The present invention relates to an information reading method, and more particularly, to a method of optically reading information, such as characters, digits, patterns or symbols. More particularly, the present invention pertains to a method of reading information recorded using a phosphor capable of emitting light in the infrared wavelength region of the spectrum when excited by suitable light.
Optical information reading methods are known in which information, such as characters, digits, patterns or symbols, is recorded using an ink which emits fluorescence and in which the recorded information is read by observing the light emitted from the ink when the ink is excited by adequate light.
Such conventional information reading methods are disclosed in, for example, Japanese Patent Laid-Open No. 33634/1979.
In this information reading method, information recorded using a phosphor which is activated to a predetermined medium by neodymium and ytterbium ions is excited by a GaAlAs light-emitting diode designed to emit light with the peak wavelength of 800 nm, and the light emitted from the stimulated phosphor is read by a photodetector designed to detect light with wavelengths in the range between 900 nm and 1100 nm.
However, in such a conventional information reading method, when the phosphor is excited by light with a wavelength near 800 nm, the photodetector detects that exciting light as well as the light emitted from the phosphor. Therefore, a filter made of, for example, InP polycrystal having a thickness of 0.5 mm must be employed so as to cut out light of a wavelength near 800 nm, so the device using this method is expensive and has a complicated structure.