Fig. 1. Pilot/Chenguang thermosensitive erasable pen writingbefore erasing (a), (d), after erasing (b), (e), and fluorescent effect (c), (f)

Fig. 2. Fluorescence excitation spectra (a, monitoring wavelength: 716 nm) and 716 nm peak intensities (b, excitating wavelength: 365 nm) of erasable pens

Fig. 3. The micromorphology of erasing part of the bottom half handwrite (a), and the micromorphology of the erased handwriting (b), at 350× magnification

Fig. 4. Angcai handwriting (a), fluorescence of Angcai handwriting (b); handwriting of lemon juice (c), fluorescence of lemon juice (d)

Fig. 5. Fluorescence exitation and emission spectra of Angcai pen and white paper with a monitoring wavelength of 447 nm and a excitating wavelength of 365 nm

Fig. 6. Reflectance spectra of white paper and lemon juice (a), original appearance of lemon juice handwriting (b) and appearance of lemon juice handwriting irradiated by 365 nm beam (c)

Fig. 7. Original appearance of the chenguong handwriting (a), being covered by Pilot (b), the infrared 850 nm appearance (c)

Fig. 8. Microscope image of covered handwriting at 350× magnification

Fig. 9. Transmissivity curves of different brand pen ink on glass (a), reflectivity curves of different brand pen ink on paper (b)

Fig. 10. LCTF imaging spectrometer (a), spectral curves of different kinds of handwriting (b)

Fig. 11. Chenguang erasable pen erasing handwriting, Angcai steganography pen handwriting, lemon juice handwriting (a), and classification result based on hyperspectral image (b)