The rarefaction of a shock‐heated solid surface into vacuum and the luminous radiation emitted by the surface were studied using a one‐dimensional hydrodynamic code. The temporal evolution of the spectral and brightness temperatures as observed by a remote detector was determined. Moreover, the time‐integrated spectral temperature was computed to simulate the actual experimental measurements. For shock speeds greater than 106cm/s and for measurement times exceeding a few picoseconds, the code predicted a significiantly lower ‘‘observed’’ temperature, in good agreement with experiments.