The optical recording media in question are suitable for writing and reading audio information, video images and data. Said recording media are mostly disc-shaped and the writing and reading of information takes place during rotation of the medium by a light beam emanating from a laser. The projection of the modulated laser beam on the recording medium brings about a change of the optical properties of said medium during the writing operation, which change can be optically detected later on during reading. Known media are those which are based on the magneto-optical or Kerr-effect and the "phase-change" mechanism. Further known media include those which contain photochromic compounds, in which the color of the compounds changes upon exposure to light of a specific wavelength. Detection of the change in color takes place by using light of a different wavelength.
An optical recording medium of the type mentioned in the opening paragraph is described in European Patent Application EP-A 312339. The medium described therein contains optically active compounds comprising an asymmetric carbon atom and hydrophobic and hydrophilic groups. Exposure to light of a wavelength of 360 nm causes the aggregation structure of said compounds to change, as a result of which a change takes place in the optical rotation. Said change in optical rotation is detected with linearly polarized light having a wavelength of 700 nm. The recorded information is erased with light having a wavelength of 560 nm, as a result of which the optical rotation returns to the original state. Erasing can also be carried out by a combined operation of irradiating the medium with light having a wavelength of 360 nm and heating it to 100.degree. C. Of essential importance is the use of three wavelengths or two wavelengths in combination with a heating step for the write, read and erase processes, because the information must not be erased during reading. As a result of this requirement, use of the known medium is rendered complicated.
USPTO Backgrounds
USPTO Backgrounds