A time-resolved fluorescence imaging (TRFI) system that images a target medium without lifetime fitting. Instead of extracting the lifetime precisely, the system images the fluorophore distribution to allow for a simple and accurate method to obtain the fluorescence image without lifetime-extraction for time-resolved fluorescence imaging. An illumination source circuit for TRFI is also disclosed that shapes the excitation pulse. In one embodiment, the illumination source comprises an LED and stub line configured for generating a linear decay profile.