  We propose a novel measurement technique capable of determining the temporal
duration of an intense laser pulse directly in its focus at full intensity. We
show that the electromagnetic radiation pattern emitted by an electron bunch
with a temporal energy chirp colliding perpendicularly with the laser pulse
exhibits a distinct dependence on the pulse's duration. As the electrons emit
radiation into an angular region determined by the ratio of their instantaneous
energy to the laser's local field strength, the temporal change of the
electrons' energy imprints information about the laser's pulse duration onto
the angular radiation distribution. We quantify the interaction by a simplified
analytical model and confirm this model's predictions by numerical simulations
of the electrons' dynamics inside a realistically focused laser field. Based on
these findings the pulse's duration can be determined to an accuracy of several
percent.
