This paper describes a multiwavelength optical pyrometer designed to measure, with a 3 ns rise-time, the evolution of a fast varying temperature as can be encountered in the detonation products of an explosive. By measuring six luminance temperatures within the visible and near infra-red spectra, this apparatus allows to determine a true surface temperature over the range 1500 to 6000 K. It has been used for studies of detonation in nitromethane, a transparent liquid explosive.