Precise surface form measurements of off-axis aspheric freeform optics

Abstract

Freeform optical components are increasingly demanded by optical manufacturers and researchers, and are always a challenging topic for metrologists. Form errors of the freeform surfaces resulting from the manufacturing process are critical, in terms of the functionality and reliability of the freeform optics. This paper presents two methodology case studies of off-axis aspheric optics using contact profilometry and non-contact scanning point interferometry. The contact method is accomplished by use of an ultra-low noise measurement platform, combined with a patented phase grating interferometry (PGI) technology and specially developed algorithms for calibration and analysis. The study shows the capability of the proposed method for high tangential slope freeform measurement. This slope measurement capability of PGI Freeform, together with its large gauge range, enables 3D form measurements for most freeform surfaces. However, for some optical surfaces non-contact measurement is preferred due to the possible surface damage caused by the stylus force of contact method. Non-contact scanning metrology is based on a patented multi-wavelength interferometry (MWLI) technology. It provides high density 3D data in short measurement times at a highly reproducible form measurement accuracy. The long-range absolute measurement capability of the MWLI sensor, together with its ultraprecision metrology platform and improved calibration routine through which the sensor accurately follows the designed shape of optical surfaces, enables precise 3D freeform surface measurements within its tangential slope measurement range.