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Abstract

A method for providing intelligible markings that are virtually invisible to the unaided eye on the surface of an article, comprises: applying onto the surface, by a jet printing process, a marking medium comprising a compatible liquid or viscous substance containing an organic laser dye (typically IR-125 in a concentration of about 0.005 to 0.05 percent by weight of the medium) that is poorly absorptive of radiation in the visible range of about 400 to 700 nanometers, is highly absorptive of radiation in the near infrared range of at least about 750 nanometers in wavelength (typically about 750 to 900 nanometers), and fluoresces in response to radiation excitation in the said near infrared range to produce fluorescent radiation of wavelengths longer than the wavelength of the excitation (typically in the range of about 800 to 1100 nanometers). Other useful laser dyes may comprise DTTCI, DNTTCI, HDITCI, DDTTCI, IR-140, DDCI-4, or IR-132.

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First Claim