There are many types of known optical recording elements. In many of the elements, the mode of operation requires a recording layer having a high absorption and the recorded marks, often referred to as pits, have low optical density or high reflectivity. The high reflectivity pits are made by ablating away the high absorption recording material, usually exposing an underlying reflective layer.
One of the currently popular forms of optical storage of information is the compact disk or CD. Digital information is stored in the form of high optical density marks or pits on an otherwise reflective background, as read with a focused laser diode operating in the 780-835 nm spectral region. This is the exact opposite of the above described optical recording materials. In this format, the optical information is most often in the form of read only memory or ROM. Optical information is not usually recorded in real time but rather is produced by press molding. In a typical process, the optical recording substrate is first press molded with a master containing the digital information to be reproduced. The thus formed information is then overcoated with a reflective layer and then with an optional protective layer. In those areas having the deformations or pits, the optical density is higher than in those areas not having the deformations.
It is desirable to produce optical recording elements which, when recorded in real time, produce a record that mimics the conventional CD on read out by generating dark marks on a reflective background.
One recently disclosed system of this type is the so called "Photo CD". In this system, conventional photographic film is first processed in a conventional manner. Then, the images from the film are scanned and digitized. The digitized information is recorded in a CD readable form on an optical recording element. Images can then be played back on a CD type player into a conventional television.
Commercially useful materials of the type described in these references have stringent requirements. The recording layer must be able to couple with incident write laser irradiation to provide features having sufficient contrast. At the same time the unmarked portion of the disk must have sufficient reflectivity to conform to the 70% CD-ROM standard (ISO/IEC 10149). The layer must also have good stability towards light, heat and humidity for acceptable shelf life. Photo CD, for example, is a consumer product and it must be capable of withstanding extreme environments. Between the time the original images are recorded on the Photo CD and the time subsequent images are recorded, the CD might be placed in strong sunlight.
Thus, there is a continuing need for optical recording materials that have the necessary optical characteristics so that they are CD compatible, can couple with incident laser irradiation to form features with sufficient contrast, meet the minimum reflectivity standard, and yet are light stable. It is to a solution to this problem that the present invention is directed.
U.S. Pat. No. 5,426,015 discloses optical recording elements having recording layers containing tetra dyes. These dyes comprise metallized azodianions with cationic dye counterions. These elements exhibit sensitivity that could be improved.