Measurements of the radiative decay of the 3P 1 state in Kr are performed in the 0.006-10 Torr pressure range in a cylindrical geometry with a radius of 0.0128 m. Due to the good accuracy of the experimental results a decrease of the fundamental mode radiative decay rate β is observed in the 0.6-4 Torr interval, where present theories, based on the impact approximation, predict a nearly constant decay rate. This decrease has not been reported previously in the literature. It is shown that the impact approximation is not valid in this optical depth range and that the duration of collision effects has to be taken into account. An analytical formula for the apparent decay rate β, in which the pressure broadening is treated regardless of the duration of the collisions, is proposed. This formula yields a precise reproduction of the observed decrease.