Bat-wing attenuation in white-light interferometry

Bat wings are removed at step discontinuities within the coherence length of the light source of a vertical-scanning interferometer. A first height profile is obtained from a correlogram using a coherence-sensing technique. A second height profile is obtained from phase measurements at the best-focus frame position of the scanner. The two profiles are compared, and phase ambiguities are removed in conventional manner. In addition, during unwrapping the differences in height between two adjacent pixels obtained both by coherence sensing and by phase measurements are compared to flambd/4. Where the inter-pixel height difference calculated by coherence sensing is smaller and the inter-pixel height difference calculated by phase is larger than flambd/4, the phase measurement is corrected by 2pi increments until both coherence and phase inter-pixel height differences are within flambd/4. This additional step removes bat-wing effects from profiles obtained by phase measurement.