3.1. Experimental luminescence proof
First of all we verified that the revealed effect was effectively
luminescence. In order to provide a qualitative description of this
observed effect, we measured a reflectance spectrum of the
considered thermal paper by employing a UV–vis micro-spectrophotometer
(S.E.E. Inc. 2100), equipped by an optical objective 15,
which allows to analyze a region of diameter equal to roughly
12 mm. The used instrument mounted a Xenon lamp impinging the
surface of the paper. For each wavelength between 240 nm and
900 nm, we measured the fraction of the input radiation which was
reflected. We assumed that the radiation which was not reflected,
was absorbed by the paper. The performed measure gives a
qualitative estimation of the spectral region of maximum
absorption for the considered paper. The detection was performed
by a CCD camera and a spectrometer based on two diffraction
gratings (the spectral resolution achieved with this instrument is
2 nm). The detection time for each step of the performed measure
was chosen in order to maximize the revealed signal.
We measured even the emission spectrum of the same paper
when the wavelength of the input radiation belongs to the found
interval of absorbed light. This measure allows to verify that there
is no overlap between the emission and the absorption region. In
order to perform this measurement we employed a function
provided by the VSC6000. The aim of this paper is not to provide a
quantitative description of the studied thermal paper. For this
reason we report only one spectrum obtained by illuminating an
highway ticket in a particular visible spectral region [see Fig. 1].
Based on this measure, it is clear that we revealed a luminescence
effect given by the substrate of thermal paper. We verified that
similar behavior can be detected for the other types of thermal
paper.
3.1. Experimental luminescence proofFirst of all we verified that the revealed effect was effectivelyluminescence. In order to provide a qualitative description of thisobserved effect, we measured a reflectance spectrum of theconsidered thermal paper by employing a UV–vis micro-spectrophotometer(S.E.E. Inc. 2100), equipped by an optical objective 15,which allows to analyze a region of diameter equal to roughly12 mm. The used instrument mounted a Xenon lamp impinging thesurface of the paper. For each wavelength between 240 nm and900 nm, we measured the fraction of the input radiation which wasreflected. We assumed that the radiation which was not reflected,was absorbed by the paper. The performed measure gives aqualitative estimation of the spectral region of maximumabsorption for the considered paper. The detection was performedby a CCD camera and a spectrometer based on two diffractiongratings (the spectral resolution achieved with this instrument is2 nm). The detection time for each step of the performed measurewas chosen in order to maximize the revealed signal.We measured even the emission spectrum of the same paperwhen the wavelength of the input radiation belongs to the foundinterval of absorbed light. This measure allows to verify that thereis no overlap between the emission and the absorption region. Inorder to perform this measurement we employed a functionprovided by the VSC6000. The aim of this paper is not to provide aquantitative description of the studied thermal paper. For this
reason we report only one spectrum obtained by illuminating an
highway ticket in a particular visible spectral region [see Fig. 1].
Based on this measure, it is clear that we revealed a luminescence
effect given by the substrate of thermal paper. We verified that
similar behavior can be detected for the other types of thermal
paper.
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