Förster (Fluorescence) Resonance Energy Transfer (FRET) has become a powerful tool to study protein-protein interactions and signal transduction in living cells. FRET is commonly read out either by detecting the ratio of the donor and acceptor intensities (sensitized emission) or by detecting the excited state lifetime of the donor, which decreases with increasing FRET (Fluorescence Lifetime IMaging or FLIM). FLIM is robust, immune to bleaching and inherently quantitative. On confocal microscopes, FLIM is typically read out by Time-Correlated Single Photon Counting (TCSPC). This requires expensive add-on hardware and is inherently very slow, necessitating accumulation of many consecutive scans to arrive at low-noise lifetime images. For this reason fast lifetime changes, such as those encountered when reading out live-cell signaling events with FRET sensors, cannot be detected. Accumulation of images and/or very slow scanning can also cause morphological artifacts in the acquired images.