Generated optically, holographic gratings generally do not display periodic errors or ghosts often found in ruled gratings. They typically have a sinusoidal groove profile and are generated by the recording of an interference pattern onto a photoresist-coated substrate. Newport's holographic gratings can provide excellent wavefront flatness and high efficiency in a single plane of polarization. Blazed gratings are generated by bombarding holographically recorded masters with a beam of ions. This etching process alters the groove profile from sinusoidal to triangular, which often increases the peak efficiency of the grating. Like their ruled counterparts, holographic gratings are most effective when used in the Littrow configuration.