sentences:  Measurement of small differences in the refractive index between the two data states.This method usually employs a phase-contrast microscope or confocal reflection microscope.No absorption of light is necessary, so there is no risk of damaging data while reading, but the required refractive index mismatch in the disc may limit the thickness (i.e., number of data layers) that the media can reach due to the accumulated random wavefront errors that destroy the focused spot quality.