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Generation of near -infrared light within the first biological optical window via frequency upconversion in Tm3+-doped PbGeO3-PbF2-CdF2, glass excited within the second biological window at 1.319 pm is reported. The upconversion emission at 800nm is the sole light signal observed in the entire ultraviolet-visible-near-infrared spectral region making it possible obtaining high contrast imaging. The dependence of the 800 ntn signal upon the sample temperature was investigated arid results showed an increase by a factor of x 2.5 in the 30-280 degrees C range. Generation of detectable 690 nm for temperatures above 100 degrees C in addition to the intense 800 am main signal was also observed. The proposed excitation mechanism for the 800 nm thulium emitting level is assigned to a multiphononassisted excitation frotn the ground-state H-3(6) to the 3H5 excited-state level, a rapid relaxation to the 3E4 level and followed by an excited-state absorption of the pump photons mediated by multiphonons connecting the 3F,1 level to the H-3(4) emitting level. (C) 2015 AIP Publishing LLC.