The upconversion of infrared (i.r.) radiation into blue luminescence has been studied in a heavy metal fluoride glass: BZYTLTm codoped with Yb3+ and Tm3+ ions as a function of Tm3+ concentration and temperature. This blue emission is centered at 478 nm and corresponds to the 1G4 --> H-3(6) transition. The dependence of the integrated intensity on the excitation power confirms that this emission is due to a three-photon absorption process. An absolute efficiency of 2 x 10(-8) has been obtained in the 0.05 mol% TmF3 and 26 mol% YbF3 codoped glass, for a 16.5 mW cm-2 i.r. excitation intensity at room temperature using a comparative method. The intensity of this emission was 20 times stronger at 10 K. The heat-treatment increases the upconversion emission intensity by a factor of 10 at room temperature.