Optical filter wavefront distortion: out-of-band to in-band predictions and the effect of the illumination source bandwidth.

The wavefront distortion (WFD) of a surface with an optical filter coating is ideally measured at the operating wavelength () and angle of incidence () of the filter. However, this is not always possible, requiring that the filter be measured at an out-of-band wavelength and angle (typically =633 and =0). Since the transmitted wavefront error (TWE) and reflected wavefront error (RWE) can depend on the measurement wavelength and angle, an out-of-band measurement may not give an accurate characterization of the WFD.