An apparatus includes a delay producing system that receives a parent electromagnetic pulse and outputs two spatially-overlapped children electromagnetic pulses having a relative and adjustable time delay between each other; a fluorescence producing device placed in a path of the children electromagnetic pulses; a guidance system that is in the path of the children pulses and is positioned between the delay producing system and the fluorescence producing device to guide the children pulses to the fluorescence producing device; a detector that receives fluorescence produced at the fluorescence producing device, where the fluorescence travels at least partly through the guidance system before reaching the detector; and a data system that receives the value of the time delay and the output of the detector and determines the electromagnetic pulse duration based on the value of the time delay and the output of the detector.