A system for monitoring tissue ablation comprising: a catheter comprising: a catheter body; and a distal tip positioned at a distal end of the catheter body, the distal tip defining an illumination cavity having one or more openings for exchange of light energy between the illumination cavity and a tissue; an ablation system in communication with the distal tip to deliver ablation energy to the distal tip; a visualization system comprising a light source, a light measuring instrument, and one or more optical fibers in communication with the light source and the light measuring instrument and extending through the catheter body into the illumination cavity of the distal tip, wherein the one or more optical fibers are configured to pass light energy in and out of the illumination chamber; a processor in communication with an ablation energy source, the light source and the light measuring instrument, the processor being programmed to: collect light reflected from a tissue illuminated to excite NADH in the tissue, while ablation energy is being applied to the tissue to form a lesion in the tissue; monitor a level of NADH fluorescence in the illuminated tissue; characterised in that said processor is further programmed to cause ablation of the tissue to stop when the level of NADH fluorescence reaches a steady state of continued NADH fluorescence for a specific period of time of 60% or more decrease in NADH from a base level in the beginning of the ablating. a catheter comprising: a catheter body; and a distal tip positioned at a distal end of the catheter body, the distal tip defining an illumination cavity having one or more openings for exchange of light energy between the illumination cavity and a tissue; a catheter body; and a distal tip positioned at a distal end of the catheter body, the distal tip defining an illumination cavity having one or more openings for exchange of light energy between the illumination cavity and a tissue; an ablation system in communication with the distal tip to deliver ablation energy to the distal tip; a visualization system comprising a light source, a light measuring instrument, and one or more optical fibers in communication with the light source and the light measuring instrument and extending through the catheter body into the illumination cavity of the distal tip, wherein the one or more optical fibers are configured to pass light energy in and out of the illumination chamber; a processor in communication with an ablation energy source, the light source and the light measuring instrument, the processor being programmed to: collect light reflected from a tissue illuminated to excite NADH in the tissue, while ablation energy is being applied to the tissue to form a lesion in the tissue; monitor a level of NADH fluorescence in the illuminated tissue; characterised in that said processor is further programmed to cause ablation of the tissue to stop when the level of NADH fluorescence reaches a steady state of continued NADH fluorescence for a specific period of time of 60% or more decrease in NADH from a base level in the beginning of the ablating. collect light reflected from a tissue illuminated to excite NADH in the tissue, while ablation energy is being applied to the tissue to form a lesion in the tissue; monitor a level of NADH fluorescence in the illuminated tissue; characterised in that said processor is further programmed to cause ablation of the tissue to stop when the level of NADH fluorescence reaches a steady state of continued NADH fluorescence for a specific period of time of 60% or more decrease in NADH from a base level in the beginning of the ablating. <EOS>