to deterministically correct the higher spatial frequency errors. The SSRI system can be integrated, with slight modifications, into existing commercially available interferometers and interferometric profilers to increase the obtainable spatial resolution.