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Reflection spectra of POFs (containing 10 wt % benzophenone) with invisible patterns printed under different duration of UV irradiation: (a) 0.5, (b) 1, (c) 2 h. Insets are the images of POFs before and after 8% applied strain. White and red spots show collection regions of the reflection spectra. (d) Difference of the peak reflection wavelength from irradiated and nonirradiated regions under 8% strain.
