NASA Goddard Space Flight Center has developed a verification method to qualify null lenses used in the precision fabrication and testing of an aspheric optic. It uses image-based wavefront sensing to verify that the null optic has been manufactured correctly and integrated correctly into the test setup. The independent characterization of the null optic prior to the aspheric figure measurement reduces uncertainties in the metrology setup and improves the accuracy in the measurement of the absolute figure of the aspheric optic.
This innovation uses image-based wave-front sensing or phase retrieval, which uses light traveling through the optical system in generally the same way that it will in its as-used configuration. It estimates the wavefront of the optical system using images collected on a light detector (which can be the same detector used by the optical system in its intended use).
This method uses several images taken through the null optic under test, with some aspect of the test setup being systematically varied from one image to the next. Usually this variation involves somewhat out-of-focus images that are taken systematically with different amounts of defocus. The light for each diversity-defocus image passes through the optical test setup and the null lens. The resulting irradiance of the transmitted beam is measured by a light detector and this image is saved on a computer where algorithms determine an estimate of the null lens's wavefront aberrations.