The invention relates to a tissue monitoring apparatus, a tissue monitoring method and an ablation lesion monitoring, measuring, and controlling automated algorithm incorporating diffuse reflectance spectroscopy (DRS) and/or Arrhenius model thermal denaturation kinetics for determining the characteristics of the lesion or the tissue, especially for identifying the transmurality of the ablation lesion. The invention pertains to a device for and method of real time monitoring of lesion formation as ablation is being carried out.