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.