text: The reading of data from 3D optical memories has been carried out in many different ways. While some of these rely on the nonlinearity of the light-matter interaction to obtain 3D resolution, others use methods that spatially filter the media's linear response. Reading methods include:
  Two photon absorption (resulting in either absorption or fluorescence). This method is essentially two-photon microscopy.
  Linear excitation of fluorescence with confocal detection. This method is essentially confocal laser scanning microscopy. It offers excitation with much lower laser powers than does two-photon absorbance, but has some potential problems because the addressing light interacts with many other data points in addition to the one being addressed.
  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.
  Second-harmonic generation has been demonstrated as a method to read data written into a poled polymer matrix.
  Optical coherence tomography has also been demonstrated as a parallel reading method.